Case Study: Conditional Advertisement
It's often useful to conditionally advertise some routes to upstream neighbors—particularly if you are trying to control which link is crossed by traffic destined to a particular network. (Refer to "Case Study: Dual-Homed Connections to the Internet" for an example.)
BGP has the capability to conditionally advertise routes; look at Figure 8-8 and work through the example that follows.
Figure 8-8. Conditional Advertisement
In this case, you want to advertise 172.28.23.0/24 to Router B as long as that link is up, but if it fails, you want to advertise this route to Router A from Router C.
Here, you would ...
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