Configuring BGP Local Preference on Juniper Routers

⁠In the BGP Path selection algorithm, if your device is not a Cisco router, the first BGP Path Attribute considered during Path Selection is the Local Preference attribute.

Local Preference is not a vendor-specific BGP Path Attribute, unlike the WEIGHT attribute, which is Cisco Proprietary.

The Local Preference attribute can be modified to allow a specific external BGP path to be chosen when you have a multi-homed environment. Local Preference can be applied inbound from an external BGP neighbor or outbound to an internal BGP neighbor. Local Preference shows the best outbound interface towards another AS. In other words, here, it shows the exit point of your AS towards another AS.

The default value of Local Preference is 100. And always, the highest one is chosen as the Best BGP path. Local Preference is a non-transitive attribute as it cannot be advertised to an external BGP neighbor. 

The local Preference attribute is a Well-known and Discretionary BGP Path Attribute. As we mentioned before, it is supported by every BGP implementation (well-known), and every BGP router recognizes it but is optionally present in the BGP Update packet (discretionary). Since Local Preference is a Well-known attribute, it must be understood by all BGP implementations irrespective of a vendor. Since Local Preference is discretionary, it may or may not be present as a mandatory option.

Let's now see how to configure Local Preference on a Juniper Router.

Here is my topology:

⁠
In this topology, I have already pre-configured my BGP neighborships. Here is my pre-config:

vMXvCP-1:
cli
configure
set routing-options router-id 1.1.1.1
set routing-options autonomous-system 1
set protocols bgp export ADVERTISE_ROUTES
set protocols bgp group eBGP type external
set protocols bgp group eBGP neighbor 192.168.12.2 peer-as 2345
set protocols bgp group eBGP neighbor 192.168.13.3 peer-as 2345
set policy-options policy-statement ADVERTISE_ROUTES term 1 from protocol direct
set policy-options policy-statement ADVERTISE_ROUTES term 1 from route-filter 1.1.1.1/32 exact
set policy-options policy-statement ADVERTISE_ROUTES term 1 then accept
commit

vMXvCP-2:
cli
configure
set routing-options router-id 2.2.2.2
set routing-options autonomous-system 2345
set protocols bgp export ADVERTISE_ROUTES
set protocols bgp group eBGP type external
set protocols bgp group eBGP neighbor 192.168.12.1 peer-as 1
set protocols bgp group iBGP type internal
set protocols bgp group iBGP peer-as 2345
set protocols bgp group iBGP local-address 2.2.2.2
set protocols bgp group iBGP neighbor 7.7.7.7  
set policy-options policy-statement ADVERTISE_ROUTES term 1 from protocol direct
set policy-options policy-statement ADVERTISE_ROUTES term 1 from route-filter 2.2.2.2/32 exact
set policy-options policy-statement ADVERTISE_ROUTES term 1 then accept
set policy-options policy-statement NHS term 1 from protocol bgp
set policy-options policy-statement NHS term 1 from route-type external
set policy-options policy-statement NHS term 1 then next-hop self
set protocols bgp group iBGP export NHS
set protocols bgp group iBGP export ADVERTISE_ROUTES
set protocols bgp advertise-inactive
commit

vMXvCP-3:
cli
configure
set routing-options router-id 3.3.3.3
set routing-options autonomous-system 2345
set protocols bgp export ADVERTISE_ROUTES
set protocols bgp group eBGP type external
set protocols bgp group eBGP neighbor 192.168.13.1 peer-as 1
set protocols bgp group iBGP type internal
set protocols bgp group iBGP peer-as 2345
set protocols bgp group iBGP local-address 3.3.3.3
set protocols bgp group iBGP neighbor 7.7.7.7
set policy-options policy-statement ADVERTISE_ROUTES term 1 from protocol direct
set policy-options policy-statement ADVERTISE_ROUTES term 1 from route-filter 3.3.3.3/32 exact
set policy-options policy-statement ADVERTISE_ROUTES term 1 then accept
set policy-options policy-statement NHS term 1 from protocol bgp
set policy-options policy-statement NHS term 1 from route-type external
set policy-options policy-statement NHS term 1 then next-hop self
set protocols bgp group iBGP export NHS
set protocols bgp group iBGP export ADVERTISE_ROUTES
set protocols bgp advertise-inactive
commit

