Aggregating stub nets on cat3750 running OSPF

[azlatkin]

I have a router (cat3750) in an OSPF normal area that is not an ABR/ASBR.

This router has multiple stub networks directly connected to it,

all having IPs 10.10.x.0/24. The router will source one LSA 1 for

each stub /24 network connected. However, with the IP numbering

scheme used in the OSPF domain it is desirable that this router

would be able to source an aggregate 10.10.0.0 /16 into the area.

The problem is that I do not want to convert this router into ASBR

(or nssa-asbr) and source the summary as LSA 5 (or 7).

I wonder if there might be a way to trick the router into sending

an aggregated LSA 1 for all of its directly connected stub networks.

Any suggestions would be welcomed.

[Harold Ritter]

Unfortunately, it is not possible to do that unless you generate a type 5 LSA from that router.

The way to generate a summary stub network within the type 1 LSA would be to assign the aggreagte address to a loopback interface for instance and to have this address define under the ospf process. But IOS will not allow you do have overlapping addresses.

So the only way I can think of is to create a summary static route (ip route 10.10.0.0 255.255.0.0 null0) and redistribute it in ospf.

Hope this helps,

[azlatkin]

Thank you Harold.

The interesting question which arises is: Why cisco IOS does not permit overlapping addressing on Loopback?

Within the routing paradigm "More specific route first" I would not see any problem with doing that.

E.g., 10.10.10.0 /24 connected

10.10.11.0 /24 connected

10.10.0.1 /16 Loopback

all packets arriving on that router with destinations 10.10.10.x, 10.10.11.x will be sent

to connected nets; all other within 10.10.0.0/16 --

to Loopback (which will act as a drop bucket).

alex

[Harold Ritter]

Alex,

I totally agree with you that conceptually this would not hurt. This flexibility has probably never been requested. Would probably be useful in this kind of scenario. You could still do the aggregation using a type 5 LSA.

Hope this helps,