BGP multihome - is Outbound policy Routing possibe ?

Answered Question
Aug 9th, 2007

hi i have two internet Links of 4 MB & 8 MB.

Link A - 4 MB

Link B - 8 MB

I am doing Multi-Home BGP for these two.

I have 2 ranges of IPs

192.168.1.0 / 24

192.168.2.0 / 24

Currently i am advertising them in such a way that from the Internet, inbound Traffic ( Using AS-Path Prepend )

Comes in via Link A for 192.168.1.0

Comes via Link B for 192.168.2.0

My requirement is

Outgoing Traffic should go out

via Link A for source network 192.168.1.0

via Link B for source network 192.168.2.0

FYI

I am getting static default routes from both ISPs via BGP

I have this problem too.
0 votes
Correct Answer by Richard Burts about 9 years 4 months ago

Anand

If you can do PBR with route tracking then it should resolve the potential problem with losing connectivity through one of the providers.

Good luck with getting this going. Let us know how it works out.

HTH

Rick

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Richard Burts Thu, 08/09/2007 - 07:41

Anand

Yes you could configure Policy Based Routing and accomplish that packets sourced from 192.168.1.0 are forwarded out link A and packets sourced from 192.168.2.0 are forwarded out link B.

Note that for this you would configure PBR on the interface(s) facing your network and not on the interfaces facing the ISPs since you configure PBR on the entering interface not on the exiting interface.

HTH

Rick

anandramapathy Thu, 08/09/2007 - 08:08

Glad to hear that Rick.

I am running 12.3(16) on my I-BGP routers.

What happens when either of the link goes down, do i have to manually remove the Policy route from my interface for the traffic to be routed via the alternate link ?

Currently i am routing all traffic via the 8 MB using local-preference.

Is this possible to be accomplished via a BGP feature where no manual intervention is required

anandramapathy Thu, 08/09/2007 - 08:22

If i define, multiple next hops, will the PBR work or do i have to define SAA & configure PBR

Richard Burts Thu, 08/09/2007 - 09:01

Anand

With Policy Based Routing if you are setting next-hop and the next hop goes out of the routing table, then PBR lets taffic be routed normally. So no manual action required. The possible sticky point is what will make the next hop go out of the routing table? Usually that is the outbound interface going into protocol down state. The difficulty is that Ethernet type interfaces generally do not go protocol down even though you may have lost connectivity through the interface. So Cisco has introduced a new capability to track availability of the next hop. This link should give you the information about how to use this:

http://www.cisco.com/en/US/products/ps6350/products_command_reference_chapter09186a0080447549.html#wp1125547

HTH

Rick

anandramapathy Thu, 08/09/2007 - 09:31

Thanks a lot for that Rick.

My IOS does not support the track feature & my ISP has not enable CDP so that i can use the verify-availability command,

The route is an old 7200 with 24 MB flash & 128 MB RAM.

Upgrading the IOS is also an issue.

Do you know of any option within BGP which can accomplish this ?

send outgoing traffic 192.168.0.1 /24 via ISP A

send outgoing traffic for 192.168.0.2 /24 via ISP B

Richard Burts Thu, 08/09/2007 - 09:47

Anand

I do not believe that anything in BGP is able to arrange outbound traffic in the way that you want.

What kind of interfaces connect your router to the ISPs?

HTH

Rick

Richard Burts Thu, 08/09/2007 - 11:29

Anand

I do not see any easy solution in this. If you can not do the check availability or tracking in the current IOS version and if you can not upgrade the IOS version, then configuring HSRP will open the possibility that if one of the provider interfaces could no longer communicate with the provider then HSRP would continue to send traffic into a black hole until someone recognizes the problem and manually changes the HSRP.

It is a decision that someone needs to make whether the benefit of being able to send traffic out both interfaces outweighs the potential problem of dropping traffic if you lose connectivity to one of the providers.

HTH

Rick

anandramapathy Fri, 08/10/2007 - 06:19

Thank you Rick. i have uploaded to a version which supports PBR with Route tracking.

Let me try this out.

Thanks a lot again for your help.

Correct Answer
Richard Burts Fri, 08/10/2007 - 07:27

Anand

If you can do PBR with route tracking then it should resolve the potential problem with losing connectivity through one of the providers.

Good luck with getting this going. Let us know how it works out.

HTH

Rick

Richard Burts Fri, 08/10/2007 - 10:12

Anand

I am glad that you got it working. And glad that my advice was helpful in this.

Thank you for using the rating system to indicate that your issue was resolved (and thanks for the rating). It makes the forum more useful when people can read about an issue and can know that they will read the resolution of the issue. I encourage you to continue your participation in the forum.

HTH

Rick

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