"match community" question

[g.raymakers]

                   Hi,

hopefully an easy question, but when one uses the "match community" command in a route-map, does the following work :

route-map bgp permit 1

match community 65000:2

or does it have to be:

ip community-list 20 permit 65000:2

!

route-map bgp permit 1

match community 20

the platform is SUP2T using IOS 15.0.1SY4

Many Thanks,

Guy

[Jose Jara · Accepted solution]

It has to be a community-list, otherwise it will not work.

Best Regards,

Jose.

[Peter Paluch · Accepted solution]

Hi Guy,

I believe that only the second option is supported, i.e. first define a community ACL and then refer to it in a route-map.

Router(config-route-map)#match community ?

  <1-99>     Community-list number (standard)

  <100-500>  Community-list number (expanded)

  WORD       Community-list name

Best regards,

Peter

[Jose Jara · Accepted solution]

Peter,

you are right. The problem is that, if by mistake you add the community directly, IOS will not give you an error message as it will believe that this is a community-list that it really does not exist. I have seen this mistake a couple of times in production networks.

Of course, this will be depend of what community number you are using, but for those in-between 1 and 500 or in the format with the two colons, IOS will not give you any error message as it will believe that they are community-lists. For the others, it gives this message:

R7(config-route-map)#match community 501

% Community name cannot have all digits

R7(config-route-map)#match community 65000

% Community name cannot have all digits

Best Regards,

Jose.

[g.raymakers]

Thanks Peter, Jose.

I also always used the community list, but noticed this other config on another network

Regardsn

Guy

[tomahawk513@gmail.com]

Great tip!  This fixed it.  Here's what I went with:

route-map BGP-OUT-RM, permit, sequence 10
Match clauses:
community (community-list filter): 100
Set clauses:
as-path prepend 65218 65218 65218 65218
Policy routing matches: 0 packets, 0 bytes
route-map BGP-OUT-RM, permit, sequence 20
Match clauses:
community (community-list filter): 200
Set clauses:
Policy routing matches: 0 packets, 0 bytes
route-map OSPF-to-BGP-RM, permit, sequence 10
Match clauses:
tag 100
Set clauses:
community 100
Policy routing matches: 0 packets, 0 bytes
route-map OSPF-to-BGP-RM, permit, sequence 20
Match clauses:
route-type internal
Set clauses:
community 200
Policy routing matches: 0 packets, 0 bytes

router bgp 65218
no synchronization
bgp log-neighbor-changes
redistribute connected
redistribute ospf 20 route-map OSPF-to-BGP-RM
neighbor 10.3.1.78 remote-as 13979
neighbor 10.3.1.78 route-map BGP-OUT-RM out
no auto-summary

ip community-list 100 permit 100
ip community-list 200 permit 200

Now the receiving router receives those routes w/ the correct AS-Path prepending, allowing it to use the most optimal route to given subnets:

ATT-WAN#show ip bgp nei 10.3.1.77 received-routes
BGP table version is 19, local router ID is 10.3.1.158
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*  10.250.1.0/24    10.3.1.77                2             0 65218 ?
*  10.254.255.32/30 10.3.1.77                4             0 65218 65218 65218 65218 65218 ?
*  10.254.255.36/30 10.3.1.77                4             0 65218 65218 65218 65218 65218 ?
*  10.254.255.40/30 10.3.1.77                3             0 65218 65218 65218 65218 65218 ?
*  10.254.255.44/30 10.3.1.77                3             0 65218 65218 65218 65218 65218 ?
*  10.254.255.200/30
                    10.3.1.77                2             0 65218 65218 65218 65218 65218 ?
*  10.254.255.204/30
                    10.3.1.77                2             0 65218 65218 65218 65218 65218 ?
*  10.254.255.208/30
                    10.3.1.77               12             0 65218 65218 65218 65218 65218 ?
   Network          Next Hop            Metric LocPrf Weight Path

Total number of prefixes 8

The project I was working on was how to optimize routing between my organization's Datacenter and DR site, the later of which is starting to host production applications, and our remote sites via our MPLS AVPN cloud.  I don't have access to the AVPN PE router (and the ISP is a huge pain) that sits in a separate ASN, so I needed a way to influence the PE router's route selection so that it passes on those routes to our remote sites.  

I was also trying to avoid a prefix list or an ACL because someone has to keep those up-to-date or we get suboptimal routing again.  If I could use tags, I could pick a choke-point between the Datacenter and DR site (that communicate using OSPF on a back-door connection).  On that choke-point I would split our OSPF instance, applying route tags when redistributing between OSPF instances.  Then, the CE WAN routers could see those tags via their OSPF instance and, when redistributing into their BGP instance, apply community values.  Finally, when advertising routes to their BGP neighbors, the WAN routers could prepend the AS-Path (or not) based on those community values.