BGP Conditional Advertisement Query

Unanswered Question
Jan 29th, 2009
User Badges:

Hi All,

I have client with 2 ISPs/Dual routers. ISP-A :10Meg & ISP-B : T1

Local public IP space advertised to ISPA & ISPB via EBGP (with ISP-B : via AS-Prepend). LAN side of the routers has HSRP config to have ISP-A Connected router as Primary and IBGP config as well.

I would like to implement 'BGP Conditional Advertisement' so that ISP-B will strictly be backup (as ISP-B does not support any communmities). I want to create the NON-EXIST map such a way when ISP-B router not seeing being advertised from ISP-A AS, then ISP-B will be used. Please see the attached info from both the routers.

(attached as the o/p copy& paste here not sitting in better way)

As the Backup router learning from backup ISP (preferred *>) as well as from IBGP via primary ISP AS#, if conditional adverisement applied and in case if BGP with Primary ISP does down, will the secondary ISP takes over (as from priamry willbe gone).

How much time normally it takes for the take over - 3mins..??



  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Mohamed Sobair Thu, 01/29/2009 - 12:49
User Badges:
  • Gold, 750 points or more


The link provided by Gui is usefull, But what do u mean by Do u mean the as (next-hop) or (Default route)?

The Conditional advertise applies IF and Only IF the Network Specified in the Non-exist Map is not in the BGP routing table, This doesnt mean the Main link has to be Down. about BGP update , the EBGP update takes 30 sec while the IBGP takes 5 sec.



fortis123 Fri, 01/30/2009 - 20:59
User Badges:

Thank you both for your replies. I have the above link, but this example shows that client is receiving different n/w from the ISPs. My case, Iam receiving default route ( only from each ISP (as customer advertise his own /24 subnet to the ISPs).

The primary router will not receive default route from ISP1  1. Link is down  2. EBGP peering issues with primary ISP. So My query is (with reference attached output from both routers- when both links are up & healthy), incase on the second router, if I create a NON-EXIST map of 'NOT RECEIVING from ISP1 AS# ONLY'  ADVERTISE --> Client network. Will that achieve what Iam trying to..?



Giuseppe Larosa Sat, 01/31/2009 - 00:14
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

Hello Mehboob,

the third example of the document shows that you can match on AS path and on IP prefix.

So you need to have an iBGP session between the two customer routers.

On R2 you will use weight to give preference to received on iBGP session.

All this should work but some testing is recommended.

Hope to help




royalblues Sat, 01/31/2009 - 01:17
User Badges:
  • Green, 3000 points or more

Yes it would work

The best thing would be to set a condition on the router connecting to ISPB to advertise the LAN subnet pool only if you receive the WAN subnet between the primary router and ISPA..something like below

ip prefix-list adv seq 10 permit

ip prefix-list cond seq 10 permit

route-map advertise permit 10

match ip address prefix-list adv

route-map condition permit 10

match ip address prefix-list cond

router bgp

neighbor advertise-map advertise non-exist-map cond



fortis123 Sat, 01/31/2009 - 21:14
User Badges:

Thanks again. Narayan, if you look at the o/p that was attached , the ISPB router not learning the any of the Subnets from ISPA (except default route)

Thank you


royalblues Sun, 02/01/2009 - 00:33
User Badges:
  • Green, 3000 points or more

Yes i saw that but the WAN subnet can be advertised from the ISPA router as well towards ISPB router


fortis123 Sun, 02/01/2009 - 06:07
User Badges:

Thank you. What is recomended way for that (redistributing..?) . Also, without modifying ISPA rtr with the existing o/p attached, if I create the route-map(s) based on default route ( on ISP-B, will that work..?




This Discussion