RFC 2174 (rfc2174) - Page 2 of 22
A MAPOS version 1 Extension - Switch-Switch Protocol
Alternative Format: Original Text Document
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