Information Technology Reference
In-Depth Information
A company might also acquire another company that might be running another routing
protocol. Figure 11-20 shows a network that has both OSPF and EIGRP routing protocols.
Routers A and B perform redistribution between OSPF and EIGRP. Both routers must fil-
ter routes from OSPF before redistributing them into EIGRP and filter routes from EIGRP
before redistributing them into OSPF. This setup prevents route feedback.
Redistribution of Routes
Router A
OSPF 100
EIGRP 150
Router B
Figure 11-20
Redistribution Between IGPs
Route feedback occurs when a routing protocol learns routes from another routing proto-
col and then announces the routes to the other routing protocol. In Figure 11-20, OSPF
should not advertise the routes it learned from EIGRP on Router A back to EIGRP on
Router B. And EIGRP should not announce the routes it learned from OSPF on Router B
back to EIGRP on Router A.
Yo u c a n u s e a c c e s s l i s t s , d i s t r i b u t i o n l i s t s , a n d r o u t e m a p s w h e n r e d i s t r i b u t i n g r o u t e s . Yo u
can use these methods to specify (select) routes for redistribution, to set metrics, or to set
other policies for the routes. They are used to prevent loops in the redistribution. They are
also used to control routes' redistribution direction. Redistribution can be accomplished
by two methods:
Two-way redistribution
■
One-way redistribution
■
In two-way redistribution, routing information is exchanged between both routing proto-
cols. No static routes are used in this exchange. Route filters are used to prevent routing
loops. Routing loops can be caused by one route protocol redistributing routes that were
learned from a second route protocol back to that second routing protocol.
One-way redistribution only allows redistribution from one routing protocol to another.
Normally, it is used in conjunction with a default or static route at the edge of a network.
Figure 11-21 shows an example of one-way redistribution. The routing information from
the WAN routes is redistributed into the campus, but campus routes are not redistributed
out to the WAN. The WAN routers use a default gateway to get back to the campus.
Other locations for one-way redistribution are from building access networks, BGP routes
or static routes into the IGP, and from VPN static routes into the IGP.
Default Metric
Yo u s h o u l d c o n f i g u r e t h e m e t r i c o f t h e r e d i s t r i b u t e d r o u t e s t o a m e t r i c o t h e r t h a n 0. Yo u c a n
configure the metric in the
redistribution
can also use the command in OSPF. IS-IS does not use the
default-metric
command. The
