On Wed, Jul 02, 2008 at 08:57:14AM -0400, Mike Johnson wrote:
> I have read docs detailing a central route reflector and a RR for each POP.
> [...]
> I assume an IGP is used somehow, however, its the internal IGP design I am
> not grasping.
Mike,
A common, but by no means the only, strategy is as follows:
1. All routers participate in a single, flat IGP. The only routes carried
in the IGP are loopbacks and links between routers. All other routes are
carried in BGP. This keeps things simple and promotes fast convergence.
2. All core routers participate in a full IBGP mesh.
3. All lower-level routers in a "region" are client peers of the cores
that serve that region (where 'region' could mean POP, city, country,
etc., depending on your network).
4. All routes advertised via BGP have their next-hop reset where they
enter the network. Typically this is on the edge routers, which are
client peers of the local core routers, but can be done anywhere. The
end result is that no matter where on the network you stand, every BGP
route has a next-hop address that corresponds to a router loopback that
you know how to reach via your IGP.
Variations on this model might include:
a. Multi-{level,area} IGP, if your network is big enough to warrant it.
b. Fewer reflectors, from "regional" all the way to "central".
c. In very large or complex networks, you might see tiered reflection or
confederations, but those should be fairly rare.
--Jeff
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