BGP multihoming single site, two isp

Hi all,

We are soon starting up bgp and multihoming. Two ISPs and we have one site. We have already PI number and ASN assign to us. A Cisco 3825 with 3 ethernet interface with IOS advanced feature pack is order. No full routing table.

Any sugestion for a config?

Regards Jan Rockstedt

Reply to
Loading thread data ...

"" wrote in news:

Hi Jan

You could do it two ways:

  1. Use one ISP as primary and one as secondary
  2. Use both ISPs and recieve default routes from both ISP along with their "own" IPs

For both ISPs you setup standard BGP settings as described in the documentation from Cisco (take a look at CCO and do a search on BGP).

With option 1 you tag the inbound routes from both ISPs, setting the local-preference for 200 on the primary routes and leave the secondary ISP with default local-preference (that'll be 100). When you advertise your own routes, do a prepend to the routes you advertise to the secondary ISP.

With option 2 you simply accept any routes advertised by the ISP and advertise your own routes with default settings.

If you need further information / help, please respond to this group.

Cheers, Lars Christensen

Reply to
Lars L. Christensen


Thanks, that is interesting to me too.

Is it possible to find out how many AS numbers to "prepend" to effectively guarantee that one path will become primary for all ISPs?

I have made a stab at 4 relating to 2 connections within a single city. How many would be needed to guarantee this globally?

Is there a limit to the number of ASs in a path and if so what is a reasonable number to limit our prepends to?


Reply to


Hi Lars,

Thanks for your respond.

I have found so many doc on the google. Many are old quit .....

Our primary isp are the link 50 Mbit and our second isp we have a 10 Mbit. So two default routes, one with static with default

100 to the 50 Mbit and one with with 200 to the 10 Mbit Redistribute static to bgp?

router bgp our-ASN network network

neighbor remote-as ISP1-ASN neighbor route-map localonly out neighbor remote-as IPS2-ASN neighbor route-map localonly out


Reply to

A few comments...

Don't worry about the age of the docs on the web! Not much has changed with BGP since version 4 was released last century. If you can track down a copy of my book, read the chapter on connecting to an ISP, it still applies.

As for your proposal, I do not recommend it if your goal is to minimize potential for black holes. Rather than using static default routes, have your ISPs advertise a default route to you (that way, you only get a default route if your link to the ISP is up). There are even better ways to protect against problems in your ISP or upstream, but they take more effort, as does setting up reasonable load balancing.

Good luck and have fun!

Reply to
Vincent C Jones Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.