User`s guide

1-30
Introduction
The architecture behind source-routing bridges is that a packet
header containing a route is inserted by the source end-station. For
the source end-station to discover a route to a destination end-
station, it must learn of a route by transmitting a special type of
packet called an explorer packet.The explorer packet is duplicated
by source-routing bridges as it discovers possible route choices.
A copy of the explorer packet is sent over every possible route.
When a source end-station discovers a route to a destination end-
station, it stores the route so that it can be used for subsequent
packets to the same destination end-station. Generally routes are
stored for approximately 15 minutes, or three times the Spanning
Tree age. However, the time can be shorter if the Spanning Tree
topology is changing.
In simplest terms, the data link layer header of a packet on a Local
Area Network (LAN) looks like the following.
Figure 1-11. Data Link Layer Packet Header
To distinguish between packets whose data link headers include
routing information and those that do not involves setting the
Routing Information Indicator (RI). The Routing Information Field
(RIF) contains the additional source-routing information.
The RI happens to be the multicast bit in the source field.
Therefore, a packet with a multicast bit set to 0 is not treated as a
source-routing packet. However, if the multicast bit is set, the
information following the usual data link layer header is assumed
to be a source-routing header.
destination source data