• This is a problem which arises because of routing loop in a network.
• It is one of the major flaw of Distance vector routing Protocol.
• This is also known as routing loop in a network.
• Routing loops occur when a packet is continually routed through the same routers over and over, in an endless circle.

Reason for Routing Loop in a network:

• The network configuration is not appropriate(static routing).
• Routing loops usually occur when any interface goes down or two-routers send updates at the same time.
• They are formed when an error occurs in the operation of the routing algorithm, and as a result, in a group of nodes, the path to a particular destination forms a loop.

Explanation with example:

• There is a 3 routers(A, B and C) each with its routing table having the next hop and hop count to reach each others.

• If the link between B and C goes down, B will get to know that it cannot reach C and hence remove its entries from routing table.

• But before B can update this information A, it receives an update from A that it(A) can reach C via B with hop count of 2.
• Upon receiving this outdated information from A, B updates its routing table that it can reach C via A with hop count of 3( A to C =2 and B to A =1, so total 3).
• Later at some point A will receive an update from B that it(B) can reach C with 3 hop count.
• So A will update its routing table that it can reach C with 4 hop count( A to B =1 and B to C =3, total 4).
• Thus this process will continue and at some point both A and B will have its routing table with hop count to infinity(as it keeps incrementing by 1).
• Hence this problem is called count to infinity and Distance vector is also called routing by rumors and is totally dependent upon neighbors.

Solution to this problem:

• Split Horizon
• Route poisoning

Categories: Networking