BGP Routing policy behaviour

[Sarafianl1980]

Hi All,

I am reviewing some configuration on one of our routers and confused by this design:

router bgp 12345

neighbor 1.2.3.4 remote-as 6789
neighbor 1.2.3.4 
neighbor 1.2.3.4 send-community
neighbor 1.2.3.4 remove-private-as
neighbor 1.2.3.4 prefix-list FX-IN in
neighbor 1.2.3.4 prefix-list FX-TUNNEL-LON-OUT out
neighbor 1.2.3.4 route-map RM-BGP-LON-OUT out
neighbor 1.2.3.4 filter-list 1 out

route-map RM-BGP-LON-OUT, permit, sequence 10
Match clauses:
ip address prefix-lists: FX-PRIMARY-LON-OUT
Set clauses:
community 6789:280

route-map RM-BGP-LON-OUT permit 20
match ip address prefix-list FX-SECONDARY-LON-OUT
set community 6789:270

ip prefix-list FX-PRIMARY-LON-OUT seq 100 permit 10.10.13.13/32

ip prefix-list FX-SECONDARY-LON-OUT seq 100 permit 10.10.80.213/32

ip prefix-list FX-TUNNEL-LON-OUT seq 150 permit 10.10.13.13/32

ip prefix-list FX-TUNNEL-LON-OUT seq 120 permit 10.10.80.213/32

What happens to local pref impact here?  They have configured the same subnet several times in two different prefix lists.  One set prefix lists has a route-map around it to set a community tag but the other does not?  What does actually mean?  What takes precedence?

Thanks

[cofee · Accepted solution]

Hi,

I don't see any explicit local preference configuration and it is only advertised to iBGP neighbors . But in my opinion this configuration is not clean and not exactly sure what the engineer who configured this is trying to do.

neighbor 1.2.3.4 prefix-list FX-IN in ( I don't see any prefix-list with this name)

neighbor 1.2.3.4 prefix-list FX-TUNNEL-LON-OUT out (This doesn't need to configured and it can be incorporated with route map RM-BGP-LON-OUT as sequence 30 without any community value and by default any other prefixes won't be advertised to this bgp neighbor)

neighbor 1.2.3.4 route-map RM-BGP-LON-OUT out

neighbor 1.2.3.4 filter-list 1 out  ( Didn't see access-list 1 in your configuration, so not sure what exactly it is doing)

[Sarafianl1980]

Thanks Cofee.

I don't understand what the intent was either?

I'm still puzzled though what the effect would be even if we incorporated that into another route-map as seq 30.  If subnet 10.10.13.13/32 matched in seq 20 with a community then applied what would the effect be on the exact same subnet 10.10.13.13/32 be in seq 30 where there is no community setting? I just cannot understand why they've duplicated it twice? 

[cofee]

That is correct. It would make no difference.