The Journey of a Network Engineer

May 18 2012   4:24AM GMT

Different methods to advertise routes into OSPF



Posted by: Sulaiman Syed
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CCIE
Cisco
redistribute
route
router

Advertising routes into protocols is one of the interesting topics. Most of big enterprise networks will have different routing domains. Thus dealing with these routes become an essential topic in CCIE exams.

Although the topic stats into OSPF, but these methods can generally be applied into any routing protocols.

Network x.x.x.x

This is the basic way of advertising routes. This command will enable the routing protocol on that interface (sending hellos, etc) and advertising to other peers of the interfaces that match the network command.

router ospf 1
network 10.0.0.1 0.0.0.0 area 0

Redistribute connected

This is the first way of advertising routes into OSPF. When done in this way, the routes would appear external in the OSPF domain (or eigrp). The interfaces that would be advertise can be controlled with route-maps. Lets see an example or redistributing a loopback 0 into OSPF.

route-map CON>OSPF permit 10
match interface loopback 0
!
router ospf 1
redistribute connected route-map CON>OSPF
!

area x range

Another method to advertise routes is using the range command. This will cause the summary to be advertised into the an area, this summary will hide the more specific routes that were used to originate the range command. For example, the loopback interface will be used with network command to advertise the route, then with the range command the summary will hide the interface subnet mask and advertise the summary address.

int loopback 0
ip add 10.0.1.1 255.255.255.0
!
router ospf 1
network 10.0.1.1 0.0.0.0 area 1
area 1 range 10.0.1.0 255.255.255.0

Redistribute [protocol]

This is another way of advertising the route into OSPF. The redistribute connected and static are just special cases of this method. A route can be originated in one routing domain, then redistributed into OSPF. this will cause the route to appear as external route. in this example, RIP is being redistributed into OSPF.

router ospf 1
redistribute rip subnets

These methods and techniques cover the majority of route advertising techniques that can be used with OSPF and any other routing protocol.

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