OSPF

Ned5150

Hey guys,

I have a quick question about OSPF convergence. I have a 6 router in an OSPF area setup in a a physical ring (A connected to B, B to C, C to D, D to E, E to F, F to A). If a T1 goes down between router C and D does the reconvergence process stop routing traffic on all routers until it is complete? (ie. Does traffic between router A and B stop even though the routing statements between the two routers don't change.)

I've read about the convergence process but I'm getting conflicting reports for routing during the convergence process. Thanks for the help guys!

Pass my CCNA on Monday with a 923. This board was a big help. Thanks!

Ned
Fresh CCNA

Danman32

Non-authorative answer: I believe it will briefly stop. Recall though, that OSPF is a link-state routing protocol and not a vector routing protocol. Routers advertise the states of the directly attached links to all other routers in the area via multicast. The other routers use this information to figure out what the network looks like. Therefore, since the routers are doing parallel processing, the convergence delay will be very short.

Sometimes I forget that OSPF is a link state protocol and try to treat it as vector. Vector protocol cascades network tables. Link state protocol sends link states to everyone immediately.

dmaftei

No. Moving traffic between interfaces is done by an independent process that has nothing to do with OSPF's convergence (or with the OSPF process for that matter).

Danman32

If the packet was originating from one of the two routers on either end of the link (source address is router A, destination address is router , then I would agree with you as there is no routing involved here.

But the routing table is built by OSPF. Since the routing table is being converged, the router may suspend determining if it should send packet between the two interfaces. It may suspend routing.

dmaftei

OSPF does not build THE routing table, it builds a OSPF routing table (similarly RIP builds a RIP routing table, IGRP builds a IGRP routing table, etc.) A separate process combines the protocol specific tables into the routing table. It is this "master" routing table that's used for the actual routing. The actual routing is acomplished by yet another process, which uses the routing table.

If a change happens in the OSPF network, the OSPF process on the router will first update its link state database, then if necessary the OSPF routing table. Then the process repsonsible for combining the protocol specific routing tables determines whether or not the routing table must be updated based on the change in the OSPF routing table; if yes, it updates it. During this time the routing process is not suspended, it continues to use the old routing table.

lwwarner

dmaftei wrote:

During this time the routing process is not suspended, it continues to use the old routing table.

It might be clearer to say that the packet switching process is not suspended. Indeed, the packet switching process doesn't give a hoot about OSPF or any other routing protocol. All it does is perform a longest prefix match against the data that is in the RIB (the routing information base, aka the routing table) now, at the instant a new packet arrives.[*] It has no way of knowing, and does not care, about the OSPF convergence status.

Bottom line, dmaftei is right, the answer to the original question is no.

[*]This assumes that process switching is in use. If a different form of packet switching is in use the nature of the lookup will be different, but the general idea is the same. For more about packet switching see: How to Choose the Best Router Switching Path for Your Network

Congratulations, Ned!

Yankee

Y'all including my old buddy dmaftei confused me a bit. If the link goes down and it was the route in use it will be removed from the routing table and that destination will be briefly unreachable as the new route is inserted. This is assuming there were not two equal routes already in the table.

Yankee

lwwarner

But that's not the original question...

dmaftei

Yankee wrote:

Y'all including my old buddy dmaftei confused me a bit. If the link goes down and it was the route in use it will be removed from the routing table and that destination will be briefly unreachable as the new route is inserted. This is assuming there were not two equal routes already in the table.

Yankee

Of course some traffic could go down the drain, but that's exactly because routing doesn't stop during convergence, isn't it?

Good to see you're still around!

Yankee

Me bad for not reading his question properly, but it was a good excuse to say hello to someone that has been posting with me on different sites for well over 5 years and that should include Darth too, right Dmaftei! He surfaces here from time to time too.

Yankee

Ned5150

Your replies have helped to clarify the issue. Thanks again guys. I really appreciate the assistance.

Ned