What is a Routing Loop?

What is a Routing Loop? A routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its intended destination network. A routing loop can occur when two or more routers have routing information that incorrectly indicates that a valid path to an unreachable destination exists.

The loop may be a result of:

Incorrectly configured static routes

Incorrectly configured route redistribution (redistribution is a process of handing the routing information from one routing protocol to another routing protocol and is discussed in CCNP-level courses)

Inconsistent routing tables not being updated due to slow convergence in a changing network

Incorrectly configured or installed discard routes.

Distance vector routing protocols are simple in their operations. Their simplicity results in protocol drawbacks like routing loops. Routing loops are less of a problem with link-state routing protocols but can occur under certain circumstances.

Note:The IP protocol has its own mechanism to prevent the possibility of a packet traversing the network endlessly. IP has a Time-to-Live (TTL) field and its value is decremented by 1 at each router. If the TTL is zero, the router drops the packet.

What are the Implications of Routing Loops?

A routing loop can have a devastating effect on a network, resulting in degraded network performance or even a network downtime.

A routing loop can create the following conditions:

Link bandwidth will be used for traffic looping back and forth between the routers in a loop. A router's CPU will be strained due to looping packets.

A router's CPU will be burdened with useless packet forwarding that will negatively impact the convergence of the network.

Routing updates may get lost or not be processed in a timely manner. These conditions would introduce additional routing loops, making the situation even worse. Packets may get lost in "black holes."

There are a number of mechanisms available to eliminate routing loops, primarily with distance vector routing protocols. These mechanisms include:

Defining a maximum metric to prevent count to infinity

Holddown timers

Split horizon

Route poisoning or poison reverse

Triggered updates