A matrix follows which details this restriction. This can restrict the use of HSRP in this multi-protocol environment. If a router is configured for DECnet or XNS, all interfaces much share and use a specified MAC address. In most cases when routers are configured to be part of a Hot Standby group, they will recognize their own native MAC address plus the hot standby group MAC address, unless they are only able to utilize one MAC address. Token Ring interfaces use functional addresses for the HSRP MAC addresses.
Functional addresses are the only general multicast mechanism available.
There are a limited number of token ring functional addresses and many of them are reserved for other functions. You may only configure 3 HSRP addresses on token ring interfaces. The authentication built into HSRP consists of the addition of a shared clear-text key within the HSRP packets. The purpose of this password is to disallow mis-configured routers from participating in an HSRP group it was not intended to participate in. Although the feature allows for a certain level of security, in practice it is possible to cause more issues that this authentication can avert.