Summary

Summary

Bridging alternatives exist for Ethernet and Token Ring media types. Traditionally, Ethernet uses transparent bridging with the following operations: learning (adding a source address/interface to a bridge table), forwarding out a single interface (known unicast traffic), flooding out all interfaces (unknown unicast, multicast and broadcast), filtering (unicast traffic where the source and destination are on the same interface side), and aging (removing an entry from the bridge table.)

Token Ring usually implements source-route bridging in which the source specifies the path for the frame to the destination. This means the source identifies a sequence of ring/bridge hops. SRT (source-route transparent bridging), another Token Ring bridge method, does both source-route bridging and transparent bridging. Source-route bridging is employed if the frame includes a routing information field. Otherwise, transparent bridging is used.

When connecting transparently bridged segments (Ethernet) to source-routed segments (Token Ring), use routing or translational bridging. If you must use bridging, translational bridging resolves a number of issues in the way that frames are constructed for the different access methods. However, translational bridges must be aware of the protocols to be translated. The best solution, though, is to use routing to interconnect mixed media networks. Chapter 11, "Layer 3 Switching," discusses using routers in a switched environment.

Token Ring switching creates two functions to segment Token Rings: the concentrator relay function and the bridge relay function. In the Catalyst, the Token Ring Concentrator Relay Function (TrCRF) defines which ports belong to a ring. A TrCRF may operate either in SRB or SRT mode. A TrCRF cannot span across Catalysts. The Token Ring Bridge Relay Function (TrBRF) provides Token Ring bridging between TrCRFs. A TrBRF can span across Catalysts to allow ports on different units to communicate through a common bridge.

Source-route switching determines whether a frame may be forwarded within the TrCRF or whether the frame needs to be sent to the TrBRF. The SRS looks for the RII to make this decision. If the RII indicates the presence of a routing information field, it forwards the frame to the TrBRF. Otherwise, the SRS keeps the frame within the TrCRF and uses transparent bridging.