RFC 2174 (rfc2174) - Page 2 of 22

A MAPOS version 1 Extension - Switch-Switch Protocol

Alternative Format: Original Text Document
< Previous
Next >
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
< Previous
Next >