OTV Decoded - A Fancy GRE Tunnel

Edit: For those of you that want to take a look first-hand at these packets, the Wireshark PCAP files referenced in this post can be found here

One of the hottest topics in networking today is Data Center Virtualized Workload Mobility (VWM). For those of you that have been hiding under a rock for the past few years, workload mobility basically means the ability to dynamically and seamlessly reassign hardware resources to virtualized machines, often between physically disparate locations, while keeping this transparent to the end users. This is often accomplished through VMware vMotion, which allows for live migration of virtual machines between sites, or as similarly implemented in Microsoft’s Hyper-V and Citrix’s Xen hypervisors.

One of the typical requirements of workload mobility is that the hardware resources used must be on the same layer 2 network segment. E.g. the VMware Host machines must be in the same IP subnet and VLAN in order to allow for live migration their VMs. The big design challenge then becomes, how do we allow for live migrations of VMs between Data Centers that are not in the same layer 2 network? One solution to this problem that Cisco has devised is a relatively new technology called Overlay Transport Virtualization (OTV).

As a side result of preparing for INE’s upcoming CCIE Data Center Nexus Bootcamp I’ve had the privilege (or punishment depending on how you look at it ;) ) of delving deep into the OTV implementation on Nexus 7000. My goal was to find out exactly what was going on behind the scenes with OTV. The problem I ran into though was that none of the external Cisco documentation, design guides, white papers, Cisco Live presentations, etc. really contained any of this information. The only thing that is out there on OTV is mainly marketing info, i.e. buzzword bingo, or very basic config snippets on how to implement OTV. In this blog post I’m going to discuss the details of my findings about how OTV actually works, with the most astonishing of these results being that OTV is in fact, a fancy GRE tunnel.

From a high level overview, OTV is basically a layer 2 over layer 3 tunneling protocol. In essence OTV accomplishes the same goal as other L2 tunneling protocols such as L2TPv3, Any Transport over MPLS (AToM), or Virtual Private LAN Services (VPLS). For OTV specifically this goal is to take Ethernet frames from an end station, like a virtual machine, encapsulate them inside IPv4, transport them over the Data Center Interconnect (DCI) network, decapsulate them on the other side, and out pops your original Ethernet frame.

For this specific application OTV has some inherent benefits over other designs such as MPLS L2VPN with AToM or VPLS. The first of which is that OTV is transport agnostic. As long as there is IPv4 connectivity between Data Centers, OTV can be used. For AToM or VPLS, these both require that the transport network be MPLS aware, which can limit your selections of Service Providers for the DCI. For OTV you can technically use it over any regular Internet connectivity.

Another advantage of OTV is that provisioning is simple. AToM and VPLS tunnels are Provider Edge (PE) side protocols, while OTV is a Customer Edge (CE) side protocol. This means for AToM and VPLS the Service Provider has to pre-provision the pseudowires. Even though VPLS supports enhancements like BGP auto-discovery, provisioning of MPLS L2VPN is still requires administrative overhead. OTV is much simpler in this case, because as we’ll see shortly, the configuration is just a few commands that are controlled by the CE router, not the PE router.

The next thing we have to consider with OTV is how exactly this layer 2 tunneling is accomplished. After all we could just configure static GRE tunnels on our DCI edge routers and bridge IP over them, but this is probably not the best design option for either control plane or data plan scalability.

The way that OTV implements the control plane portion of its layer 2 tunnel is what is sometimes described as “MAC in IP Routing”. Specifically OTV uses Intermediate System to Intermediate System (IS-IS) to advertise the VLAN and MAC address information of the end hosts over the Data Center Interconnect. For those of you that are familiar with IS-IS, immediately this should sound suspect. After all, IS-IS isn’t an IP protocol, it’s part of the legacy OSI stack. This means that IS-IS is directly encapsulated over layer 2, unlike OSPF or EIGRP which ride over IP at layer 3. How then can IS-IS be encapsulated over the DCI network that is using IPv4 for transport? The answer? A fancy GRE tunnel.

