Convergence in dynamic routing protocols???

[vipin.vikraman]

Hi,,

                   Could  any one help me in better understanding  about convergence in dynamic routing protocols  , i mean when  i have a network connected with many routers , what exactly happens  inside  each router when it is trying to converge before data forwarding actually happens. say i have four routers and alltogether they  are conneted to 8 networks , so does convergence simply means that each router should learn routes to those 8 networks through some dynamic routing protocol  and when thats is done  can i say ,, ok now my network is converged , and data transfer will happen soon ,  is it as simple as that ???

Vipin Vikraman

[smitesh kharecha]

Hi Vipin,

Convergence in simple terms means, all of the routers in your networks are aware about all the IP subnets (which are advertised / not blcoked, etc) and also which is the shortest / least cost path to reach them.

Data transfer can happen ( in case of L3 network) before the convergence, however; that data forwarding can lead to router dropping it if forwarding router doesn't know how to reach that subnet or even routing loops.

HTH,

Smitesh

[Richard Burts]

Vipin Vikraman

Your understanding of convergence is pretty good. When running a dynamimc routing protocol the router receives advertisements from its neighbors about what they know and then the router applies a methodology specific to that routing protocol to use the advertisements to build its routing table. (OSPF uses the Dijkstra algorithm, EIGRP uses DUAL algorithm, etc). When the routers have received and processed their advertisements and all routers have a consistent understanding of the network then we say that they have converged. Then is there is a change in the network (a new network is added or a network goes down) then the routing protocol must send advertisements, the routers must process the advertisements, and the process of adapting to the changes is the process of convergence.

As Smitesh points out, the router can forward traffic before convergence is complete - so not being converged does not stop forwarding traffic. But forwarding packets when not converged may lead to using less than optimum paths, or may lead to dropped packets. So we like routing protocols that converge quickly such as EIGRP and OSPF.

HTH

Rick

[vipin.vikraman]

Thanks rick and smitesh

[smitesh kharecha · Accepted solution]

Hi Vipin,

If your query is resolved, can you please close this thread; so that older community member don't waste their time coming to this post and finding this to be already answered.

Thanks,

Smitesh