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Most routing protocols have
metric structures and algorithms that are not compatible with other
protocols. In a network where multiple routing protocols are
present, the exchange of route information and the capability to
select the best path across the multiple protocols is critical.
In order for routers to select the best path when they learn two
or more different routes to the same destination from two different
routing protocols, Cisco uses administrative distance, which
defines the believability of a routing protocol. Each routing
protocol is prioritized in order of most to least (believable)
reliable using a value called administrative distance.
This criterion is the first a router uses to determine which
routing protocol to believe if two protocols provide route
information for the same destination. The more believable protocol
is selected, even when it advertises a suboptimal route.
What Protocol to Believe?
Figure
lists the believability (administrative distance)
of the protocols that Cisco supports. Note that the smaller the
administrative distance, the more reliable the protocol. For
example, if the router received a route to network 10.2.2.0 from
IGRP and then received a route to the same network from OSPF, IGRP
is more believable, so the IGRP version of the route would be added
to the routing table.
When
using route redistribution, occasionally there may be a need to
modify the administrative distance of a protocol so that it has
preference. If you want the router to select RIP-learned routers
rather than IGRP-learned routes to the same destination, for
example, then you must increase the administrative distance for IGRP.
After
an ASBR selects the routing protocol to which to listen, it must be
able to translate the metric of the received route from the source
routing protocol into the other routing protocol. If an ASBR
receives a RIP route, for example, it will have a hop count as a
metric. To redistribute it into OSPF, however, the hop count must be
translated into a cost value. The cost value you define during
configuration is referred to as the seed or default metric.
After
the seed metric is established, the metric will increment normally
within the AS. The exceptions are OSPF E2 routes, as discussed
previously, which hold their default metric regardless of how far
they are propagated across an AS.
You
should take some precautions when a loop exists between two ASs, as
shown in Figure .
Consider setting the default metric on the incoming
redistributed route to something higher than what currently exists
in the receiving protocol. This way, you get some degree of
automatic protection from route loops.
Suppose
a route loop exists where a routing protocol is getting preferred
routes that it originally sourced. If those routes are in at a low
metric, they can be preferred over the original route.
So,
imagine a router Z in a RIP domain knows that network A, a route
native to the RIP domain of which you are a member, is currently at
a cost of three hops from my point of view. Now suppose that the
route to A has been redistributed to some foreign protocol and is
now coming back into the RIP domain from the outside and router Z is
only one hop away from the point of redistribution.
If
the redistributing router sets the metric for this new route to
network A to one, the router Z cost to A is now only two hops, and
it now points toward the redistribution router. If the
redistribution router sets the metric to four hops, for example,
then all will be well, even though technically a route loop is
occurring. Setting the incoming default metric low may create a
black hole for routes.
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