Understanding Multiple Spanning Tree Protocol

Introduction

Over time I was thinking of putting together the two blog posts made in the past about MSTP and adding more clarification for MSTP multi-region section. This new blog post recaps the information posted previously and provides more details this time. Additionally, it discusses some MSTP design-related questions. Both single-region and multiple-region MSTP configurations are reviewed in the post. The reader is assumed to have a good understanding of classic STP and RSTP protocols as well as Cisco's PVST/PVST+ implementations.

Table of Contents

Due to the large size of the document, a table of contents is provided for the ease of navigation.

Historical Review
Logical and Physical Topologies
Implementing MSTP
Caveats in MSTP Design
MSTP Single-Region Configuration Example
Common and Internal Spanning Tree (CIST)
Common Spanning Tree (CST)
Mapping MSTI’s to CIST
MSTP Multi Region Design Considerations
Interoperating with PVST+
Scenario 1: CIST Root and CIST Regional Root
Scenario 2: MSTIs and the Master Port
Scenario 3: PVST+ and MSTP Interoperation
Conclusions
Further Reading

Historical Review

In the beginning, there was IEEE STP protocol, which was preceded by DEC and IBM STP variants. All of them utilized the same logic originally proposed by Radia Perlman in the 80s, while she was working in DEC. The IEEE version was adapted for use with multiple VLANs using 802.1q frames tagging. A shared spanning-tree, sometimes called Mono Spanning Tree (MST) by Cisco, or more often - Common Spanning Tree (CST) was used to create a single loop-free topology. The drawback of this approach is inability to perform VLAN traffic engineering across redundant links: if a link is blocked, it is blocked for all VLANs. Another issue related to STP construction - more traffic is forwarded over the links closer to the root bridge, which puts higher demand on the root bridge resources - both in terms of CPU and links capacity utilization.

To overcome these limitations, Cisco introduced proprietary Per-VLAN Spanning Tree Protocol (PVST), using separate STP instances per VLAN. Initially, PVST was created to be used with Cisco's proprietary ISL encapsulation only, but the later PVST+ version allowed for tunneling PVST BPDUs over 802.1q trunks and IEEE STP domains. PVST allowed for using different logical topology with every VLAN, enhancing basic Layer 2 traffic engineering. Every VLAN may use its own root bridge and forwarding topology allowing for more fair resource utilization. This method has some limitations as it does not deal with the actual network link capacities and utilization, but rather statistically multiplexes VLANS to different topologies. However, this is the limitation inherent to any load-balancing method based on STP. The main problem of PVST was that with the number of VLANs growing, PVST becomes a waste of switch resources and management burden. This is because the number of different logical topologies is usually much smaller than the number of active VLANs.

With time, PVST adopted fast-convergence properties introduced by IEEE RSTP protocol, but the core feature of keeping a separate copy of STP per VLAN did not change. Seeing the problems associated with the PVST approach, Cisco came up with the idea of decoupling the concepts of STP instances and VLANs. The initial implementation was called MISTP (Multiple Instances Spanning Tree) and later evolved into IEEE 802.1s standard called MSTP (Multiple Spanning Trees Protocol).

Logical and Physical Topologies

The core idea of MSTP is utilizing the fact that a redundant physical topology only has a small amount of different spanning-trees (logical topologies). The figure below shows a ring topology of three switches and three different spanning trees that may result from different root bridge placements.

Instead of running an STP instance for every VLAN, MSTP runs a number of VLAN-independent STP instances (representing logical topologies) and then the administrator maps each VLAN to the most appropriate logical topology (STP instance). The number of STP instances is kept to a minimum (saving switch resources), but the network capacity is utilized in more optimal fashion, by using all possible paths for VLAN traffic.

The switch logic for VLAN traffic forwarding has changed a little bit. In order for a frame to be forwarded out of a port, two conditions must be met: first, VLAN must be active on this port (e.g. not filtered) and second, the STP instance the VLAN maps to, must be in non-discarding state for this port. The second property is normally enforced automatically, as MAC addresses are not learned on discarding ports. It is worth reminding that due to multiple logical topologies active on a port, the port could be blocking for one instance and forwarding for another (note that in (R)PVST+ a port is either forwarding or discarding for a VLAN). The figure below demonstrates six VLANs using two MSTP instances, thus reducing the number of STP trees that would be required with PVST from 6 to 2.

Implementing MSTP

The following is a set of implementation-related questions that a theoretical implementation needs to address:

• Logical Topology Calculation. How to build multiple STP instances (logical topologies) in a single physical topology? Should we run multiple STP instances sending their BPDUs independently? If yes, then how would we distinguish every instance's BPDUs? VLAN tags cannot be utilized for that purpose, as STPs are not bound to VLANs anymore.
• Information Distribution. What protocol should be used to distribute VLAN to instance mapping tables among switches? Should VLAN IDs be placed in BPDUs along with respective instance numbers?
• Consistency Check. How to ensure the VLANs to instance mapping is consistent across all switches? That is, how would a switch know that another switch maps VLAN X to the same instance?

MISTP vs MSTP

Original Cisco MISTP pre-standard implementation was sending separate BPDUs for each instance. Every BDPU contained an instance number and a list of VLANs, mapped to send switches to this particular instance - that allowed for consistency checks between the switches. The table mapping VLANs to instance numbers has to be configured on each switch separately. There was no automated mechanism to distribute VLAN to instance mappings between the switches.

The final implementation adopted by the IEEE 802.1s standard made this mechanics more elegant and simple. Before we proceed with discussing IEEE's implementation, let's define MSTP region as a collection of switches, sharing the same view of physical topology partitioning into a set of logical topologies. For two switches to become members of the same region, the following attributes must match:

• Configuration name.
• Configuration revision number (16 bit value).
• The table of 4096 elements that map the respective VLANs to STP instance numbers.

The IEEE 802.1s implementation does not send BDPUs for every active STP instance separately, nor does it encapsulate VLAN numbers list configuration messages. Instead, a special STP instance number 0 called Internal Spanning Tree (IST aka MSTI0, Multiple Spanning Tree Instance 0) is designated to carry all STP-related information. The BPDUs for IST contain all standard RSTP-style information for the IST itself, as well as carry additional informational fields. Among those fields are configuration name, revision number and a hash value calculated over VLANs to MSTI mapping table contents. Using just this condensed information switches may detect mis-configuration in VLAN mappings by comparing the hash value received from the peer with the local value.

M-Records

By default, all VLANs are mapped to the IST. This represents the case of classic IEEE RSTP with all VLANs sharing the same spanning-tree. Other MSTP instances could be enabled, and they are referred to as Multiple Spanning Tree Instances (MSTIs). Every MSTI assigns its own priorities to the switches and uses its own link costs to come up with a private logical topology, separate from the IST. Since MSTP does not send MSTI's information in separate BPDUs, this information is piggybacked into the IST's BPDUs using special M-Record fields (one for every active MSTI). Using TLV’s (Type-Length-Value) those fields carry root priority, designated bridge priority, port priority and root path cost among others.

