BGP routes learning

[Romanpreet Singh]

BGP routes Insertion

Hi everybdy,

I was practising this simple BGP lab (above fig.) on GNS nothing much fancy now and encountered some issues. The details are:-

on R1 the output of sh ip bgp command is:

Network          Next Hop            Metric LocPrf Weight Path

*>i30.1.1.1/32      1.1.1.2                  0    100      0 200 300 i

*>i30.1.2.1/32      1.1.1.2                  0    100      0 200 300 i

*> 50.1.1.1/32      0.0.0.0                  0         32768 i

*>i100.1.1.1/32     2.2.2.3                  0    100      0 200 400 ?

*>i100.1.2.1/32     2.2.2.3                  0    100      0 200 400 ?

on R2 this is output of sh ip bgp command

Network          Next Hop            Metric LocPrf Weight Path

*> 30.1.1.1/32      10.1.1.2                               0 200 300 i

*> 30.1.2.1/32      10.1.1.2                               0 200 300 i

*>i50.1.1.1/32      1.1.1.1                  0    100      0 i

*>i100.1.1.1/32     3.3.3.4                  0    100      0 200 400 ?

*                   10.1.1.2                               0 200 300 400 ?

*>i100.1.2.1/32     3.3.3.4                  0    100      0 200 400 ?

*                   10.1.1.2                               0 200 300 400 ?

R2#

on R3 output of that command is

Network          Next Hop            Metric LocPrf Weight Path

*  30.1.1.1/32      11.1.1.2                               0 200 400 300 i

*>i                 3.3.3.3                  0    100      0 200 300 i

*  30.1.2.1/32      11.1.1.2                               0 200 400 300 i

*>i                 3.3.3.3                  0    100      0 200 300 i

*>i50.1.1.1/32      2.2.2.2                  0    100      0 i

*> 100.1.1.1/32     11.1.1.2                               0 200 400 ?

*> 100.1.2.1/32     11.1.1.2                               0 200 400 ?

The problem is that on breaking the R1 - R2 link the 30.1 prefix from R1's routing table goes missing. As per my understading R1 should learn that route from R3 now as they as also neighbors but this is not happening. Please help.

Roman

[Manouchehr Omari]

what do you mean by "breaking the R1 - R2 link the 30.1 prefix from R1's routing table goes missing"

as per you topolgy the 30.1 is on R6, isn't it?

Please describe what problem exactly you are facing?

[Henrik Grankvist]

Nothing is broken, it's just how BGP works.

If you look at R3's output, it shows the ">"-symbol for the 30.1.1.1 network and 30.1.2.1 network with next hop 3.3.3.3 (which I guess is R2?).

R3 will then only advertise its best route to 30.1.x.1, which is the one through R2, and that is the one you see on R1.

[Romanpreet Singh]

When all links are workin see the R1 bgp table, it has routes to 30.1.1.1 & 30.1.2.1 n/ws from 1.1.1.2 i.e from R2. But when I shut interface connecting R1 - R2, there is no route to both 30.1 n/ws on R1. And as per my understanding R3 should advertise those routes to R1 now. But it doesn't.

[blau grana · Accepted solution]

Hello Roman,

There could be two problems which depend on your configuration.

First ->

You shut interfaces between R1 and R2. Best route for both 30.1 prefixes in AS 100 is via R2 router. R2 will advertise best routes to R3 but R3 will not advertise them to R1 because of BGP split horizont rule -> iBGP learned routes are not advertised to other iBGP peers.

This is probably reason why you do not see 30.1 routes on R1 router. If you manipulate for example LOCAL_PREF to prefer R3, R1 will install 30.1 prefixes to routing table.

Second ->

If you eliminate problem with BGP split horizont rule (route reflector, iBGP full mesh) R1 will not install 30.1 prefixes to routing table even if they will be in BGP table. This is just my opinion cause I do not know exactly your setup, but from picture I assume that you configured BGP sessions based on IP addresses of interfaces. It is not recommended for iBGP sessions if you have redundant paths.

So if you shut interface link between R1 and R2 and R1 will learn 30.1 routes from R3 [best route for AS 100 is still via R2] R1 will not have route to next hop for BGP learned routes -> this is requirement for installation BGP route to routing table.

Best Regards

Please rate all helpful posts and close solved questions

[Romanpreet Singh]

Thanks everybdy for clearing my doubts !:)