Route redistribution can be tricky
business, with several pitfalls:
- Routing loops - Depending
on how you use redistribution, routers can send routing
information received from one AS back into the AS. The feedback
is similar to the split-horizon problem that occurs in
distance-vector technologies.
- Incompatible routing
information - Each routing protocol uses different metrics.
Because these metrics cannot be translated exactly into a
different protocol, path selection using the redistributed route
information may not be optimal.
- Inconsistent convergence time
- Different routing protocols converge at different rates. For
example, RIP converges slower than EIGRP, so if a link goes
down, the EIGRP network will learn about it before the RIP
network.
These potential trouble spots can be
avoided with careful planning and implementation. Be sure to follow
these important guidelines when configuring route redistribution:
- Be familiar with your network.
There are many ways to implement redistribution, so knowing your
network will enable you to make the best decision.
- Do not overlap routing protocols.
Do not run two different protocols in the same internetwork.
Instead, have distinct boundaries between networks that use
different routing protocols.
- Use one-way redistribution with
multiple boundary routers. If more than one router serves as a
redistribution point, use one-way redistribution to avoid
routing loops and convergence problems. Consider using default
routes in the domains that do not import external routes.
- Use two-way redistribution with a
single boundary router. Two-way redistribution works smoothly
when redistribution is configured on a single boundary router in
the internetwork. If you have multiple redistribution points, do
not use two-way redistribution unless you enable a mechanism to
reduce the chances of routing loops. A combination of default
routes, route filters, and distance modifications can be used to
combat routing loops.
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