Information Technology Reference
In-Depth Information
Figure 2-2
Prefix Propagation in a Multi-AS Topology
AS
65000
Prefix
Propagation
R1
R2
R4
R3
AS
65001
R5
R6
AS
65002
Continuing with Figure 2-2, assume that R3 needs to propagate the prefix to R7 in AS 65002.
There are a couple of options to achieve that.
One option is to have R3 redistribute all the BGP prefixes into the IGP, which advertises
them to R4, R5, and R6. Next, have R5 and R6 redistribute these prefixes back into BGP,
and advertise them to their respective eBGP neighbors, R7 and R8. There are a few issues
with this strategy.
IGPs were not designed to handle the number of routes that would be involved. The full
Internet table has more than 100,000 prefixes. The periodic refresh of prefix information
that many IGPs require could further result in network instability, additional system resource
consumption, and significant bandwidth requirements on a regular basis for routing updates.
The increased number of prefixes results in a greater probability of route flapping, which
can lead to significant stability and convergence issues.