MSTI Tree Construction

Similar to RSTP, every switch emits its own configuration BPDUs, one every Hello seconds. The BDPUs have full information about the IST and carry M-Records for every active MSTI . Using the RSTP convergence mechanics (Proposal & Agreement bits), separate STP instances are constructed for the IST and every MSTI. It is important to notice that fundamental STP timers such as Hello, ForwardTime, MaxAge could only be tuned for the IST. All other instances (MSTI’s) inherit the timers from the IST - this is the natural result of all MSTI information being piggybacked in IST BPDUs.

MSTP has a special mechanism to age out old information out of the domain. The IST BDPUs have a special field called Remaining Hops. The IST root sends BPDUs with hop count equal to MaxHops (configurable value) and every downstream switch decrements the hop count field on reception of IST BPDU. As soon as hop count becomes zero, the information in BPDU is ignored, and the switch may start declaring itself as a new IST root. The classic MaxAge and ForwardDelay timers are still used when MSTP interacts with RSTP, STP or (R)PVST+ bridges.

STP Dispute

Cisco switches have long been implementing LoopGuard feature that allows for blocking the non-designated port when it loses the flow of STP BPDUs. This is helpful for detecting unidirectional links (normally on fiber optical links) and preventing Layer 2 loops that could go undetected by STP. Cisco's implementation of MSTP allows for detecting unidirectional conditions, by comparing the downstream port state reported in BPDUs. If the upstream switch sends superior root bridge information to the downstream bridge but receives the BPDUs with Designated bit set, the upstream switch concludes that the downstream does not hear its BPDU’s. The upstream switch then blocks the downstream port and marks it as an STP dispute link.

Caveats in MSTP Design

There are some issues that may arise due to the fact that spanning-tree instances are not mapped one-to-one to VLANs. With PVST, pruning a VLAN on a link would also disable the corresponding STP on the same link. Since MSTIs are decoupled from VLANs, every MSTI is running on every link in the region. The MSTI’s differ in their decisions to make this link forwarding or blocking. By pruning VLANs you may end up in situations where VLAN is not enabled on the link where the corresponding MSTI is forwarding OR enabled on the link where the corresponding MSTI is blocking. Consider the following example to illustrate this idea:

In this topology, VLANs are manually pruned on trunks. Since the filtering is not consistent with the respective MSTI blocking decisions, VLAN2's traffic is blocked between SW1 and SW2. To avoid this situation, do not use static “VLAN pruning” method of distributing VLANs across trunks when you have MSTP enabled. A situation equivalent to the one described is when ports connecting the switches are access ports. MSTP runs on these ports and have logical topologies either blocking or forwarding on the ports. Depending on VLANs to MSTI mapping, a given VLAN could be blocked on the access ports due to MSTP decision - even though the access VLANs are different, they use the same STP. To avoid this problem, do not run MSTP on access-ports and use them for connecting "stub" devices only – e.g. hosts and leaf switches.

MSTP Single-Region Configuration Example

Now that we have a basic understanding of how MSTP works inside a region let's create a sample configuration. Consider the following physical topology of three switches:

The topology has the following VLANs: 1, 10, 20, 30, 40, 50, 60. Our goals for this scenario are:

• Make VLANs 10,20,30 follow the link from SW3 to SW1.
• Make VLANs 40,50,60 follow the link from SW3 to SW2.
• If any of the above links fail, the affected VLANs should fall-back to the other link.

To accomplish this, we create two MSTIs - number 1 and 2. SW1 will be the root for instance 1 and SW2 will be the root for instance 2. As for the IST (MSTI0), we make SW3 the root switch for it (though it's not recommended to assign root roles to access switches). As for VLAN to MSTI mappings, VLAN 1 will remain mapped to the IST. Remaining VLANs 10, 20 and 30 would map MSTI1, while VLANs 40, 50 and 60 would map to MSTI2. Here is the configuration:

SW1:

spanning-tree mode mst

!

spanning-tree mst configuration

 name REGION1

 instance 1 vlan 10, 20, 30

 instance 2 vlan 40, 50, 60

!

! Root for MSTI1

!

spanning-tree mst 1 priority 8192

!

interface FastEthernet0/13

 switchport trunk encapsulation dot1q

 switchport mode trunk

!

interface FastEthernet0/16

 switchport trunk encapsulation dot1q

 switchport mode trunk

SW2:
spanning-tree mode mst
!
spanning-tree mst configuration
name REGION1
instance 1 vlan 10, 20, 30
instance 2 vlan 40, 50, 60
!
! Root for MSTI 2
!
spanning-tree mst 2 priority 8192
!
interface FastEthernet0/13
switchport trunk encapsulation dot1q
switchport mode trunk
!
interface FastEthernet0/16
switchport trunk encapsulation dot1q
switchport mode trunk

SW3:
spanning-tree mode mst
!
spanning-tree mst configuration
name REGION1
instance 1 vlan 10, 20, 30
instance 2 vlan 40, 50, 60
!
! Root for the IST
!
spanning-tree mst 0 priority 8192
!
interface FastEthernet0/13
switchport trunk encapsulation dot1q
switchport mode trunk
!
interface FastEthernet0/16
switchport trunk encapsulation dot1q
switchport mode trunk

The following show commands will demonstrate the effect our configuration has on traffic forwarding:

SW1#show spanning-tree mst configuration

Name      [REGION1]

Revision  0     Instances configured 3

Instance Vlans mapped
-------- ---------------------------------------------------------------------
0 1-9,11-19,21-29,31-39,41-49,51-59,61-4094
1 10,20,30
2 40,50,60
-------------------------------------------------------------------------------

SW1#show spanning-tree mst

##### MST0 vlans mapped: 1-9,11-19,21-29,31-39,41-49,51-59,61-4094
Bridge address 0019.5684.3700 priority 32768 (32768 sysid 0)
Root address 0012.d939.3700 priority 8192 (8192 sysid 0)
port Fa0/16 path cost 0
Regional Root address 0012.d939.3700 priority 8192 (8192 sysid 0)
internal cost 200000 rem hops 19
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/13 Desg FWD 200000 128.15 P2p
Fa0/16 Root FWD 200000 128.18 P2p

##### MST1 vlans mapped: 10,20,30
Bridge address 0019.5684.3700 priority 8193 (8192 sysid 1)
Root this switch for MST1

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/13 Desg FWD 200000 128.15 P2p
Fa0/16 Desg FWD 200000 128.18 P2p

