BGP Route Preference

[jeremyarcher]

We are working with a ISP to provide two different Internet connections between two sites for redunancy.

The provider has set up private BGP on both sides and we have two different /28 networks associated with these circuits - one per side.

Our preference is that by default, one of these /28 networks routes to one Internet connection and the other network routes, by default, to the other Internet connection.  However, both sides need to be able to advertise both networks in the even of a failure on one of the connections.

We are able to accomplish this but I'm curious how I set the preference for each side and which network is treated as the "prefered path" for each subnet.

Since one connection is quite a bit faster than the other I'm concerned that this will cause all traffic for both subnets to be routed to this faster conneciton.

So, my question is, Is there a way to set a route preference, with the BGP configuration, for each subnet (/28) that would prefer one sub-optimal path over another while still seeing both optional routes within the BGP route table?

Thanks in advance,

Jeremy

[Jon Marshall]

Jeremy

Yes, you can use AS PATH prepending to achieve this. Part of the BGP best path selection process is to look at the number of AS's in the AS PATH attribute (the shorter the better). So for the route you don't want to use you simply add multiple copies of your own AS PATH. If the preferred route is then not available this route would used instead.

See this link for more details and config example -

http://blog.ioshints.info/2008/02/bgp-essentials-as-path-prepending.html

Jon

[jeremyarcher]

Thank you!  I'll try it this afternoon.

[jeremyarcher]

Jon,

Is there any advantage to using BGP Communities that are mentioned in that article?

[lgijssel]

Hi Jeremy,

The link below will provide a lot of practical info regarding your question:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml

regards,

Leo

[Mohamed Sobair]

Jeremy,

You need to have both /28 subnets each advertised towards one specific link.

and you would also need to advertise the whole block /27 on each link. This way your Internet cloud would always prefer the longest Match subnet /28 advertised by each link , on the same time, both would be redundant and advertised out another link if one fails. (For Incoming direction). this requires running IBGP between your edge routers as well.

Let us know if this answers your question or if your require additional assistance.

Regards,

Mohamed