The next portion that is significant about OTV’s operation is how it actually sends packets in the data plane. Assuming for a moment that the control plane “just works”, and the DCI edge devices learn about all the MAC addresses and VLAN assignments of the end hosts, how do we actually encapsulate layer 2 Ethernet frames inside of IP to send over the DCI? What if there is multicast traffic that is running over the layer 2 network? Also what if there are multiple sites reachable over the DCI? How does it know specifically where to send the traffic? The answer? A fancy GRE tunnel.

Next I want to introduce the specific topology that will be used for us to decode the details of how OTV is working behind the scenes. Within the individual Data Center sites, the layer 2 configuration and physical wiring is not relevant to our discussion of OTV. Assume simply that the end hosts have layer 2 connectivity to the edge routers. Additionally assume that the edge routers have IPv4 connectivity to each other over the DCI network. In this specific case I chose to use RIPv2 for routing over the DCI (yes, you read that correctly), simply so I could filter it from my packet capture output, and easily differentiate between the routing control plane in the DCI transport network vs. the routing control plane that was tunneled inside OTV between the Data Center sites.

What we are mainly concerned with in this topology is as follows:

OTV Edge Devices N7K1-3 and N7K2-7

These are the devices that actually encapsulate the Ethernet frames from the end hosts into the OTV tunnel. I.e. this is where the OTV config goes.

DCI Transport Device N7K2-8

This device represents the IPv4 transit cloud between the DC sites. From this device’s perspective it sees only the tunnel encapsulated traffic, and does not know the details about the hosts inside the individual DC sites. Additionally this is where packet capture is occurring so we can view the actual payload of the OTV tunnel traffic.

End Hosts R2, R3, Server 1, and Server 3

These are the end devices used to generate data plane traffic that ultimately flows over the OTV tunnel.

Now let’s look at the specific configuration on the edge routers that is required to form the OTV tunnel.

N7K1-3:
vlan 172
name OTV_EXTEND_VLAN
!
vlan 999
name OTV_SITE_VLAN
!
spanning-tree vlan 172 priority 4096
!
otv site-vlan 999
otv site-identifier 0x101
!
interface Overlay1
otv join-interface Ethernet1/23
otv control-group 224.100.100.100
otv data-group 232.1.2.0/24
otv extend-vlan 172
no shutdown
!
interface Ethernet1/23
ip address 150.1.38.3/24
ip igmp version 3
ip router rip 1
no shutdown

N7K2-7:
vlan 172
name OTV_EXTEND_VLAN
!
vlan 999
name OTV_SITE_VLAN
!
spanning-tree vlan 172 priority 4096
!
otv site-vlan 999
otv site-identifier 0x102
!
interface Overlay1
otv join-interface port-channel78
otv control-group 224.100.100.100
otv data-group 232.1.2.0/24
otv extend-vlan 172
no shutdown
!
interface port-channel78
ip address 150.1.78.7/24
ip igmp version 3
ip router rip 1

As you can see the configuration for OTV really isn’t that involved. The specific portions of the configuration that are relevant are as follows:

Extend VLANs

These are the layer 2 segments that will actually get tunneled over OTV. Basically these are the VLANs that you virtual machines reside on that you want to do the VM mobility between. In our case this is VLAN 172, which maps to the IP subnet 172.16.0.0/24.

Site VLAN

Used to synchronize the Authoritative Edge Device (AED) role within an OTV site. This is for is when you have more than one edge router per site. OTV only allows a specific Extend VLAN to be tunneled by one edge router at a time for the purpose of loop prevention. Essentially this Site VLAN lets the edge routers talk to each other and figure out which one is active/standby on a per-VLAN basis for the OTV tunnel. The Site VLAN should not be included in the extend VLAN list.

Site Identifier

Should be unique per DC site. If you have more than one edge router per site, they must agree on the Site Identifier, as it’s used in the AED election.

Overlay Interface

The logical OTV tunnel interface.

