OSPF link update-Basic Query

[avilt]

I am new to OSPF. I have 2 basic querys on OSPF link update.

1. How does one router communicates with every router in the OSPF area. Is it that each router communicates only with its directly connected neighbours? example.

ROUTERA-------ROUTERB-----ROUTERC-----ROUTERD----ROUTERE

Lets say link on RouterB connected to ROUTERA goes down. How does "ROUTERE" get to know about this? Is the communication is from RouterB to ROUTERC then to ROUTERD and then to ROUTERE?

2. How does router handle(forward) multicast packets?

Thanks

[globalnettech]

Hello Avil,

the way OSPF communicates depends on how your network is set up. In your case, it looks like all routers are connected back-to-back with point-to-point links. OSPF uses the concept of adjacencies: an OSPF router has an adjacency with a connected router, which means that they have the exact same view of the entire network. If you do a 'show ip ospf neighbor' on your RouterA and your RouterE, it tells you the adjacencies these routers have with other connected routers. Now let's say the link between RouterA and RouterB goes down. RouterC notifies RouterD, and RouterD notifies RouterE immediately by exchanging link-state packets.

Keep in mind that on a multiaccess segment (where multiple OSPF routers are directly connected on the same segment), the concept of DR (Designated Router) and BDR (Backup Designated Router) comes into play: on a multiaccess segment, a DR and a BDR are elected, and all other routers have full adjacencies only with the DR and the BDR.

Regarding the multicast traffic: it is handled just as unicast traffic (unless you specifically block it).

Does that make sense ? If you are just starting with OSPF, have a look at the link below, which contains a pretty good introduction of the basic concepts.

OSPF Design Guide

http://www.cisco.com/warp/public/104/2.html#4.0

HTH,

GNT

[avilt]

Thank you very much.

That means in order to run OSPF between ROUTERA and ROUTERE, all the intermediate routers must run OSPF right?

What if there is a firewall in between ROUTERA and ROUTERE?

ROUTERA-------ROUTERB-----ROUTERC-----ROUTERD----ROUTERE

[sw1]

Hi Avil,

Yes you would have to run OSPF between ROUTERA through to ROUTERE.

If a firewall is between them then you would need to allow the OSPF protocol through the firewall.

Question for GlobalNettech -

In the setup as provided by Avil - ROUTERA----ROUTERB--ROUTERC-ROUTERD, if the link between ROUTERA & ROUTERB went down wouldn't ROUTERB send out an LSA to ROUTERC then from ROUTERC to ROUTERD etc??

Regards

Steve

[avilt]

Hi Steve,

Is it that I have to allow OSPF thru the firewall or run the OSPF on firewall itself? If we do not run OSPF on firewall, it will not be contiguous. Correct me if I am wrong.

Thanks

[yzhu]

the router Multicast packets TTL always=1 , it never forwarded from A to E.

[avilt]

Hi Steve,

Is it that I have to allow OSPF thru the firewall or run the OSPF on firewall itself? If we do not run OSPF on firewall, it will not be contiguous. Correct me if I am wrong.

Thanks