Routing Loops – Ciscco Certifed Support Technici

It is possible, from time to time, for a routing loop to form in a network. Figure 3-13 illustrates.

Figure 3-13 A Routing Loop

If host A sends a packet toward F, then

1.  Router B forwards the packet to D.

2.  Router D forwards the packet to E.

3.  Router E forwards the packet to C.

4.  Router C forwards the packet to A.

This is called a routing or forwarding loop. This loop can happen for a few moments when a link or device either fails or is added to the network—called a microloop. This can also happen because of misconfigurations.

IP packets carry a time-to-live (TTL). Routers decrement the TTL

by 1 when processing a packet; if a packet is received with a TTL of 0, it is discarded. The TTL prevents a packet from being forwarded around in the network “forever.”

Physical frames do not have a TTL, so a forwarding loop in a switched network is more likely to cause a network failure in a switched network than a routed network.

Asymmetric Paths

IP networks are packet-based, which means each packet can travel a different path, or packets between two different hosts can travel on two different paths. Figure 3-14 illustrates.

Figure 3-14 Asymmetric Routing

In Figure 3-14, packets traveling from host A to F can travel along the [B,C,E] path or the [B,D,E] path. Every other packet A sends to F could travel along a different path, or router B could choose to send all the packets traveling to host F along one of these two paths. Normally, in IP routing, router B will choose one path because spitting the traffic along two paths will reduce the performance of the host A to F communication.

When host F sends packets to A, it also may travel on either of the available paths. There is no particular reason for routers B and E to choose the same path, however. In fact, these two routers are not even aware of which path the other router has chosen. It is often true, then, that packets will travel consistently on one path in one direction and consistently on another path in the opposite direction.

Using different paths in each direction is called asymmetric routing; it is very common for two hosts to communicate over an asymmetric path of this kind.