User`s guide

1-18

Introduction

Figure 1-3. Source Routing Example

In the example in Figure 1-3, a data packet traveling from station C

on LAN 43 through bridge B to station A on LAN 7 must specify

the full route it is to take. The source station is responsible for

specifying the route, hence the term “source routing.”

Bridges in a source routing network must be conﬁgured with the

LAN numbers (normally 1 to 4095) to which it is connected and a

bridge number (normally 1 to 15). The network administrator

chooses the numbers; the LAN numbers must be unique in the

source routed network and the bridge numbers must be unique

between each pair of LANs.

Source routing workstations need not be conﬁgured with route

information; instead they discover the best route to a destination

through the use of explorer frames. In the Figure 1-3 example,

station C might ﬁrst transmit an empty explorer frame. Bridge B

would add 43-B-7 as its portion of the route, and then transmit the

explorer on all other LANs. When the packet reaches station A, it

can reverse the route to send a reply back to C. When C receives

the reply, both stations have all of the routing information needed

to converse, with no further explorer frames needed.

Part of the original intent of source routing bridging was to enable

LANs to be richly connected by low-performance, low-cost

bridges. As shown in Figure 1-4, source routing allows an end

station to choose a less-congested path through a chain of bridges,

Ring

address 43 B 7 data

data packet

Station A Station CBridge B