OTV Join Interface

The physical link or port-channel that you use to route upstream towards the DCI.

OTV Control Group

Multicast address used to discover the remote sites in the control plane.

OTV Data Group

Used when you’re tunneling multicast traffic over OTV in the data plane.

IGMP Version 3

Needed to send (S,G) IGMP Report messages towards the DCI network on the Join Interface.

At this point that’s basically all that’s involved in the implementation of OTV. It “just works”, because all the behind the scenes stuff is hidden from us from a configuration point of view. A quick test of this from the end hosts shows us that:

R2#ping 255.255.255.255
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 255.255.255.255, timeout is 2 seconds:

Reply to request 0 from 172.16.0.3, 4 ms
Reply to request 1 from 172.16.0.3, 1 ms
Reply to request 2 from 172.16.0.3, 1 ms
Reply to request 3 from 172.16.0.3, 1 ms
Reply to request 4 from 172.16.0.3, 1 ms

R2#traceroute 172.16.0.3
Type escape sequence to abort.
Tracing the route to 172.16.0.3
VRF info: (vrf in name/id, vrf out name/id)
1 172.16.0.3 0 msec * 0 msec

The fact that R3 responds to R2’s packets going to the all hosts broadcast address (255.255.255.255) implies that they are in the same broadcast domain. How specifically is it working though? That’s what took a lot further investigation.

To simplify the packet level verification a little further, I changed the MAC address of the four end devices that are used to generate the actual data plane traffic. The Device, IP address, and MAC address assignments are as follows:

The first thing I wanted to verify in detail was what the data plane looked like, and specifically what type of tunnel encapsulation was used. With a little searching I found that OTV is currently on the IETF standards track in draft format. As of writing, the newest draft is draft-hasmit-otv-03. Section 3.1 Encapsulation states:

3. Data Plane

3.1. Encapsulation

The overlay encapsulation format is a Layer-2 ethernet frame
encapsulated in UDP inside of IPv4 or IPv6.

The format of OTV UDP IPv4 encapsulation is as follows:

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| IHL |Type of Service| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Protocol = 17 | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source-site OTV Edge Device IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination-site OTV Edge Device (or multicast) Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port = xxxx | Dest Port = 8472 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP length | UDP Checksum = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|R|R|R|I|R|R|R| Overlay ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Instance ID | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Frame in Ethernet or 802.1Q Format |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

A quick PING sweep of packet lengths with the Don’t Fragment bit set allowed me to find the encapsulation overhead, which turns out to be 42 bytes, as seen below:

R3#ping 172.16.0.2 size 1459 df-bit

Type escape sequence to abort.
Sending 5, 1459-byte ICMP Echos to 172.16.0.2, timeout is 2 seconds:
Packet sent with the DF bit set
.....
Success rate is 0 percent (0/5)

R3#ping 172.16.0.2 size 1458 df-bit

Type escape sequence to abort.
Sending 5, 1458-byte ICMP Echos to 172.16.0.2, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms

None of my testing however could verify what the encapsulation header was though. The draft says that the transport is supposed to be UDP port 8472, but none of my logging produced results showing that any UDP traffic was even in the transit network (save for my RIPv2 routing ;) ). After much frustration, I finally broke out the sniffer and took some packet samples. The first capture below shows a normal ICMP ping between R2 and R3.

MPLS? GRE? Where did those come from? That’s right, OTV is in fact a fancy GRE tunnel. More specifically it is an Ethernet over MPLS over GRE tunnel. My poor little PINGs between R2 and R3 are in fact encapsulated as ICMP over IP over Ethernet over MPLS over GRE over IP over Ethernet (IoIoEoMPLSoGREoIP for short). Let’s take a closer look at the encapsulation headers now:

In the detailed header output we see our transport Ethernet header, which in a real deployment can be anything depending on what the transport of your DCI is (Ethernet, POS, ATM, Avian Carrier, etc.) Next we have the IP OTV tunnel header, which surprised me in a few aspects. First, all documentation I read said that without the use of an OTV Adjacency Server, unicast can’t be used for transport. This is true... up to a point. Multicast it turns out is only used to establish the control plane, and to tunnel multicast over multicast in the data plane. Regular unicast traffic over OTV will be encapsulated as unicast, as seen in this capture.

