BGP Community Local AS

Lessons Discussion
12

Hello Stefanita,

I just made some changes to this lesson:

I fixed this in the topology picture.

If you want connectivity between the different routers, you’ll have to advertise some more networks yes. R3, R4, R5, and R6 don’t know about 192.168.12.0/24 but R1 also doesn’t know about any of the subnets in between R2, R3, R4, R5, and R6. For this example, it doesn’t matter since the only thing I wanted to demonstrate is how the local-as community works.

I double checked this. According to this document:

It states:

Paths that contain AS_CONFED_SEQUENCE and AS_CONFED_SET are local to the confederation. Therefore, these paths are treated as internal paths. There is no distinction between Confederation External and Confederation Internal.

So, there is no difference between “external” and “internal” here, which is why R5 uses the lowest BGP Router ID to choose the path. Here’s what R5 has now:

R5#show ip bgp 1.1.1.1/32
BGP routing table entry for 1.1.1.1/32, version 4
Paths: (2 available, best #1, table default)
  Advertised to update-groups:
     2         
  Refresh Epoch 1
  (23) 1
    192.168.12.1 (metric 3) from 3.3.3.3 (3.3.3.3)
      Origin IGP, metric 0, localpref 100, valid, confed-external, best
      rx pathid: 0, tx pathid: 0x0
  Refresh Epoch 1
  (23) 1
    192.168.12.1 (metric 3) from 4.4.4.4 (4.4.4.4)
      Origin IGP, metric 0, localpref 100, valid, confed-internal
      rx pathid: 0, tx pathid: 0

It chooses 3.3.3.3 as the best path. Let’s change the router ID:

R3(config)#router bgp 23
R3(config-router)#bgp router-id 33.33.33.33

Here’s R5 again:

R5#show ip bgp 1.1.1.1/32
BGP routing table entry for 1.1.1.1/32, version 5
Paths: (2 available, best #2, table default)
  Advertised to update-groups:
     3         
  Refresh Epoch 1
  (23) 1
    192.168.12.1 (metric 3) from 3.3.3.3 (33.33.33.33)
      Origin IGP, metric 0, localpref 100, valid, confed-external
      rx pathid: 0, tx pathid: 0
  Refresh Epoch 1
  (23) 1
    192.168.12.1 (metric 3) from 4.4.4.4 (4.4.4.4)
      Origin IGP, metric 0, localpref 100, valid, confed-internal, best
      rx pathid: 0, tx pathid: 0x0

You can see R5 doesn’t care whether it’s confed-internal or confed-external, it picks the path with lowest router ID.

I hope this has been helpful!

Rene

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