MAXAGE of OSPF LSA

[henrybb]

Hi,

I change OSPF router-id of the router and clear ip ospf process. And I sh ip ospf database,router link and network link will change from old router-id to the new router-id. But Summary Net Link will not change router-id. Summary Net Link of Old router-id and new router-id will co-exist in the database. Summary Net Link of old router-id will disappear when it reach MaxAge.

I can't understand this. Why does Summary Net Link of old router-id wait time for MaxAge ? Why not it disappear right now after I change the router-id ?

12.1.1.2 is new router-id and 1.1.1.2 is old router-id.

#sh ip ospf data

OSPF Router with ID (12.1.1.2) (Process ID 1)

Router Link States (Area 0)

Link ID ADV Router Age

3.3.3.3 3.3.3.3 281

12.1.1.2 12.1.1.2 1364

Net Link States (Area 0)

Link ID ADV Router Age

2.2.2.3 3.3.3.3 974

Summary Net Link States (Area 0)

Link ID ADV Router Age

1.1.1.1 12.1.1.2 1364

1.1.1.1 1.1.1.2 2381

1.1.2.0 12.1.1.2 1364

1.1.2.0 1.1.1.2 2381

thanks!

[ruwhite]

OSPF works this way for two reasons:

-- A router should never flush information it didn't originate until that information reaches maxage, since no other router than the originating router really knows if the information is still valid.

-- A router that is disconnected from the network and reconnected will find that part of it's information is still in the network when it reconnects, reducing the amount of time it takes to resynchronixe its database with the surrounding routers.

In this case, you're changing the router id, which is, in effect, the same as removing the router from the network, and reconnecting it as a different router. The old router is no longer around to flush all of it's old information, and no other router connected to the network can flush it until it max ages.

HTH

:-)

Russ.W

[henrybb]

Russ.W,

thanks for your reply.

But LSA 1(router link) of the router which router-id was changed was flushed right now . I can't understand what's difference between LSA 1 and LSA 3. Why LSA 3 must wait for MaxAge and LSA 1 do not.

[ruwhite]

Ah, I missed that part of the question--sorry. I assume, from the way the question looks, that the type 3 is being generated by the router you're changing the router id on (?). If so, then the same thing applies as the original answer to the type 1 generated by the router. We flush the type 1 in some situations when you change the router id, but we don't flush the type 3, so it will "hang out" until it times out (max ages). Again, the router that originated the type 3 no longer exists in the network to flush it.

Generally, these old LSAs don't do anything other than take up space in the router's databases--they don't fit into the SPF tree, since there's no connectivity to them.

:-)

Russ.W

[henrybb]

Russ.W

Sorry for I didn't express exactly.

LSA 1, LSA 3 is generated by the router which I change the router-id. And the router is ABR.

You said "We flush the type 1 in some situations when you change the router id, but we don't flush the type 3" . Could you please to explain why did that ? Why not flush type 3 LSA,too ?

thanks!

[henrybb]

If there is two LSA 3 for the same network. One is old router-id,another is new router-id. Is it possible that it make routing loop ? How did OSPF know which is useable LSA 3 ?

thanks!

[ruwhite]

Because the old router id won't be connected to anything when all the other routers run SPF, there's no chance of a routing loop forming.

:-)

Russ.W

[henrybb]

Russ.W,

thanks for your reply.

I still can't understand Why not flush type 3 LSA ? Is there any advantage to do that ?

[amit-singh]

Henry,

LSA is the router LSA which is being sent by each and every router in its particular area. LSA 3 is originated by ABR )area boundry router) so only ABR can flush this LSA not any other router as told by Russ in previous post.

So if you change the router ID on any router in a particular area it will flush that as it generates that LSA. but LSA 3 will only be flushed by ABR.

HTH,

-amit singh

[henrybb]

amit singh,

The router which I change router-id is ABR router. So I can't understand why it does not flush the LSA 3 after I change router-id.And it does flush LSA 1 and LSA 2.