The next header after IP is GRE. In other words, OTV is basically the same as configuring a static GRE tunnel between the edge routers and then bridging over them, along with some enhancements (hence fancy GRE). The OTV enhancements (which we’ll talk about shortly) are the reason why you wouldn’t just configure GRE statically. Nevertheless this surprised me because even in hindsight the only mention of OTV using GRE I found was here. What’s really strange about this is that Cisco’s OTV implementation doesn’t follow what the standards track draft says, which is UDP, even though the authors of the OTV draft are Cisco engineers. Go figure.

The next header, MPLS, makes sense since the prior encapsulation is already GRE. Ethernet over MPLS over GRE is already well defined and used in deployment, so there’s no real reason to reinvent the wheel here. I haven’t verified this in detail yet but I’m assuming that the MPLS Label value would be used in cases where the edge router has multiple overlay interfaces, in which case the label in the data plane would quickly tell it which overlay interface the incoming packet is destined for. This logic is similar to MPLS L3VPN where the bottom of the stack VPN label tells a PE router which CE facing link the packet is ultimately destined for. I’m going to do some more testing later with a larger more complex topology to actually verify this fact though, as all data plane traffic over this tunnel is always sharing the same MPLS label value.

Next we see the original Ethernet header, which is sourced from R2’s MAC address 0000.0000.0002 and going to R3’s MAC address 0000.0000.0003. Finally we have the original IP header and the final ICMP payload. The key with OTV is that this inner Ethernet header and its payload remain untouched, so it looks like from the end host perspective that all the devices are just on the same LAN.

Now that it was apparent that OTV was just a fancy GRE tunnel, the IS-IS piece fell into place. Since IS-IS runs directly over layer 2 (e.g. Ethernet), and OTV is an Ethernet over MPLS over GRE tunnel, then IS-IS can encapsulate as IS-IS over Ethernet over MPLS over GRE (phew!). To test this, I changed the MAC address of one of the end hosts, and looked at the IS-IS LSP generation of the edge devices. After all the goal of the OTV control plane is to use IS-IS to advertise the MAC addresses of end hosts in that particular site, as well as the particular VLAN that they reside in. The configuration steps and packet capture result of this are as follows:

R3#conf t

Enter configuration commands, one per line.  End with CNTL/Z.

R3(config)#int gig0/0

R3(config-if)#mac-address 1234.5678.9abc

R3(config-if)#

*Aug 17 22:17:10.883: %LINK-5-CHANGED: Interface GigabitEthernet0/0, changed state to reset

*Aug 17 22:17:11.883: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to down

*Aug 17 22:17:16.247: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to up

*Aug 17 22:17:17.247: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up

The first thing I noticed about the IS-IS encoding over OTV is that it uses IPv4 Multicast. This makes sense, because if you have 3 or more OTV sites you don’t want to have to send your IS-IS LSPs as replicated Unicast. As long as all of the AEDs on all sites have joined the control group (224.100.100.100 in this case), the LSP replication should be fine. This multicast forwarding can also be verified in the DCI transport network core in this case as follows:

N7K2-8#show ip mroute
IP Multicast Routing Table for VRF "default"

(*, 224.100.100.100/32), uptime: 20:59:33, ip pim igmp
Incoming interface: Null, RPF nbr: 0.0.0.0
Outgoing interface list: (count: 2)
port-channel78, uptime: 20:58:46, igmp
Ethernet1/29, uptime: 20:58:53, igmp

(150.1.38.3/32, 224.100.100.100/32), uptime: 21:00:05, ip pim mrib
Incoming interface: Ethernet1/29, RPF nbr: 150.1.38.3
Outgoing interface list: (count: 2)
port-channel78, uptime: 20:58:46, mrib
Ethernet1/29, uptime: 20:58:53, mrib, (RPF)

