BGP Conditional Advertisement Query

mvsheik123 · ‎09-23-2008

Hi all,

We have 2 edge routers ebgp peering with 2 ISPS (primary & backup). ASPath condition on second ISP still showing some incoming traffic. But I want secondary to be pure backup.ISPs do nt support communities. So Iam looking at 'conditional ' advt.

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094309.shtml

Iam wondering how I can apply in my scenario. ISP1 & 2 assgned us with /27 for LAN, but we are using/advertising our own (6.9.6.0/24) subnet. The ' ip route' from both rtrs:

*****************************************

Primary:

Gateway of last resort is 27.10.6.48 to network 0.0.0.0

15.35.14.0/27 is subnetted, 1 subnets

B 15.35.14.0 [200/0] via 6.9.6.2, 6w3d

27.10.9.0/32 is subnetted, 2 subnets

S 27.10.6.49 [1/0] via 8.21.9.101

S 27.10.6.48 [1/0] via 8.21.9.101

6.0.0.0/24 is subnetted, 1 subnets

C 6.9.6.0 is directly connected, FastEthernet0/0

8.0.0.0/30 is subnetted, 1 subnets

C 8.21.9.100 is directly connected, FastEthernet0/1

B* 0.0.0.0/0 [20/0] via 27.10.6.48, 2w1d

Backup:

Gateway of last resort is 23.23.53.17 to network 0.0.0.0

15.35.14.0/27 is subnetted, 1 subnets

S 15.35.14.0 is directly connected, Null0

6.0.0.0/24 is subnetted, 1 subnets

C 6.9.6.0 is directly connected, FastEthernet0/0

23.23.53.0/30 is subnetted, 1 subnets

C 23.23.53.16 is directly connected, Serial0/2/0

B* 0.0.0.0/0 [20/0] via 23.23.53.17, 6w3d

**********************************

We are receiving default route from carriers. I might be wrong here, but from the above o/p, I think, we are not receving any routes from ISP1 to add conditional advt config. Please suggest.

Thank you

MS

Giuseppe Larosa · ‎09-25-2008

Hello Mehboob,

in the last example in the document you linked there is a interesting suggestion:

to add a match as-path to the route-map so that you can check if a route is lost from a specific neighbor

in your case you need to build

route-map NON-EXIST permit 10

match ip address prefix-list only-default

match as-path 1

!

ip as-path access-list 1 permit ^AS_ISP1

ip prefix-list only-default permit 0.0.0.0/0

route-map ADVERTISE permit 10

match ip address 60

!

to be used on neighbor to ISP2

Hope to help

Giuseppe

mvsheik123 · ‎09-26-2008

Hi,

Thank you for your reply. I actually did some more research on this I was thinking of doing with the default route advertisement. I found a good link explains the same with configs..:

http://ciscoconfigs.net/index.php?option=com_content&task=view&id=26&Itemid=29

In the backup rtr(connecting to 2nd ISP) Iam seeing a default route learned with path info having primary ISP AS# (due to IBGP between rtrs).

* i0.0.0.0 6.9.6.3 0 100 0 2050 i

So iam planning to add a 'conditional config' based on this.(as long as RTR2 learning 0.0.0.0 with AS#ISP1 do not advertise my nw to ISP2). I think that helps.

Thank you for your quick reply and suggestion.

Thanks

MS