3Com 520 - Manual (Page 59)

Bridging and Routing Concepts A-5
Figure A-2 Multiple Remote Bridges
Multiple Paths Between Bridged LANs
With only a single physical path between LANs, the
network is susceptible to link and bridge failures. In
the event of a failure, the connection between any of
the LANs upstream or downstream of the point of
failure will be broken. A more resilient network of
interconnected LANs can be established by providing
more than one link between any two of the LANs.
Normally, this network would soon encounter serious
problems resulting from a loop, around which frames
could endlessly travel if precautions aren’t taken by
the bridges. Over ISDN links a proprietary form of
loop control is implemented. On WAN leased line
links the Spanning Tree Protocol (STP) prevents data
loops.
On startup, the bridges send out frames to enquire if
there are other bridges on the network. By
exchanging information, the bridges block ports that
cause the loops and ensure that there is only ever
one active path through the network. If one of the
links or bridges fail, the other bridges detect this and
reconfigure their ports so that there is once again an
active data path through the network.
Rc.bk : RCAPPA.FRM Page 5 Thursday, July 10, 1997 9:53 AM