##### MST2 vlans mapped: 40,50,60
Bridge address 0019.5684.3700 priority 32770 (32768 sysid 2)
Root address 001e.bdaa.ba80 priority 8194 (8192 sysid 2)
port Fa0/13 cost 200000 rem hops 19

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/13 Root FWD 200000 128.15 P2p
Fa0/16 Altn BLK 200000 128.18 P2p

SW1#show spanning-tree mst interface fastEthernet 0/13

FastEthernet0/13 of MST0 is designated forwarding
Edge port: no (default) port guard : none (default)
Link type: point-to-point (auto) bpdu filter: disable (default)
Boundary : internal bpdu guard : disable (default)
Bpdus sent 561, received 544

Instance Role Sts Cost Prio.Nbr Vlans mapped
-------- ---- --- --------- -------- -------------------------------
0 Desg FWD 200000 128.15 1-9,11-19,21-29,31-39,41-49,51-59
61-4094
1 Desg FWD 200000 128.15 10,20,30
2 Root FWD 200000 128.15 40,50,60

SW1#show spanning-tree mst interface fastEthernet 0/16

FastEthernet0/16 of MST0 is root forwarding
Edge port: no (default) port guard : none (default)
Link type: point-to-point (auto) bpdu filter: disable (default)
Boundary : internal bpdu guard : disable (default)
Bpdus sent 550, received 1099

Instance Role Sts Cost Prio.Nbr Vlans mapped
-------- ---- --- --------- -------- -------------------------------
0 Root FWD 200000 128.18 1-9,11-19,21-29,31-39,41-49,51-59
61-4094
1 Desg FWD 200000 128.18 10,20,30
2 Altn BLK 200000 128.18 40,50,60

The link cost values are much higher than the default STP costs (IEEE standard values), and MSTIx is called MSTx (e.g. IST is MST0). Aside from that, note the term “Regional Root” which is to be explained in details below.

Common and Internal Spanning Tree (CIST)

As mentioned before, every MSTP region runs a special instance of spanning-tree known as IST or Internal Spanning Tree (=MSTI0). This instance mainly serves the purpose of disseminating STP topology information for MSTIs. IST has a root bridge, elected based on the lowest Bridge ID (Bridge Priority + MAC address). The situation changes with multiple MSTP regions in the network. When a switch detects BPDU messages sourced from another region (or STP/PVST+ BPDU), it marks the corresponding port as MSTP boundary. For convenience, we would call all other ports “internal”. A switch that has boundary ports is known as a boundary switch.

In the figure below you can see three MSTP regions interconnected in “ring” topology using a pair of links between every pair of regions. The links connecting the regions connect the boundary ports. Since every switch has a connection to some other region, all switches are boundary. Notice the simplified notation for link costs and bridge priorities. We will use those to demonstrate how the CIST is constructed. For simplicity, assume that all link costs inside the region are the same value of 1.

When multiple regions connect together, every region needs to construct its own IST and all regions should build one common CIST spanning across the regions. To see how this is accomplished, first have a look at the structure of MSTP BPDU. On the figure below, notice MSTP uses protocol version 3 as opposed to RSTP's version 2. Version 4 is reserved to SPT - Shortest Path Tree - new loop prevention and packet bridging standard defined in the emerging IEEE 802.1aq document.

The MSTP BPDU contains two important blocks of information. One, highlighted in red, is related to CIST Root and CIST Regional Root election. As you will see later, CIST Root is elected among all regions and CIST Regional Root is elected in every region. The green block outlines the information about CIST Regional Root (which becomes the IST Root in the presence of multiple regions). The CIST Internal Root path cost is the intra-region cost to reach the CIST Regional Root. It is important to keep in mind that IST Root = CIST Regional Root in cases where multiple regions interoperate. This transformation is explained further in the text. Now, to define the CIST Root and CIST Regional Root roles:

• CIST Root is the bridge that has the lowest Bridge ID among ALL regions. This could be a bridge inside a region or a boundary switch in a region.
• CIST Regional Root is a boundary switch elected for every region based on the shortest external path cost to reach the CIST Root. Path cost is calculated based on costs of the links connecting the regions, excluding the internal regional paths. CIST Regional Root becomes the root of the IST for the given region as well.

CIST Root Bridges Election Process

• When a switch boots up, it declares itself as CIST Root and CIST Regional Root and announces this fact in outgoing BPDUs. The switch will adjust its decision upon reception of better information and continue advertising the best known CIST Root and CIST Regional Root on all internal ports. On the boundary ports, the switch advertises only the CIST Root Bridge ID and CIST External Root Path Cost thus hiding the details of the region’s internal topology.
• CIST External Root Path Cost is the cost to reach the CIST Root across the links connecting the boundary ports – i.e. the inter-region links. When a BPDU is received on an internal port, this cost is not changed. When a BPDU is received on a boundary port, this cost is adjusted based on the receiving boundary port cost. As a result, the CIST External Root Path Cost is propagated unmodified inside any region.
• Only a boundary switch could be elected as the CIST Regional Root, and this is the switch with the lowest cost to reach the CIST Root. If a boundary switch hears better CIST External Root Path cost received on its internal link, it will relinquish its role of CIST Regional Root and start announcing the new metric out of its boundary ports.
• Every boundary switch needs to properly block its boundary ports. If the switch is a CIST Regional Root, it elects one of the boundary ports as the “CIST Root port” and blocks all other boundary ports. If a boundary switch is not the CIST Regional Root, it will mark the boundary ports as CIST Designated or Alternate. The boundary port on a non-regional-root bridge becomes designated only if it has superior information for the CIST Root: better External Root Path cost or if the costs are equal to CIST Regional Root Bridge ID. This follows the normal rules of the STP process.
• As a result of CIST construction, every region will have one switch having single port unblocked in the direction of the CIST Root. This switch is the CIST Regional Root. All boundary switches will advertise the region’s CIST Regional Root Bridge ID out of their non-blocking boundary ports. From the outside perspective, the whole region will look like a single virtual bridge with the Bridge ID = CIST Regional Root ID and single root port elected on the CIST Regional Root switch.
• The region that contains the CIST Root will have all boundary ports unblocked and marked as CIST designated ports. Effectively the region would look like a virtual root bridge with the Bridge ID equal to CIST Root and all ports being designated. Notice that the region with CIST Root has CIST Regional Root equal to CIST Root as they share the same lowest bridge priority value across all regions.

Have a look at the diagram below. It demonstrates the CIST topology calculated from the physical topology we outlined above. First, SW1-1 is elected as the CIST Root as it has the lowest Bridge ID among all bridges in all regions. This automatically makes region 1 a virtual bridge with all boundary ports unblocked. Next, SW2-1 and SW3-1 are elected as the CIST Regional Roots in their respective regions. Notice that SW3-1 and SW2-3 have equal External Costs to reach the CIST Root but SW3-3 wins the CIST Regional Root role due to lower priority. Keep in mind that in the topology with multiple MSTP regions, every region that does not contain the CIST Root has to change the IST Root election process and make IST Root equal to CIST Regional Root.