(150.1.78.7/32, 224.100.100.100/32), uptime: 21:00:05, ip pim mrib
Incoming interface: port-channel78, RPF nbr: 150.1.78.7
Outgoing interface list: (count: 2)
port-channel78, uptime: 20:58:46, mrib, (RPF)
Ethernet1/29, uptime: 20:58:53, mrib

(*, 232.0.0.0/8), uptime: 21:00:05, pim ip
Incoming interface: Null, RPF nbr: 0.0.0.0
Outgoing interface list: (count: 0)

Note that N7K1-3 (150.1.38.3) and N7K2-7 (150.1.78.7) have both joined the (*, 224.100.100.100). A very important point about this is that the control group for OTV is an Any Source Multicast (ASM) group, not a Source Specific Multicast (SSM) group. This implies that your DCI transit network must run PIM Sparse Mode and have a Rendezvous Point (RP) configured in order to build the shared tree (RPT) for the OTV control group used by the AEDs. You technically could use Bidir but you really wouldn't want to for this particular application. This kind of surprised me how they chose to implement it, because there are already more efficient ways of doing source discovery for SSM, for example how Multicast MPLS L3VPN uses the BGP AFI/SAFI Multicast MDT to advertise the (S,G) pairs of the PE routers. I suppose the advantage of doing OTV this way though is that it makes the OTV config very straightforward from an implementation point of view on the AEDs, and you don’t need an extra control plane protocol like BGP to exchange the (S,G) pairs before you actually join the tree. The alternative to this of course is to use the Adjacency Server and just skip using multicast all together. This however will result in unicast replication in the core, which can be bad, mkay?

Also for added fun in the IS-IS control plane the actual MAC address routing table can be verified as follows:

N7K2-7# show otv route

OTV Unicast MAC Routing Table For Overlay1

VLAN MAC-Address Metric Uptime Owner Next-hop(s)
---- -------------- ------ -------- --------- -----------
172 0000.0000.0002 1 01:22:06 site port-channel27
172 0000.0000.0003 42 01:20:51 overlay N7K1-3
172 0000.0000.000a 42 01:18:11 overlay N7K1-3
172 0000.0000.001e 1 01:20:36 site port-channel27
172 1234.5678.9abc 42 00:19:09 overlay N7K1-3

N7K2-7# show otv isis database detail | no-more
OTV-IS-IS Process: default LSP database VPN: Overlay1

OTV-IS-IS Level-1 Link State Database
LSPID Seq Number Checksum Lifetime A/P/O/T
N7K2-7.00-00 * 0x000000A3 0xA36A 893 0/0/0/1
Instance : 0x000000A3
Area Address : 00
NLPID : 0xCC 0x8E
Hostname : N7K2-7 Length : 6
Extended IS : N7K1-3.01 Metric : 40
Vlan : 172 : Metric : 1
MAC Address : 0000.0000.001e
Vlan : 172 : Metric : 1
MAC Address : 0000.0000.0002
Digest Offset : 0
N7K1-3.00-00 0x00000099 0xBAA4 1198 0/0/0/1
Instance : 0x00000094
Area Address : 00
NLPID : 0xCC 0x8E
Hostname : N7K1-3 Length : 6
Extended IS : N7K1-3.01 Metric : 40
Vlan : 172 : Metric : 1
MAC Address : 1234.5678.9abc
Vlan : 172 : Metric : 1
MAC Address : 0000.0000.000a
Vlan : 172 : Metric : 1
MAC Address : 0000.0000.0003
Digest Offset : 0
N7K1-3.01-00 0x00000090 0xCBAB 718 0/0/0/1
Instance : 0x0000008E
Extended IS : N7K2-7.00 Metric : 0
Extended IS : N7K1-3.00 Metric : 0
Digest Offset : 0

So at this point we see what our ICMP PING was actually ICMP over IP over Ethernet over MPLS over GRE over IP over Ethernet, and our routing protocol was IS-IS over Ethernet over MPLS over GRE over IP over Ethernet :/ What about multicast in the data plane though? At this point verification of multicast over the DCI core is pretty straightforward, since we can just enable a multicast routing protocol like EIGRP and look at the result. This can be seen below:

R2#config t

Enter configuration commands, one per line.  End with CNTL/Z.

