|
Redistribution, although
powerful, increases the complexity and potential for routing
confusion, so it should be used only when absolutely necessary. The
key issues that arise when using redistribution follow:
--- Depending
on how you employ redistribution, routers can send routing
information received from one AS back into the AS, as shown in the
main figure. The feedback is similar to the split-horizon problem
that occurs in distance vector technologies.
In the main figure,
information about network 172.16 crosses the AS boundary router (ASBR)
A to the EIGRP net. Because this internetwork has a loop, the same
information propagates back to the RIP network through ASBR B.
- Incompatible
routing information
--- Because
each routing protocol uses different metrics to determine the best
path (for example, RIP uses hops and OSPF uses cost), path
selection using the redistributed route information may not be
optimal. Because the metric information about a route cannot be
translated exactly into a different protocol, the path a router
chooses may not be the best.
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.
To understand why
some of these problems occur, you must understand how the Cisco
routers convert the metrics used when importing routes from one AS
into another; this conversion is discussed later.
Note: Redistribution
is the capability for boundary routers connecting different ASs to
exchange and advertise routing information received from one AS to
the other AS.
The next sections
discuss how a router can select a path when it learns routes to a
given destination from different routing protocols. Understanding
how path selection occurs in this environment is important to
prevent and troubleshoot routing feedback and poor path selection.
|