So, guys, now let us wind up with the last topic of this section, which is convergence.
So all devices must update their routing tables in a consistent way whenever there is a change in the
topology.
Now the change in the topology could be addition or removal of new devices.
It could be sudden or abrupt failure of any node of the topology or any link of the topology.
Right.
Or it could be an admin may choose to change the cost of any link of this topology.
Right?
For example, the admin may configure that the cost of the link which is connecting node A and b instead
of four as 14.
Right.
The admin has all the liberty to do so.
So now let us see that.
How a typical interior gateway protocol reacts whenever there is a change in the topology for any reason.
So let us suppose that the link which connects the Node A and B abruptly fails, right?
So when this link fails, it means that the transmission of any kind of packet from A to B and B to
A has completely ceased.
So it means that the membership information that A is maintaining about neighbor B in its local neighborhood
database would eventually time out.
Right.
Because it is not receiving hello packets from the neighbor Node B anymore.
And the same is true for the Node B, also the neighbor information about node A maintained in the local
neighborhood database of the node B would going to be eventually time out or deleted.
So when this happens, then A will withdraws B's entry from its neighbor ship database and regenerate
and flood its packets in the entire topology.
Right.
Similarly be will withdraw as entry from its local neighborship database and regenerate and flood its
updated link State packets.
Right.
So when Node A and Node B will flood their respective link state packets, which are now new and updated
link state packets, then every other device that is including A and B themselves would receive these
new link state packets of A and B, and they will eventually update their link state database.
Right.
So now, because every router has been informed that the link connecting A and B do not exist anymore,
the final result is that that every device attains the new consistent view of the network graph because
they have updated their link state database from the updated link.
State packets originated from the router A and B, right.
And every other device of the topology.
Then retriggers SPF algorithm and update routing tables.
So this whole exercise is called convergence.
From the point of change in the topology to the point when every other device in the topology updates
its routing table as a result of receiving updated link state packets from other devices of the topology.
And finally updating their routing table is called convergence.
So now let us suppose that at time t is equal to zero.
The link connecting Node A and B fails, right?
And at time T plus one, all other device let us assume, receives the new link state packets from A
and b right that is node as D and c receives the updated link state packets of node A and B.
At time t plus one.
So it simply means that t one is the transient time interval during which the traffic which could be
flowing across this topology can be subjected to loops right until the node D or C or S receives the
updated link state packets from the node A and B nodes D and C would continue to believe that the link
connecting Node A and B is still a part of the topology, so it simply means that's D and C would have
a different logical view of the topology for a delta time.
T one.
Right.
And because of the different view of the logical topology, the routing loops can occur that we have
already discussed in the previous lecture video.
Right.
So this is a limitation of interior gateway protocol, that whenever there is a change in the topology
for a small time duration T1, if there a stream of the traffic flow across the topology from say,
device as to the device B, then that traffic flow stream could be disrupted momentarily.
If time T1 is large, then we say that the protocol has a poor convergence, lesser the value of T1,
better the convergence of the protocol.