Real World scenario?

dtakhar

what would happen if there was a new lsa and ospf topology changed during a massive data transfer. would the data be interrupted? since lsa happen every 30 mins, how would the convergence effect normal data stream. would the path change? it would have to if a router went down?

networker050184

No, OSPF SPF recalculations on their own are not enough to cause traffic to drop. You need to keep in mind (or learn if you haven't yet) the separation of data and forwarding planes.

james43026

Hmm, I'm curious about this one. If OSPF had to perform a SPF recalculation then the forwarding plane would be affected as well, as CEF would have to be updated as soon as a change in the routing table occurs. Which could cause enough delay to drop packets.

Does this mean that the entire FIB and adjacency table doesn't need to be cleared when an SPF recalculation takes place, and that only entries pertaining to the update are affected?

networker050184

Once the routing table (data plane) is updated the decision is sent to the forwarding table (forwarding plane) which updates the destination prefix. It's not like the forwarding table sits in a state with no route in the interim so packets continue to be forwarded regardless. Of course delays, high CPU etc. can come in to play here. Tables being wedged, any number of things that could result in the forwarding table unable to forward packets to a certain destination. Or the destination is no longer there but the FIB has not yet been updated. Plenty of reasons why a packet could be dropped obviously, but a route update alone isn't one of them. Modern hardware can reroute without drop in most instances.

Yes, the forwarding table is only updated if need be.

Dieg0M

If a router went down in the case of a failure and there is a new LSA and the OSPF topology changed then yes there would interruption in the data transfer. The interruption would be the duration of the convergence time of OSPF (Detection of the failure, propagation of the new LSA, time to perform the SPF calculation and the time to update the forwarding table. That is why most data transfers are done through TCP for re transmissions purposes in case of packet loss.

In the case of the 30 minute LSA refresh timer of OSPF, it will not cause any packet loss as the route to the destination will never be removed from the data-plane and the destination will still be reachable.

*Edit: I did not include the time to update the routing table from the forwarding table since this is done at the same time.