R2(config)#router eigrp 1

R2(config-router)#no auto-summary

R2(config-router)#network 0.0.0.0

R2(config-router)#end

R2#
R3#config t

Enter configuration commands, one per line.  End with CNTL/Z.

R3(config)#router eigrp 1

R3(config-router)#no auto-summary

R3(config-router)#network 0.0.0.0

R3(config-router)#end

R3#

*Aug 17 22:39:43.419: %SYS-5-CONFIG_I: Configured from console by console

*Aug 17 22:39:43.423: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 172.16.0.2 (GigabitEthernet0/0) is up: new adjacency
R3#show ip eigrp neighbors

IP-EIGRP neighbors for process 1

H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq

                                            (sec)         (ms)       Cnt Num

0   172.16.0.2              Gi0/0             11 00:00:53    1   200  0  1

Our EIGRP adjacency came up, so multicast obviously is being tunneled over OTV. Let’s see the packet capture result:

We can see EIGRP being tunneled inside the OTV payload, but what’s with the outer header? Why is EIGRP using the ASM 224.100.100.100 group instead of the SSM 232.1.2.0/24 data group? My first guess was that link local multicast (i.e. 224.0.0.0/24) would get encapsulated as control plane instead of as data plane. This would make sense because control plane protocols like OSPF, EIGRP, PIM, etc. you would want those tunneling to all OTV sites, not just the ones that joined the SSM feeds. To test if this was the case, the only change I needed to make was to have one router join a non-link-local multicast group, and have the other router send ICMP pings. Since they’re effectively in the same LAN segment, no PIM routing is needed in the DC sites, just basic IGMP Snooping, which is enabled in NX-OS by default. The config on the IOS routers is as follows:

R2#config t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#ip multicast-routing
R2(config)#int gig0/0
R2(config-if)#ip igmp join-group 224.10.20.30
R2(config-if)#end
R2#

R3#ping 224.10.20.30 repeat 1000 size 1458 df-bit

Type escape sequence to abort.
Sending 1000, 1458-byte ICMP Echos to 224.10.20.30, timeout is 2 seconds:
Packet sent with the DF bit set

Reply to request 0 from 172.16.0.2, 1 ms
Reply to request 1 from 172.16.0.2, 1 ms
Reply to request 2 from 172.16.0.2, 1 ms

The packet capture result was as follows:

This was more as expected. Now the multicast data plane packet was getting encapsulated in the ICMP over IP over Ethernet over MPLS over GRE over IP *Multicast* over Ethernet OTV group. The payload wasn’t decoded, as I think even Wireshark was dumbfounded by this string of encapsulations.

In summary we can make the following observations about OTV:

OTV encapsulation has 42 bytes of overhead that consists of:

New Outer Ethernet II Header - 14 Bytes
New Outer IP Header - 20 Bytes
GRE Header - 4 Bytes
MPLS Header - 4 Bytes

OTV uses both Unicast and Multicast transport

ASM Multicast is used to build the control plane for OTV IS-IS, ARP, IGMP, EIGRP, etc.
Unicast is used for normal unicast data plane transmission between sites
SSM Multicast is used for normal multicast data plane transmission between sites
Optionally ASM & SSM can be replaced with the Adjacency Server

GRE is the ultimate band-aid of networking

Now the next time someone is throwing around fancy buzzwords about OTV, DCI, VWM, etc. you can say “oh, you mean that fancy GRE tunnel”? ;)

I’ll be continuing this series in the coming days and weeks on other Data Center and specifically CCIE Data Center related technologies. If you have a request for a specific topic or protocol that you’d like to see the behind the scene’s details of, drop me a line at bmcgahan@ine.com.

Happy Labbing!