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Routing Loop

A routing loop occurs when a packet, trying to reach a destination, is forwarded between two routers whose routes to that destination point to the other router, creating a loop that continues until the packet's time-to-live expires and the packet is dropped.

Routing loops are bad for a network because the end result of a routing loop is that packets are forwarded around and around and around until the time to live expires and they are deleting. Each time a packet is forwarded back around the loop, it multiplies the number of packets crossing that link. They are also very easy to create. Routing loops occur most often when one of two routers has no route for a destination and uses its default route to forward the packet back to the router the packet was received from, creating a loop.

Loops get created when

• Static Routing was misconfigured by the administrator
• The wrong default gateway was used
• The routing protocol used has not yet converged
• Administrative changes to the routing protocol result in a loop

Routing loops are bad because:

• Routing loops increase network traffic load by forwarding packets within the loop until they die of old age
• Routing loops waste router resources (buffers, memory, CPU cycles)
• Routing loops prevent packets from reaching their destination
• Routing loops result in packet loss

Routing protocols are designed to prevent routing loops, but they don't always succeed.

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