This topic is to discuss the following lesson:
Hi Rene,
Good Day 
I want to know about convergence after topology change . My question is “After Topology change how a router know a prefix have to be remove from its database” ??
br//zaman
Hello Mohammad
There are several ways a router will be informed of a topology change.
If a router has one of its directly connected networks go down, that is, a cable is unplugged for example, the router will immediately know this destination is unavailable, and all destinations through which it used that interface will no longer be available. So the router removes the directly connected network from its routing table, and also begins sending out requests to neighbours to find alternative routes to the directly connected network as well as to the networks that were reached via this interface. Neighbours will receive this information and will respond with alternative routes if there are any.
Secondly, a router may be informed of remote routes that have become unavailable via route information packets that are sent from neighbouring routers. These packets contain information that states that these routers are no longer the next hop routers for the destination networks. The local router will then begin the process of searching for alternative routes to the destinations by communicating with all other neighbours. Once this process completes, the network converges again either with alternative routes to the destinations in question or by removing any destinations for which routes were not found.
I hope this has been helpful!
Laz
Hi Laz,
What is the job of LS age ?? Thx
br//zaman
Hello Mohammad
According to Cisco:
After a default of 30 minutes the router that originated the entry resends the LSA, with a higher sequence number, in a link-state update (LSU), to verify that the link is still active.
If however, an LSA is expected after 30 minutes and it doesn’t arrive, the information of the LSA is retained until the LS age or the LSA maximum age is reached. This by default is 60 minutes. So it’s kind of like a dead timer, where if no LSA is resent after 60 minutes, the specific LSA is removed from consideration by the OSPF routing calculation.
I hope this has been helpful!
Laz
Hi Rene,
So Incremental SPF works only on LSA-1 & LSA-2 . It doesnt work on other LSAs .Correct me if I am wrong .Thx
br//zaman
1 Like
Hello Zaman
This is a very good question! Shows you’re thinking very analytically. So, changes in LSA types 1 and 2 will always trigger a full SPF calculation unless the Incremental SPF is enabled. Changes in LSA types 3, 4, 5 and 7 will trigger what is called a partial SPF which is different than an incremental SPF calculation. A partial SPF calculation only involves the leaves of the SPF tree. Any changes in these LSAs indicate changes ONLY in the leaves. The SPF tree itself remains unchanged with changes to these LSAs. This is the default behaviour of a Cisco OSPF router.
So to specifically answer your question, yes, incremental SPF will be triggered only with changes to LSA types 1 and 2.
To be a little more clear as to the difference between Incremental and partial SPF, here’s a more detailed explanation of the difference.
* Partial SPF involves changes only to the leaves. No links are changed.
* In Incremental SPF, the SPF tree will be changed to discover an alternative best path after a link failure. The farther the incremental SPF is run from the root of the tree, the more savings in time and resources compared to a full SPF. If the change is close to the root, there is little performance improvement over full SPF.
I hope this has been helpful!
Laz
By default within an area , Full spf run when we have link failure (even its not in our SPT) ,Metric change, New node addition and delete etc . But i want to know one thing that , when the full spf run , do all router exchanges each other the complete LSDB once again and if they do then the ls age is going to be updated and the sequence number is going to be different …Right.,…Kindly correct me …!!
with ISPF , we will run the ISPF for that particular change .,.Right…
Hello Narad
Yes that is correct. Whenever a new LSA arrives with new information, an OSPF router will run the SPF algorithm. Now each router creates its own SPT, so any change in the network has the potential to change the local router’s SPT.
OSPF routers will exchange information using LSAs. The full LSDB is not sent, but only the changes that have been detected. Even so, under normal circumstances, OSFP will still run a full STP calculation.
The way in which they are exchanged and how OSPF routers respond to them can be seen in the following lessons:
Incremental SPF is an improvement in the efficiency of OSPF such that only the portions of the SPT that have changed are included in the calculation. It does not depend on whether a single LSA or the whole LSDB is exchanged, but has to do with how the information received is interpreted.
I hope this has been helpful!
Laz