Common Spanning Tree (CST)

From the above information, we may conclude that CIST essentially has a two-level hierarchy. The first level treats all regions as “virtual bridges” and operates with the External Root Path Cost. The first-level spanning tree roots in the CIST Root Bridge and encompasses the virtual bridges. This spanning-tree is known as CST or Common Spanning Tree. The CST connects all boundary ports and perceives every region as a single virtual bridge with the Bridge ID equal to CIST Regional Root Bridge ID.

CST is the construct where MSTP interoperates with the IEEE STP/RSTP regions as well. The legacy switch regions join their STP instance with the CST and perceive MSTP regions as “transparent” virtual bridges, staying unaware of their internal topology. Thus, connecting to IEEE STP/RSTP domains extended the CST. MSTP discovers the appropriate STP version on a boundary link by listening to external and switches to the respective mode of operations (e.g. RSTP/STP). It may happen so that a switch with the lowest Bridge ID belongs to a RSTP/STP region. This situation results in all MSTP regions electing local CIST Regional Roots and considering the new CIST Root located outside the MSTP “domain”.

The second level of CIST hierarchy consists of the various MSTP regional ISTs. Every MSTP region builds an IST instance using the internal path costs and following the optimal “internal” topology, using the CIST Regional Root as the IST Root. The changes to CST may affect the IST’s in every region, as those changes may result in re-electing the new CIST Regional Roots. Changes to the region's internal topologies normally do not affect the CST, unless those changes partition the region.

Mapping MSTI’s to CIST

MSTIs are constructed independently in every region, but they have to be mapped to the CIST at the boundary ports. This means inability to load-balance VLAN traffic on the boundary links by mapping VLANs to different instances. All VLANs use the same non-blocking boundary ports, which are either upstream or downstream with respect to the CIST Root. This statement is only valid with respect to the CST paths connecting the regional virtual bridges. Inside any region VLANs follow the internal topology paths, based on the respective MSTI configurations.

The MSTIs have no idea of the CIST Root whatsoever; they only use internal paths and internal MSTI roots to build the spanning trees. However, all MSTP instances see the root port (towards the CIST Root) of the CIST Regional Bridge as a special Master Port connecting them to the CIST Root bridge. This port serves the purpose of the “gateway” linking MSTI's to other regions. Recall that switches do not send M-Records (MSTI information) out of boundary ports, only CIST information. Thus, the CIST and MSTI's may converge independently and in parallel. The Master Port will only begin forwarding when all respective MSTI ports are in sync and forwarding to avoid temporary bridging loops.

MSTP Multi-Region Design Considerations

Ethernet is known for its broadcast nature that tends to propagate faults across the whole Layer 2 domain. There are three main problems with Ethernet that affect MSTP designs:

• Unknown unicast flooding results in traffic surges under topology changes. Those are either the result of asymmetric routing or persistent topology changes. Every topology change causes massive invalidation of MAC address tables and unicast traffic flooding. This process is the result of Ethernet topology unawareness - the bridges don't know MAC addresses location.
• Broadcast and Multicast flooding. This is a separate problem as many core protocols (ARP, IGP, PIM) rely on multicasting or broadcasting. Those packets should be delivered to every node in a broadcast domain and under intense loads, networks could be congested at every point.
• Spanning-Tree Convergence. MSTP uses the RSTP procedure for STP re-negotiation. Since it is based on distance-vector behavior, it is prone to some convergence issues, such as counting to infinity (old information circulation). This is especially noticeable in larger topologies with 10+ switches and under special conditions, such as failure of the root bridge.

The concept of MSTP region allows for bounding STP re-computations. Since MSTIs in every region are independent, any change affecting MSTI in one region will not affect MSTIs in other regions. This is a direct result of the fact that M-Record information is not exchanged between the regions. However, the CIST recalculations affect every region and might be slow converging. This is why it is a good idea not to map any VLAN to CIST and avoid connecting MSTP regions to IEEE STP domains.

Topology changes in MSTP are treated the same way as in RSTP. That is, only non-edge links going to the forwarding state will cause a topology change and the switch detecting the change will flood this information through the domain. However, a single physical link may be forwarding for one MSTI and blocking for another. Thus, a single physical change may have different effects on MSTIs and the CIST. Topology changes in MSTIs are bounded to a single region, while topology changes to the CIST propagate through all regions. Every region treats the TC notification from another region as “external” and applies them to CIST-associated ports only.

A topology change to CST (the tree connecting the virtual bridges) will affect all MSTIs in all regions and the CIST. This is due to the fact that new links becoming forwarding between the virtual bridges may change all paths in the topology and thus require massive MAC address re-learning. Thus, from the standpoint of topology change, something happening to the CST will have a massive impact on flooding in the set of interconnected MSTP regions.

The above observations advise a good design rule for MSTP networks – separate “meshy” topologies in their own regions and interconnect regions using “sparse” mesh, keeping in mind balance between redundancy and topology changes effect. This is an adaptation of a well-known design principle - separate complexity from complexity to keep networks more stable and isolate fault domains. In addition, exposing a lot of links to CST will reduce your load-balancing choices, as CST supports only one STP instance. You want to avoid designs like the one diagrammed below, which effectively disabled load balancing on the mesh of links that belong to CST. The reason is that now the full-mesh of links belongs on CST and it elects only one unblocked path between the two regions.

Even though region partitioning offers better fault isolation it still does not eliminate well-known Ethernet issues such as unicast and broadcast flooding. Those may still occur and disrupt network connectivity. For example, unicast flooding could be caused by unidirectional traffic and broadcast flooding may be a result of transient bridging loops when a root bridge fails. Transient bridging loops are reality with RSTP/MSTP especially in larger topologies due to various synchronization problems resulting in count to infinity behavior. This problem is especially dangerous when a root bridge crashes and the remaining topology contains loops – old information may circulate until its aged out using hop counting (counting to infinity).

Interoperating with PVST+

Per its design, PSVST+ runs a separate STP instance for every VLAN. On the contrary, MSTP maps VLANs to MSTIs, so one-to-one mapping between VLAN and STP instances no longer holds true. How should an MSTP switch operate on a border link connected to the PVST+ domain?

MSTP runs multiple MSTIs inside a region and maps them all to CIST on the border ports. The interoperation model needs to ensure that internal MSTI’s could be aware of changes in any of PVST+ trees. It's hard to automatically map VLAN-bounded STP’s to the MSTI’s and so the simplest way to accomplish the desired behavior is to join ALL PVST+ trees with the CST.

