How NOT to advertise a network on enabled interface (OSPF or EIGRP)

[Ivan Mamka]

Hello,

How NOT to advertise network on interface where EIGRP or OSPF is enabled? I was reading one forum that in OSPF it's to be done through distribute lists (didn't find full answer) and in EIGRP it's possible.

Example: In picture attached, let's say all routers are in same area (ospf/eigrp) and I would like R10 not to know anything about

network 100.1.1.0/30

distribute-list route-map????

Thanks!

[paul driver · Accepted solution]

Hello

Try the following:

R10
ip prefix-list ACL deny 100.1.1.0/30
ip prefix-list ACL permit 0.0.0.0/0 le 32

OSPF

router ospf X
distribute-list prefix ACL in

EIGRP
router eigrp xxx
distribute-list prefix ACL in <interface>

[Giuseppe Larosa]

Hello @Ivan Mamka ,

Paul Driver has provided examples of configuration.

Here I just want to add that in OSPF the distribute-list  affects only the routes installed in the IP routing table of the local router but it is not able to alter / modify the OSPF Router LSA. The OSPF Router LSA is propagated unchanged to all routers in the same area (LSA flooding).

As a results of this the filtered prefix is actually advertised to all routers in same OSPF area and to avoid to create a black-hole the same distribute-list has to be applied to all routers in the same AREA.

Area Border Routers can then be configured to avoid propagation of that prefix to other areas starting from backbone area 0.

In the case of EIGRP the distribute-list is effective and allows to make the change in a single node. Because EIGRP has distance vector roots.

Hope to help

Giuseppe

[Ivan Mamka]

Thanks Paul, that did the job!

Thanks Giuseppe, that makes more clear.

Have one side question - command in OSPF with OUT option, can it be used in anything? I mean if it can't filter outbound LSA/routes, then what is it for?

(example distribute-list prefix acl1 out)

Thanks in advance!

[Giuseppe Larosa]

Hello @Ivan Mamka ,

>> command in OSPF with OUT option, can it be used in anything?

either the command is not accepted either is not effective at all given the link state nature of OSPF

Hope to help

Giuseppe