Simple BGP prepending

So, Having a baby seems to suck up a lot of time... Anyway,

Getting back to my CCNP studies, lets look at some simple BGP path selection: prepending.

First lets glance at BGP route selection process.
1. Highest weight (local/cisco proprietary)
2. Highest Local Preference (propagated in IBGP, stripped from EBGP)
3. Originated by Local Router
4. Prefer shortest AS path
5. Lowest Origin Code
6. Prefer lowest MED
7. EBGP over IBGP
8. Closest IGP neighbor
9. Oldest EBGP
10. Lowest BGP neighbor ID
11. Lowest Neighbor IP address


So, down at step four we are looking at how to manipulate traffic coming into our AS.

Patch selection is by AS hops, so if we have two paths into our AS we are going to make it appear to use extra AS hops through one of the paths.

We can use a trusty route map to prepend these extra AS'.

We'll use BGP AS 65000 as us, 65010 (10.1.1.1) as the desired path, and 65020 (10.2.2.2) as the less desirable path.

#router bgp 65000
#neighbor 10.2.2.2 route-map PrependASMapSample out
#
#route-map PrependASMapSample permit 10
#set as-path prepend 65000 65000 65000

Now, lets first notice, you need to prepend your own AS number, don't use someone else's. I have used the neighbor command on the LESS DESIRABLE AS and attached a route-map that adds three extra AS hops. Now when this adjacent AS 65020 wants to send you traffic, you appear as 4 hops instead of just 1. If the path through 65010 is only three hops away, you just directed traffic through the more desirable ISP.