Duplicating Effort

It is an error to use routing daemons to duplicate the effort of another forwarding mechanism (for example, a hardware router, or another pair of routing daemon neighbors. (This error is actually a variation of the error described in Neighbors on the Same Network.)

Consider the situation in Routing Daemons and Duplication. Two mechanisms forward messages between the two networks—the hardware router and a pair of routing daemons (A.a.bad.com and B.b.bad.com). When a program on network a.bad.com sends a message, routing daemon A forwards it to its neighbor B, which redistributes it on network b.bad.com. When the hardware router receives the redistributed message, it forwards it back to network a.bad.com, where A detects the duplication and produces an error message.

This kind of error can occur in either broadcast or multicast situations. However, it is especially common in environments where hardware routers enable multicast routing. Upgrading a hardware router can trigger this error.

Upgrading rvrd from release 5 to release 6 (or later) provides another fertile environment for this error. When both routing daemons run concurrently in the same network, be careful to avoid duplicate service.

To repair the situation, remove one of the routing daemons, or disable hardware multicast routing.

Figure 68: Routing Daemons and Duplication