Token Ring

Document ID: 12266

You can have the "encapsulation failed" occur on Token Ring, for any protocol, for two reasons:

1. There is no mapping from the Network Protocol address to the MAC level address. For many protocols, this means "not finding an entry in the ARP" table.
2. For interfaces with "multiring" enabled for the protocol, not finding an entry in the RIF cache to map from the MAC level address to the RIF string can cause failure.

When the failures occur, check what is shown in the "show rif" display.

Having an entry removed from the RIF cache is actually normal behavior if the machine you are talking to has not been spoken to by the router for some time. The RIF cache has a timeout of 15 minutes. If a machine which has not been accessed for a while needs to be referenced, the first attempt will fail since the entry is not in the RIF cache. An explorer will be sent out from the router to find the machine, but the packet that needed to be sent will be dropped. When that remote machine finally replies, its entry is re-inserted into the RIF cache, so it can now be accessed by future packets from the router. This is probably why in some cases, where a subsequent client query may succeed -- the initial query caused failure. After an explorer is sent to find the machine, that machine was put into the RIF cache, and then the second one succeeds.

This is how token ring works. You can always increase the amount of time entries stay in the RIF cache by modifying the "rif cache timeout" parameter or for machines that you must have first-success access to, you can place static RIF entries into the router.