By connecting PVST+ trees to the CST, the solution ensures that changes in any of PVST+ STP instances will affect the CST and all MSTIs as a consequence. While not the optimal solution, it ensures that no changes go unnoticed and no black holes occur in a single VLAN due to the topology changes. As with the IEEE STP, every tree in PVST+ domain perceives MSTP regions like virtual bridges with multiple boundary ports. A topology change in any of PVST+ trees will affect the CST and impact every MSTI instance in all MSTP regions. This behavior makes the MSTP topology less stable and fully exposed to changes in PVST+ domain.

The MSTP implementation simulates PVST+ by replicating CIST BPDUs on the link facing the PVST+ domain and sending those BPDUs on ALL VLANs active on the trunk. The MSTP switch consumes all BDPUs received from PVST+ domain and processes them using the CIST instance. The PSVT+ domain sees the MSTP domain as a PVST+ bridge with all per-VLAN instances claiming the CIST Root as the root of their STP. With respect to the common STP Root elected between MSTP and PVST+ the two following options are possible:

• MSTP domain (either a single region or multiple regions) contains the root bridge for ALL VLANs. This means the CIST Root Bridge ID is better than any PVST+ STP root Bridge ID. If there is only one MSTP region connecting to PVST+ domain, then all boundary ports on the virtual-bridge will be unblocked and could be used by PVST+ trees. This is the preferred design, as administrators can manipulate uplink costs on the PVST+ side and obtain optimal traffic engineering results. On the figure below, VLANs 2 and 3 have their STP costs adjusted so that they select different uplinks connected to MSTP region’s boundary ports. Since the CIST Root is inside the MSTP region, both boundary ports are non-blocking designated and thus the load balancing scheme works fine.

• PVST+ domain contains the root bridges for ALL VLANs. This is only true if all PVST+ root bridges Bridge IDs for all VLANs are better than the MSTP CIST Root Bridge ID. This is not the preferred design, since all MSTIs map to CIST on the border link, and you cannot load-balance the MSTIs as they enter the PVST+ domain.

Cisco implementation does not support the second option. MSTP domain should contain the bridge with the best Bridge ID, to ensure that the CIST Root is also the root for all PVST+ trees. In any other case, the MSTP border switch will complain and place the ports that receive superior BPDUs from PVST+ region in root-inconsistent state. To fix this issue, ensure that PVST+ domain does not have any bridges with Bridge IDs better than the CIST Root Bridge ID.

And lastly a few words on MSTP and PVST interoperations. They operate in exactly the same manner and follow the same rules as PVST+ interoperation, just ISL is used for trunking encapsulation.

Scenario 1: CIST Root and CIST Regional Root

In this scenario, we configure four switches in two regions. The first region consists of one switch (SW1) and the second region consists of three switches: SW2, SW3 and SW4. SW1 is the CIST Root thanks to its lowest priority; SW2 is the CIST Regional Root for REGION234. We modify link costs in REGION234 to make SW3 prefer a path to SW2 via SW4 and not via the directly connected link.

SW1:

spanning-tree mode mst

!

! Minimum Priority among all bridges

!

spanning-tree mst 0 priority 4096

!

spanning-tree mst configuration

 name REGION1

 exit

SW2:
spanning-tree mode mst
spanning-tree mst configuration
name REGION234
exit
!
! SW2 priority is less than SW4’s, but it has better cost to CIST Root
!
spanning-tree mst 0 priority 16384
!
! This is the active boundary port
!
interface FastEthernet 0/13
spanning-tree mst 0 cost 50
!
interface FastEthernet 0/14
spanning-tree mst 0 cost 200
!
interface FastEthernet 0/16
spanning-tree mst 0 cost 100

SW3:
spanning-tree mode mst
spanning-tree mst configuration
name REGION234
exit
!
interface FastEthernet 0/16
spanning-tree mst 0 cost 100
!
interface FastEthernet 0/19
spanning-tree mst 0 cost 10

SW4:
spanning-tree mode mst
spanning-tree mst configuration
name REGION234
exit
!
! SW4 has better IST priority but higher cost to the CIST Root
!
spanning-tree mst 0 priority 8192
!
interface FastEthernet 0/13
spanning-tree mst 0 cost 100
!
interface FastEthernet 0/16
spanning-tree mst 0 cost 10
!
interface FastEthernet 0/19
spanning-tree mst 0 cost 10

In the above configuration we adjusted link costs to ensure the following:

• MSTP REGION 234 elects SW2 as the CIST Regional Root due to its shortest path to the CIST Root. SW2 is the CIST Regional Root for REGION234, even though SW4 has better switch priority inside the region.
• SW3 selects the path through SW4 to reach the internal root bridge due to the shortest path cost to SW1 (CIST Regional Root) via SW4.

Verifications for the above configuration follow:

SW1#show spanning-tree mst 0

##### MST0 vlans mapped: 1-4094
Bridge address 0019.55e6.6800 priority 4096 (4096 sysid 0)
Root this switch for the CIST
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/13 Desg FWD 200000 128.15 P2p
Fa0/14 Desg FWD 200000 128.16 P2p
Fa0/19 Desg FWD 200000 128.21 P2p

SW1 says it is the CIST Root Bridge and all of its ports are unblocked (designated). The bridge priority is set to 4096 – this will allow us to distinguish this switch in the show outputs below. The ports are not marked as boundary, since SW1 is not receiving any BPDUs on these ports – all downstream ports suppress sending their own BPDUs since SW1 is the root bridge. You may confirm this using the following debugging commands:

SW1#debug spanning-tree mstp bpdu receive 

MSTP BPDUs RECEIVEd dump debugging is on

SW1#

Next try dumping the BPDUs being sent by SW1. Notice that they all have Mum_mrec set to zero which means “zero M-records”. SW1 claims itself as the CIST Root and CIST Regional root on all ports. The cost to both roots is set to zero.

SW1#debug spanning-tree mstp bpdu transmit 

MSTP BPDUs TRANSMITted dump debugging is on

MST[0]:-TX Fa0/13 BPDU Prot:0 Vers:3 Type:2
MST[0]: Role : Desg Flags[AFL] Age:0 RemHops:20
MST[0]: CIST_root: 4096.0019.55e6.d380 Cost :0
MST[0]: Reg_root : 4096.0019.55e6.d380 Cost :0
MST[0]: Bridge_ID: 4096.0019.55e6.d380 Port_ID:32783
MST[0]: max_age:20 hello:2 fwdelay:15
MST[0]: V3_len:64 region:REGION1 rev:0 Num_mrec: 0

MST[0]:-TX Fa0/14 BPDU Prot:0 Vers:3 Type:2
MST[0]: Role : Desg Flags[AFL] Age:0 RemHops:20
MST[0]: CIST_root: 4096.0019.55e6.d380 Cost :0
MST[0]: Reg_root : 4096.0019.55e6.d380 Cost :0ll
MST[0]: Bridge_ID: 4096.0019.55e6.d380 Port_ID:32784
MST[0]: max_age:20 hello:2 fwdelay:15
MST[0]: V3_len:64 region:REGION1 rev:0 Num_mrec: 0

