BGP - Community question

[opers13]

I'm multihomed between two ISPs and I have a /24 from both ISPs.  Both ISPs advertise each others /24, all works fine.

ISP-A  BGP ISP-B

   |                  |

   |                  |

RTR1 ------- RTR2

ISP-A = 1.1.1.0/24

ISP-B = 2.2.2.0/24

I trying to send an AS prepend community to ISP-A making 2.2.2.0/24 less preferable on ISP-A, which worked just fine but I noticed that in the process 1.1.1.0/24 stopped being advertised on ISP-A.

How can I send the community to ISP-A for 2.2.2.0/24 and still advertise 1.1.1.0/24?

RTR1 - CONFIG

router bgp 1234

bgp log-neighbor-changes

network 2.2.2.0 mask 255.255.255.0

network 1.1.1.0

neighbor 5.5.5.5 remote-as 2222

neighbor 5.5.5.5 soft-reconfiguration inbound

neighbor 5.5.5.5 send-community

neighbor 5.5.5.5 route-map PEND-ISP-B-INBOUND out

route-map PEND-ISP-B-INBOUND permit 10

match ip address 163

set community 423433099

access-list 163 permit ip host 2.2.2.0 host 255.255.255.0

[Giuseppe Larosa]

Hello opers13,

you need to add a second block to the route map like

route-map PEND-ISP-B-INBOUND permit 20

match ip address 164

you should use

access-list  164 permit ip host 1.1.1.0 host 255.255.255.0

the reason is that the route-map used in this way has an implicit deny any so you need an additional block to advertise additional routes that do not match the first route-map clause

Hope to help

Giuseppe

[alexng]

so just add another ACL(164) and another route-map permitting 1.1.1.0/24?

[Giuseppe Larosa]

Hello alexng,

you are requested to add a second block/clause to the SAME route-map this is what you need, the use of the ACL is highly recommended as your scenario is multihomed to avoid to advertise routes from one provider to the other one.

Follow the instructions of the previous post and you should be fine.

Hope to help

Giuseppe

[Richard Burts]

Route maps are a bit like access lists in that there is an implicit deny at the end. In the route map if it is not permitted then it will not be advertised. So you certainly need another statement in the route map. You could have that statement use another access list to permit 1.1.1.0 or you could just let the statement permit any.

HTH

Rick

[edit] I see the post from Giuseppe and his comment about using explicit access list in a multi homed environment is well taken.