BGP not advertising route

[jpclowes]

Hello all,

I hope the diagram explain what it is I am trying to do, I have 4 routers R1 – R4 – R3 and R4 and they receive via EBGP a route to 161.163.160.0/22 this route can be seen in the IP table of each of R1 – 4. I am attempting to install a backup route to this network via R1 by configuring a static route to its WAN connection and then prepending the AS path, this route can be seen in R2 but R2 does not advertise this route to either 3 or 4.

R2>sh ip bgp 161.163.160.0

Multipath: iBGP

Advertised to update-groups:

     1

2856 65352 65439, (received & used)

   161.163.164.247 (metric 153856) from 161.163.164.247 (161.163.164.247)

     Origin incomplete, metric 0, localpref 100, valid, internal, best

     Community: 4282908783

2856 65352 65439, (received & used)

   161.163.164.249 (metric 154112) from 161.163.164.249 (161.163.164.249)

     Origin incomplete, metric 0, localpref 100, valid, internal

     Community: 4282908783

65438 65438 65438 65438 65438 65438 65438 65438 65438, (received & used) ••ß PREPENDED route

   161.163.164.14 from 161.163.164.14 (161.163.164.244)

     Origin IGP, metric 0, localpref 100, valid, external

The prepended route is not advertised

R2>sh ip bgp neighbors 161.163.164.249 advertised-routes | include 161.163.160.0

Thank you,

Jonathan

[Vasileios Bouloukos MSc, ITIL, CCIE · Accepted solution]

hi Jonathan,

The prepended route is not advertised by R2 to R3 and R4 since this route is also advertised by R3,4 to R2.

The R2 selects the route received by R3,R4 as best, so do not advertise back the route received by the same node (BGP rule).

If R3,R4 does not have the route, they learn the prepended route by R2.

Hope that helps,

Vasilis

[jpclowes]

It does yes although I think I answered the question myself if I took the time to read my post :-( R2 advertise the route to other peers just not the peers it receives the BEST path from.

Thank you Vasilis

[Vasileios Bouloukos MSc, ITIL, CCIE]

You are welcome Jonathan,

Thanks for the rating!