MST[0]:-TX Fa0/19 BPDU Prot:0 Vers:3 Type:2
MST[0]: Role : Desg Flags[AFL] Age:0 RemHops:20
MST[0]: CIST_root: 4096.0019.55e6.d380 Cost :0
MST[0]: Reg_root : 4096.0019.55e6.d380 Cost :0
MST[0]: Bridge_ID: 4096.0019.55e6.d380 Port_ID:32789
MST[0]: max_age:20 hello:2 fwdelay:15
MST[0]: V3_len:64 region:REGION1 rev:0 Num_mrec: 0

Inspect CIST statistics on SW2:

SW2#show spanning-tree mst 0

##### MST0 vlans mapped: 1-4094
Bridge address 001b.8f0c.2a00 priority 16384 (16384 sysid 0)
Root address 0019.55e6.6800 priority 4096 (4096 sysid 0)
port Fa0/13 path cost 50
Regional Root this switch
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/13 Root FWD 50 128.15 P2p Bound(RSTP)
Fa0/14 Altn BLK 200 128.16 P2p Bound(RSTP)
Fa0/16 Desg FWD 10 128.18 P2p
Fa0/19 Desg FWD 200000 128.21 P2p

SW2 is the CIST Regional Root Bridge (CIST Root) with the priority value of 16384. SW2 learns that SW1 is the CIST Root with the priority value of 4096. The boundary root port is Fa 0/13, elected based on regular STP rules (shortest path, lowest upstream port priority). The other boundary uplink is blocking. Both ports show up as “Boundary” since they face the other MSTP domain. Of course, SW2 is the Regional Root due to the fact that it has the shortest path to the CIST Root. Dump the BPDUs being sent by SW2:

SW2#debug spanning-tree mstp bpdu transmit 

MSTP BPDUs TRANSMITted dump debugging is on

MST[0]:-TX Fa0/16 BPDU Prot:0 Vers:3 Type:2
MST[0]: Role : Desg Flags[FL] Age:1 RemHops:20
MST[0]: CIST_root: 4096.0019.55e6.d380 Cost :50
MST[0]: Reg_root :16384.0019.564c.c580 Cost :0
MST[0]: Bridge_ID:16384.0019.564c.c580 Port_ID:32786
MST[0]: max_age:20 hello:2 fwdelay:15
MST[0]: V3_len:64 region:REGION234 rev:0 Num_mrec: 0

MST[0]:-TX Fa0/19 BPDU Prot:0 Vers:3 Type:2
MST[0]: Role : Desg Flags[FL] Age:1 RemHops:20
MST[0]: CIST_root: 4096.0019.55e6.d380 Cost :50
MST[0]: Reg_root :16384.0019.564c.c580 Cost :0
MST[0]: Bridge_ID:16384.0019.564c.c580 Port_ID:32789
MST[0]: max_age:20 hello:2 fwdelay:15
MST[0]: V3_len:64 region:REGION234 rev:0 Num_mrec: 0

SW2 only sends BPDUs out of its designated ports that are Fa 0/16 and Fa 0/19. The output also signifies that SW2 announces SW1 as the CIST Root and itself as the regional root. Notice the CIST External Root Path Cost and the CIST Regional Root Cost. Now, check the MSTI0 statistics on SW3:

SW3#show spanning-tree mst 0

##### MST0 vlans mapped: 1-4094
Bridge address 000c.85be.c680 priority 32768 (32768 sysid 0)
Root address 0019.55e6.6800 priority 4096 (4096 sysid 0)
port Fa0/19 path cost 50
Regional Root address 001b.8f0c.2a00 priority 16384 (16384 sysid 0)
internal cost 20 rem hops 18
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/16 Altn BLK 100 128.16 P2p
Fa0/19 Root FWD 10 128.19 P2p

SW3 sees SW1 as the CIST Root and SW2 as the CIST Regional Root. Since SW2 is also the root for IST, SW3 needs to select the root port to reach it. It selects the link via SW4 as our cost manipulations made this path more preferred. The internal cost to reach the CIST Regional root is 10+10=20. The CIST External Root Path Cost is 50, as it is not incremented when transported from SW2.

SW4#show spanning-tree mst 0

##### MST0 vlans mapped: 1-4094
Bridge address 000d.2840.ab00 priority 8192 (8192 sysid 0)
Root address 0019.55e6.6800 priority 4096 (4096 sysid 0)
port Fa0/16 path cost 50
Regional Root address 001b.8f0c.2a00 priority 16384 (16384 sysid 0)
internal cost 10 rem hops 19
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/13 Altn BLK 100 128.13 P2p Bound(RSTP)
Fa0/16 Root FWD 10 128.16 P2p
Fa0/19 Desg FWD 10 128.19 P2p

SW4 has lower bridge priority than SW2, but it is not elected as the CIST Regional Root, for SW4 path cost to the CIST Root is worse. Notice the External and Internal root path costs values – 50 and 10 respectively. Pay attention to the fact that SW4’s port Fa 0/13 is marked as alternate and blocked while the root port toward the CIST Regional Root is unblocked.

Scenario 2: MSTIs and the Master Port

In this scenario we add another STP instance to REGION234. We also create two VLANs 10 and 20 in all switches and map them to MSTI 1 in REGION234.

SW1:

vlan 10,20

SW2, SW3 & SW4:
vlan 10,20
!
spanning-tree mst configuration
instance 1 vlan 10, 20

We are concerned with the show commands for new MSTI 1 in REGION 234. Notice that we didn’t do any path manipulations and simply mapped the new VLANs to MSTI 1. It was not even necessary to create new VLANs, the configuration would work without VLANs ever existing as MSTIs are separated from the VLANs.

