RFC 2174 (rfc2174) - Page 2 of 22
A MAPOS version 1 Extension - Switch-Switch Protocol
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RFC 2174 MAPOS June 1997 2. Constraints in Designing SSP SSP is a unified routing protocol supporting both unicast and broadcast/multicast. The former and the latter are based on the Distance Vector [6][7] and the spanning tree[8] algorithm, respectively. In MAPOS version 1, a small number of switches is assumed in a segment. Thus, unlike DVMRP(Distance Vector Multicast Routing Protocol)[8], TRPB(Truncated Reverse Path Broadcasting) is not supported for simplicity. This means that multicast frames are treated just the same as broadcast frames and are delivered to every node. In MAPOS version 1, there are two constraints regarding design of the broadcast/multicast routing algorithm; (1) there is no source address field in MAPOS HDLC frames (2) there is no TTL(Time To Live) field in MAPOS HDLC frames to prevent forwarding loop. To cope with the first issue, VRPB(Virtual Reverse Path Broadcast) algorithm is introduced. In VRPB, all broadcast and multicast frames are assumed to be generated by a node under a specific switch called VSS(Virtual Source Switch). VSS is the switch which has the smallest switch number in a MAPOS network. Each switch determine its place in the spanning tree rooted from VSS independently. Whenever a switch receives a broadcast/multicast frame, it forwards the frame to all upstream and downstream switches except for the one which has sent the frame to the local switch. To cope with the second issue, the forward delay timer is introduced. Even if a switch finds a new VSS, it suspends forwarding for a time period. This timer ensures that all the switches have a consistent routing information and that they are synchronized after a topology change. 3. Unicast Routing in SSP This section describes the address structure of MAPOS version 1 and the SSP unicast routing based on it. Murakami & Maruyama Informational
