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You can configure the boundary router
for two-way redistribution, as shown in Figures
- .
Notice that the syntax of the
metric
keyword varies depending on the
routing protocol that it uses. For RIP, OSPF, and BGP, the
metric
option is followed by a single
number that represents the metric value (hop count, cost, and so
on). For IGRP and EIGRP, the
metric
option is followed by five values
that represent bandwidth, delay, reliability, load, and MTU.
Note: Whenever there is a
major network that is subnetted, you need to use the keyword
subnets
to redistribute protocols into
OSPF. Without this keyword, OSPF redistributes only major nets that
are not subnetted. For example, to inject EIGRP routes, including
subnets, into an OSPF area, use the command
redistribute eigrp 24 metric 100
subnets.
In Figures
- , RIP
is configured to import EIGRP routes and distribute them into the
RIP domain with a seed metric of 2 (hops).
Mutual redistribution will result in
RTC installing 11 routes in its table (click on the topology Figure
to view RTC's routing table).
Unlike EIGRP, RIP does not
differentiate between external and internal routes. Also, note that
RTB's seed metric has resulted in a metric of two hops for all the
redistributed routes, even though two of these networks are actually
three hops away.
After configuring two-way
redistribution, RTC and RTA have only 11 routes, while the boundary
router (RTB) has 12. What is going on here? The answer lies in RTB's
directly connected routes:
172.16.0.0/16 (missing from RTA's
table)
172.24.0.0/16 (missing from RTC's table)
Recall that the network
command identifies not only which interfaces to run the routing
protocol on, but also which directly connected networks will be
included in routing updates. Look carefully at Figures
- . RTB's
RIP process is configured to advertise the connected network
172.16.0.0, while its EIGRP process is configured to advertise the
connected network 172.24.0.0.
To bring RTA and RTC's routing tables
up to a complete 12 routes, we can configure both of RTB's routing
processes to include the two connected networks using the
network
command. However, that will result in a RIP process running in the EIGRP
AS and an EIGRP process running in the RIP domain. This solution
will generate needless overhead. Redistribution offers a much more
efficient and elegant solution. You can configure RTB to
redistribute its connected routes using a default metric, as
discussed in the following sections.
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