SW2#show spanning-tree mst 1

##### MST1 vlans mapped: 10,20
Bridge address 001b.8f0c.2a00 priority 32769 (32768 sysid 1)
Root this switch for MST1

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/13 Mstr FWD 200000 128.15 P2p Bound(RSTP)
Fa0/14 Altn BLK 200000 128.16 P2p Bound(RSTP)
Fa0/16 Desg FWD 200000 128.18 P2p
Fa0/19 Desg FWD 200000 128.21 P2p

There is no Regional Root for MSTI1. Just regular root, which is the root of MSTI, elected using the regular STP rules. In our case, the root is SW4 and the root port is Fa 0/19. Next note the special “Master Port”. This is the uplink port of CIST Regional Root. All MSTIs map to CIST here, and follow the single path. This port is also forwarding and provides the path upstream to the CIST Root for all MSTIs and their mapped VLANs. It is interesting to dump the MSTP BPDUs sent and received by SW2:

SW2#debug spanning-tree mstp bpdu receive 

MSTP BPDUs RECEIVEd dump debugging is on

MST[0]: RX- Fa0/13 repeated designated BPDU Prot:0 Vers:3 Type:2
MST[0]: Role : Desg Flags[AFL] Age:0 RemHops:20
MST[0]: CIST_root: 4096.0019.55e6.d380 Cost :0
MST[0]: Reg_root : 4096.0019.55e6.d380 Cost :0
MST[0]: Bridge_ID: 4096.0019.55e6.d380 Port_ID:32783
MST[0]: max_age:20 hello:2 fwdelay:15
MST[0]: V3_len:64 region:REGION1 rev:0 Num_mrec: 0

MST[0]: RX- Fa0/14 repeated designated BPDU Prot:0 Vers:3 Type:2
MST[0]: Role : Desg Flags[AFL] Age:0 RemHops:20
MST[0]: CIST_root: 4096.0019.55e6.d380 Cost :0
MST[0]: Reg_root : 4096.0019.55e6.d380 Cost :0
MST[0]: Bridge_ID: 4096.0019.55e6.d380 Port_ID:32784
MST[0]: max_age:20 hello:2 fwdelay:15
MST[0]: V3_len:64 region:REGION1 rev:0 Num_mrec: 0

Nothing special in received BPDUs, only SW1 claiming itself as the CIST Root/CIST Regional Root. Look at the BPDUs that SW2 is sending though:

SW2#debug spanning-tree mstp bpdu transmit

MSTP BPDUs TRANSMITted dump debugging is on

MST[0]:-TX Fa0/16 BPDU Prot:0 Vers:3 Type:2
MST[0]: Role : Desg Flags[AFL] Age:1 RemHops:20
MST[0]: CIST_root: 4096.0019.55e6.d380 Cost :50
MST[0]: Reg_root :16384.0019.564c.c580 Cost :0
MST[0]: Bridge_ID:16384.0019.564c.c580 Port_ID:32786
MST[0]: max_age:20 hello:2 fwdelay:15
MST[0]: V3_len:80 region:REGION234 rev:0 Num_mrec: 1

MST[1]:-TX> Fa0/16 MREC
MST[1]: Role : Desg Flags[AFL] RemHops:20
MST[1]: Root_ID :32769.0019.564c.c580 Cost :0
MST[1]: Bridge_ID:16385.0019.564c.c580 Port_id:146

MST[0]:-TX Fa0/19 BPDU Prot:0 Vers:3 Type:2
MST[0]: Role : Desg Flags[AFL] Age:1 RemHops:20
MST[0]: CIST_root: 4096.0019.55e6.d380 Cost :50
MST[0]: Reg_root :16384.0019.564c.c580 Cost :0
MST[0]: Bridge_ID:16384.0019.564c.c580 Port_ID:32789
MST[0]: max_age:20 hello:2 fwdelay:15
MST[0]: V3_len:80 region:REGION234 rev:0 Num_mrec: 1

MST[1]:-TX Fa0/19 MREC
MST[1]: Role : Desg Flags[AFL] RemHops:20
MST[1]: Root_ID :32769.0019.564c.c580 Cost :0
MST[1]: Bridge_ID:16385.0019.564c.c580 Port_id:149

The output now shows one M-Record attached to the IST BPDU. This M-Record specifies SW2 as the root bridge for IST1 – you can tell that by looking at the Cost field.

Scenario 3: PVST+ and MSTP Interoperation

In this scenario, we configure SW1 to use PSVT+ and see how it interworks with MSTP. First, configure SW1 as the root bridge for all VLANs and make it win over any bridge in MSTP region.

SW1:

spanning-tree mode pvst

spanning-tree vlan 1-4094 priority 4096

Let's see what happens on SW2:

%SPANTREE-2-PVSTSIM_FAIL: Superior PVST BPDU received on VLAN 5

SW2#show spanning-tree mst 0

##### MST0 vlans mapped: 1-9,11-19,21-4094
Bridge address 001b.8f0c.2a00 priority 16384 (16384 sysid 0)
Root address 0019.55e6.6800 priority 4097 (4096 sysid 1)
port Fa0/13 path cost 50
Regional Root this switch
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/13 Desg BLK 50 128.15 P2p Bound(PVST)
Fa0/14 Desg BKN*200 128.16 P2p Bound(PVST) *PVST_Inc
Fa0/16 Root FWD 100 128.18 P2p Bound(RSTP)
Fa0/19 Altn BLK 200000 128.21 P2p Bound(RSTP)

Notice the syslog message in the beginning. It says that while emulating PVST+ operation the MSTP code encountered the situation where PVST+ domain claims itself as a root for one or more VLANs. That is, the PVST+ Bridge has better BID than the current CIST Root. As a result, even though the MSTP considers the new root as the “legitimate” new CIST Root, it blocks the uplink port as PVST Simulation Inconsistent. It is interesting to notice that SW1 is still considered to be the CIST Root and SW2 is the CIST Regional Root but all ports to the CIST Root are blocked! Check the flow of BPDUs received from SW1. The first is the VLAN 1 BPDUs perceived on SW2 as IEEE STP BPDUs. They claim SW1 as the Root Bridge – you may see an extended system ID carrying the VLAN number of 1.

SW2#debug spanning-tree mstp bpdu receive

MSTP BPDUs RECEIVEd dump debugging is on

MST[0]: RX- Fa0/13 repeated designated BPDU Prot:0 Vers:0 Type:0
MST[0]: Flags[] Age:0
MST[0]: CIST_root: 1.0019.55e6.d380 Cost :0
MST[0]: Bridge_ID: 1.0019.55e6.d380 Port_ID:32783
MST[0]: max_age:20 hello:2 fwdelay:15

MST[0]: RX- Fa0/14 repeated designated BPDU Prot:0 Vers:2 Type:2
MST[0]: Role : Desg Flags[FL] Age:0
MST[0]: CIST_root: 1.0019.55e6.d380 Cost :0
MST[0]: Bridge_ID: 1.0019.55e6.d380 Port_ID:32784
MST[0]: max_age:20 hello:2 fwdelay:15

There are other BPDUs received on SW2 – due to the fact that 802.1Q is the trunking encapsulation SW2 receives PVST+ BPDUs for VLANs 10 and 20:

SW2#debug spanning-tree bpdu receive 

Spanning Tree BPDU Received debugging is on

STP: MST0 rx BPDU: config protocol = mstp, packet from FastEthernet0/13 , linktype IEEE_SPANNING , enctype 2, encsize 17
STP: enc 01 80 C2 00 00 00 00 19 55 E6 D3 8F 00 26 42 42 03
STP: Data 00000000010001001955E6D380000000000001001955E6D380800F0000140002000F00

