OSPF Summarization - Null Route

In OSPF, the discard route is created automatically whenever a summary route is configured, there are two types of summary routes: Internal and External. When internal summary routes are configured, OSPF will inject an internal discard route, and when an external summary route is configured, the OSPF process will create an external discard route. The discard routes are created to stop forwarding loops.

R2(config)#router ospf 1
R2(configrouter)#no discard-route internal
The discard route that we are discarding is the result of summarizing the internal routes, therefore we need to specify internal.

R4(config)#router ospf 1
R4(configrouter)#no discard-route external
In the above command we are discarding the external discard-route that was created as a result of summarizing the external routes.

OSPF Summarization

In OSPF, summarization can be configured on two types of routers: ABR/s and/or ASBRs. The internal OSPF routes can only be summarized on ABRs whereas the external (redistributed) routes can only be summarized on ASBRs. When summarizing internal routes on ABRs the “area xx range” command must be used, where xx is the area id. Summarization on ASBR can be accomplished by using the “summary address” command.

Advertise OSPF Loopback with Correct Mask

If a task is asking to ensure that the loopback interfaces are advertised with their correct mask, one way to accomplish this is to change their network type to point-to-point:

int l0
ip ospf network point-to-point

EIGRP Variance

The advertised distance must be lower than the local router’s feasible distance. The multiplier (Variance command’s value) times the local best metric for the destination must be greater than or equal to the metric through the next router. The path must be in the routing table for the router to use it.

Leak Map

The leakmap was introduced in IOS 12.3(14)T and it must be configured under the physical interface and NOT a subinterface. Configuring leakmap option allows us to advertise a component route (One or more specific networks of a summary route) that would otherwise be suppressed by the manual summarization.

There are three rules to remember:

• If the leakmap is configured to reference a routemap that does not exist, only the summary route is advertised and the more specific routes are suppressed.
• If the leakmap is configured to reference a routemap and the routemap is referencing an accesslist that does NOT exist, then the summary route plus all the specific routes are advertised.
• If the leakmap is configured to reference a routemap and the routemap matches on an accesslist, all the permitted networks by the accesslist will be advertised along with the summary route.

R2(config)#int F0/0
R2(configif)#ip summary-address eigrp 100 2.2.4.0 255.255.252.0 leakmap TEST21
R2(config)#routemap TEST21 permit 10
R2(configroutemap)#match ip addr 99
R2(configroutemap)#exit