BGP community

[Anukalp S]

Hi , please help to understand below config.Can you please confirm either  prefix 10.110.10.0/24 will be recieved OR both 10.110.10.0/24 & 10.110.11.0/24

router#  router bgp 64512

  neighbor 10.11.131.2 route-map route in

route-map route permit 10

match ip address prefix-list allow

match community 15

set community no-export additive

!

route-map route deny 30

!

ip prefix-list allow seq 10 permit 10.110.10.0/24

!

router# sh ip bgp community-list 15

Network Next Hop Metric LocPrf Weight Path
>i 10.110.10.0/24 X.X.X.X  0 100 0 4103 65525 i
>i 10.110.11.0/24 X.X.X.X 0 100 0 4103 65525 i

[paul driver]

Hello

The route-map will be read for both match statements only, So if an match isnt found for either match statements then the set command isnt applicable.

[Anukalp S]

Hi Paul, Thanks but which prefix will be  received in BGP table  as route map is applied inbound in bgp, only 10.110.10.0/24  OR both 10.110.10.0/24 & 10.110.11.0/24

[Giuseppe Larosa]

Hello @Anukalp S ,

to be able to answer to your questions we would need to know:

a) what community value(s) are permitted by community-list 15 invoked in the route-map with match community 15

b) we should know what BGP community value(s) one ore more are attached to the two prefixes . This level of detail is given by

show ip bgp 10.110.10.0

show ip bgp 10.110.11.0

if the prefix 10.110.10.0/24 is a match for community-list 15 as explained above it has a BGP community value that matches one of the values permitted by community-list 15 then it is a match for the first route-map clause.

Edit:

by reviewing original post we can say that both prefixes match community-list 15 but only 10.110.10.0/24 is also a match for the prefix-list.

So prefix 10.110.10.0/24 is accepted and can be advertised to other iBGP peers but not to eBGP peers.

Prefix 10.110.11.0/24 matches the last route-map clause deny 30 and so it is filtered inbound

Hope to help

Giuseppe

[Anukalp S]

Hi Giuseppe,

Thanks, so here prefix-list matching prefix  10.110.10.0/24 will win over community matching both 10.110.10.0/24 & 10.110.11.0/24 thus only prefix 10.110.10.0/24 will be accepted.

[Giuseppe Larosa · Accepted solution]

Hello @Anukalp S ,

the first route-map clause has two match criteria and as explained by @paul driver  both match criteria must be met in order to have a match in that route-map clause.  It is a boolean AND:

if ( the prefix match communily-list 15 ) AND (prefix-list allow) then it is a match it is permitted and the NO_EXPORT community is added to it

prefix 10.110.11.0/24 is not permitted by prefix-list allow so it is not able to pass the combined criteria in AND.

There is not a winner match, simply the prefix-list is more selective then the community-list. Both match must be satisfied to have a match for the route-map ( the boolean AND operation reported  above)

Hope to help

Giuseppe

[Anukalp S]

Thanks Giuseppe for clarification