vMXvCP-4:
cli
configure
set routing-options router-id 4.4.4.4
set routing-options autonomous-system 2345
set protocols bgp export ADVERTISE_ROUTES
set protocols bgp group eBGP type external
set protocols bgp group eBGP neighbor 192.168.46.6 peer-as 6
set protocols bgp group iBGP type internal
set protocols bgp group iBGP peer-as 2345
set protocols bgp group iBGP local-address 4.4.4.4
set protocols bgp group iBGP neighbor 7.7.7.7
set policy-options policy-statement ADVERTISE_ROUTES term 1 from protocol direct
set policy-options policy-statement ADVERTISE_ROUTES term 1 from route-filter 4.4.4.4/32 exact
set policy-options policy-statement ADVERTISE_ROUTES term 1 then accept
set policy-options policy-statement NHS term 1 from protocol bgp
set policy-options policy-statement NHS term 1 from route-type external
set policy-options policy-statement NHS term 1 then next-hop self
set protocols bgp group iBGP export NHS
set protocols bgp group iBGP export ADVERTISE_ROUTES
set protocols bgp advertise-inactive
commit

vMXvCP-5:
cli
configure
set routing-options router-id 5.5.5.5
set routing-options autonomous-system 2345
set protocols bgp export ADVERTISE_ROUTES
set protocols bgp group eBGP type external
set protocols bgp group eBGP neighbor 192.168.56.6 peer-as 6
set protocols bgp group iBGP type internal
set protocols bgp group iBGP peer-as 2345
set protocols bgp group iBGP local-address 5.5.5.5
set protocols bgp group iBGP neighbor 7.7.7.7
set policy-options policy-statement ADVERTISE_ROUTES term 1 from protocol direct
set policy-options policy-statement ADVERTISE_ROUTES term 1 from route-filter 5.5.5.5/32 exact
set policy-options policy-statement ADVERTISE_ROUTES term 1 then accept
set policy-options policy-statement NHS term 1 from protocol bgp
set policy-options policy-statement NHS term 1 from route-type external
set policy-options policy-statement NHS term 1 then next-hop self
set protocols bgp group iBGP export NHS
set protocols bgp group iBGP export ADVERTISE_ROUTES
set protocols bgp advertise-inactive
commit

vMXvCP-6:
cli
configure
set routing-options router-id 6.6.6.6
set routing-options autonomous-system 6
set protocols bgp export ADVERTISE_ROUTES
set protocols bgp group eBGP type external
set protocols bgp group eBGP neighbor 192.168.46.4 peer-as 2345
set protocols bgp group eBGP neighbor 192.168.56.5 peer-as 2345
set policy-options policy-statement ADVERTISE_ROUTES term 1 from protocol direct
set policy-options policy-statement ADVERTISE_ROUTES term 1 from route-filter 6.6.6.6/32 exact
set policy-options policy-statement ADVERTISE_ROUTES term 1 then accept
commit

vMXvCP-RR:
set routing-options router-id 7.7.7.7
set routing-options autonomous-system 2345
set protocols bgp group iBGP local-address 7.7.7.7
set protocols bgp group iBGP cluster 7.7.7.7
set protocols bgp group iBGP peer-as 2345
set protocols bgp group iBGP neighbor 2.2.2.2
set protocols bgp group iBGP neighbor 3.3.3.3
set protocols bgp group iBGP neighbor 4.4.4.4
set protocols bgp group iBGP neighbor 5.5.5.5
set protocols bgp advertise-inactive
commit

Based on my configs above: 

1. vMXvCP-1 establishes an external BGP neighborship with vMXvCP-2 and vMXvCP-3. 

2. vMXvCP-6 establishes an external BGP neighborship with vMXvCP-4 and vMXvCP-5. 

3. vMXvCP-RR establishes an internal BGP neighborship with vMXvCP-2, vMXvCP-3, vMXvCP-4, and vMXvCP-5.