STP: MST0 Fa0/13:0000 00 00 01 0001001955E6D380 00000000 0001001955E6D380 800F 0000 1400 0200 0F00
STP: MST0 rx BPDU: config protocol = mstp, packet from FastEthernet0/14 , linktype IEEE_SPANNING , enctype 2, encsize 17
STP: enc 01 80 C2 00 00 00 00 19 55 E6 D3 90 00 27 42 42 03
STP: Data 000002021E0001001955E6D380000000000001001955E6D38080100000140002000F00

STP: MST0 Fa0/14:0000 02 02 1E 0001001955E6D380 00000000 0001001955E6D380 8010 0000 1400 0200 0F00
STP: MST0 rx BPDU: config protocol = mstp, packet from FastEthernet0/13 , linktype SSTP , enctype 3, encsize 22
STP: enc 01 00 0C CC CC CD 00 19 55 E6 D3 8F 00 32 AA AA 03 00 00 0C 01 0B
STP: Data 0000000001000A001955E6D38000000000000A001955E6D380800F0000140002000F00

STP: MST0 Fa0/13:0000 00 00 01 000A001955E6D380 00000000 000A001955E6D380 800F 0000 1400 0200 0F00
STP: MST0 rx BPDU: config protocol = mstp, packet from FastEthernet0/14 , linktype SSTP , enctype 3, encsize 22
STP: enc 01 00 0C CC CC CD 00 19 55 E6 D3 90 00 32 AA AA 03 00 00 0C 01 0B
STP: Data 000002021E000A001955E6D38000000000000A001955E6D38080100000140002000F00

STP: MST0 Fa0/14:0000 02 02 1E 000A001955E6D380 00000000 000A001955E6D380 8010 0000 1400 0200 0F00
STP: MST0 rx BPDU: config protocol = mstp, packet from FastEthernet0/13 , linktype SSTP , enctype 3, encsize 22
STP: enc 01 00 0C CC CC CD 00 19 55 E6 D3 8F 00 32 AA AA 03 00 00 0C 01 0B
STP: Data 00000000010014001955E6D380000000000014001955E6D380800F0000140002000F00

STP: MST0 Fa0/13:0000 00 00 01 0014001955E6D380 00000000 0014001955E6D380 800F 0000 1400 0200 0F00
STP: MST0 rx BPDU: config protocol = mstp, packet from FastEthernet0/14 , linktype SSTP , enctype 3, encsize 22
STP: enc 01 00 0C CC CC CD 00 19 55 E6 D3 90 00 32 AA AA 03 00 00 0C 01 0B
STP: Data 000002021E0014001955E6D380000000000014001955E6D38080100000140002000F00

The above output shows that both ports receive IEEE native STP BPDUs along with PVST+ SSTP BPDUs for VLAN numbers 0xA (10) and 0x14 (20). Now check how MSTI1 sees the inconsistent port:

SW2#show spanning-tree mst 1

##### MST1 vlans mapped: 10,20
Bridge address 001b.8f0c.2a00 priority 32769 (32768 sysid 1)
Root this switch for MST1

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/13 Mstr BKN*200000 128.15 P2p Bound(PVST) *PVST_Inc
Fa0/14 Altn BLK 200000 128.16 P2p Bound(PVST)
Fa0/16 Desg FWD 200000 128.18 P2p
Fa0/19 Desg FWD 200000 128.21 P2p

Here we see that the Master Port is blocked as well, due to the PSVT simulation inconsistency. To resolve this issue you need to ensure that the MSTP domain contains the root bridge for all PVST+ trees. This is accomplished by tuning priority value for the CIST to a number lower than any PVST+ bridge priority.

SW1:

spanning-tree mode pvst

spanning-tree vlan 1-4094 priority 8192

SW2:
spanning-tree mst 0 priority 4096

Now SW2 is the new CIST Root. Look at the show command output again:

SW2#show spanning-tree mst 0

##### MST0 vlans mapped: 1-9,11-19,21-4094
Bridge address 001b.8f0c.2a00 priority 4096 (4096 sysid 0)
Root this switch for the CIST
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/13 Desg FWD 50 128.15 P2p Bound(PVST)
Fa0/14 Desg FWD 200 128.16 P2p Bound(PVST)
Fa0/16 Desg FWD 100 128.18 P2p
Fa0/19 Desg FWD 200000 128.21 P2p

SW2#show spanning-tree mst 1

##### MST1 vlans mapped: 10,20
Bridge address 001b.8f0c.2a00 priority 32769 (32768 sysid 1)
Root address 000d.2840.ab00 priority 32769 (32768 sysid 1)
port Fa0/19 cost 200000 rem hops 19

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/13 Desg FWD 200000 128.15 P2p Bound(PVST)
Fa0/14 Desg FWD 200000 128.16 P2p Bound(PVST)
Fa0/16 Desg FWD 200000 128.18 P2p
Fa0/19 Root FWD 200000 128.21 P2p

SW1#show spanning-tree vlan 1

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 4096
Address 001b.8f0c.2a00
Cost 19
Port 15 (FastEthernet0/13)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 8193 (priority 8192 sys-id-ext 1)
Address 0019.55e6.6800
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/13 Root FWD 19 128.15 P2p
Fa0/14 Altn BLK 19 128.16 P2p
Fa0/19 Altn BLK 19 128.21 P2p

All SW2 ports are now designated. SW2 correctly emulates the PVST+ interactions, and SW1 sees SW2 as the root of all PVST+ instances. SW1 will then block one of its redundant uplink based on the regular STP rules. In this situation, traffic may flow between the PVST+ and MSTP domains and you can achieve optimal load-balancing using the PSVT+ cost tuning on SW1.

Conclusions

MSTP was designed to overcome one major problem with classic STP protocol – inability to use blocked links for traffic forwarding due to a single STP instance present. This is accomplished by running multiple spanning trees in a topology and mapping VLANs to different trees for traffic forwarding. Even though this feature does not allow for precise and optimal traffic engineering it improves redundant link utilization. By using regions, MSTP allows for isolating different physical topologies from each other while maintaining Layer 2 connectivity between the regions. However, even with improved fault isolation, MSTP still suffers from the problems inherent to Ethernet topology – unicast and broadcast flooding and slow spanning-tree convergence. This limits MSTP deployments to small Layer 2 domains, such as single access-distribution switch block. Larger MSTP deployments should be planned carefully and require strict administrative control. As a suggestion, Private VLANs could be used for larger domains to minimize traffic flooding.

To learn more from our INE experts on subjects similar to MSTP, check our blog list below:

Related Reading

MSTP Tutorial Part I: Inside a Region
MSTP Tutorial Part II: Outside a Region
Understanding STP and RSTP Convergence
IEEE 802.1 Series Standards
MSTP Configuration Guide
RFC 5517: Cisco System’s Private VLANs