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Route redistribution can be complicated
and have several disadvantages as shown in the following:
- Routing loops – Depending on how redistribution is used,
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. Use the following important guidelines when
configuring route redistribution:
- Be familiar with the network – There are many ways to
implement redistribution and being familiar with the network helps to
make the best decisions.
- 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.
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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 there
are multiple redistribution points, do not use two-way redistribution
unless a mechanism to reduce the chances of routing loops is enabled.
A combination of default routes, route filters, and distance
modifications can be used to combat routing loops.
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Lab Activity
e-Lab Activity: Modify Administrative Distance by Using the
Distance Command
This lab is to configure the RouterA to
apply a custom administrative distance to all OSPF routes.
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