Currently, without any Local Preference configuration changes, vMXvCP-6 is seeing a default Local Preference of 100 and is choosing vMXvCP-4 as the primary exit for reaching the Loopback (1.1.1.1/32) of vMXvCP-1 based on some other BGP path selection attribute.

root@vMX-6# run show route protocol bgp 1.1.1.1 detail    

inet.0: 12 destinations, 18 routes (12 active, 0 holddown, 0 hidden)

1.1.1.1/32 (2 entries, 1 announced)

        *BGP    Preference: 170/-101

                Next hop type: Router, Next hop index: 590

                Address: 0xcee9e30

                Next-hop reference count: 8

                Source: 192.168.46.4

                Next hop: 192.168.46.4 via ge-0/0/2.0, selected

                Session Id: 0x141

                State: <Active Ext>

                Local AS:     6 Peer AS:  2345

                Age: 3:59

                Validation State: unverified

                Task: BGP_2345.192.168.46.4+54892

                Announcement bits (1): 0-KRT

                AS path: 2345 1 I

                Accepted

                Localpref: 100

                Router ID: 4.4.4.4

         BGP    Preference: 170/-100

                Next hop type: Router, Next hop index: 589

                Address: 0xcee9d10

                Next-hop reference count: 9

                Source: 192.168.56.5

                Next hop: 192.168.56.5 via ge-0/0/0.0, selected

                Session Id: 0x140

                State: <Ext>

                Inactive reason: Local Preference

                Local AS:     6 Peer AS:  2345

                Age: 44

                Validation State: unverified

                Task: BGP_2345.192.168.56.5+179

                AS path: 2345 1 I

                Accepted

                Localpref: 100

                Router ID: 5.5.5.5

Now let's go and configure local preference on VMXvCP-6 to ensure that vMXvCP-5 is chosen as the primary path to reach the Loopback (1.1.1.1/32) of vMXvCP-1. 

vMXvCP-6:
cli
configure
set protocols bgp group eBGP neighbor 192.168.56.5 import LP_FROM_R5
set policy-options policy-statement LP_FROM_R5 term 1 from protocol bgp
set policy-options policy-statement LP_FROM_R5 term 1 from route-filter 1.1.1.1/32 exact
set policy-options policy-statement LP_FROM_R5 term 1 then local-preference 101
commit

As you can see above, I configured VMXvCP-6. I applied the policy statement inbound from the external BGP neighbor vMXvCP-5 as Local Preference cannot be configured on vMXvCP-5 outbound to vMXvCP-6 as Local Preference is non-transitive.

This configuration results in VMXvCP-6 now preferring VMXvCP-5 as the primary exit router to reach 1.1.1.1/32. No changes were made in any other routers.

root@vMX-6# run show route protocol bgp 1.1.1.1 detail    

inet.0: 12 destinations, 18 routes (12 active, 0 holddown, 0 hidden)

1.1.1.1/32 (2 entries, 1 announced)

        *BGP    Preference: 170/-100

                Next hop type: Router, Next hop index: 589

                Address: 0xcee9d10

                Next-hop reference count: 9

                Source: 192.168.56.5

                Next hop: 192.168.56.5 via ge-0/0/0.0, selected

                Session Id: 0x140

                State: <Ext>

                Inactive reason: Local Preference

                Local AS:     6 Peer AS:  2345

                Age: 44

                Validation State: unverified

                Task: BGP_2345.192.168.56.5+179

                AS path: 2345 1 I

                Accepted

                Localpref: 101

                Router ID: 5.5.5.5

        BGP    Preference: 170/-101

                Next hop type: Router, Next hop index: 590

                Address: 0xcee9e30

                Next-hop reference count: 8

                Source: 192.168.46.4

                Next hop: 192.168.46.4 via ge-0/0/2.0, selected

                Session Id: 0x141

                State: <Active Ext>

                Local AS:     6 Peer AS:  2345

                Age: 3:59

                Validation State: unverified

                Task: BGP_2345.192.168.46.4+54892

                Announcement bits (1): 0-KRT

                AS path: 2345 1 I

                Accepted

                Localpref: 100

                Router ID: 4.4.4.4

I hope this was informative, and thank you.

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