Page 621

The Catalyst 5000 Internal Structure

591

Note that although Figure 11.1 denotes a single flow of packets that move

counterclockwise, in a real loop, the initial broadcast is also forwarded
clockwise. Although different cable colors have been used in this example,
both cables are within the same VLAN.

Although Figure 11.1 reflects shared media connected to switches, a switch/

bridge loop can occur in an all-switched network. This diagram simplifies the

physical connections involved by moving them "outside the box."

Logically, an administrator could avoid the entire loop issue by removing

one of the two bridges/switches. Because only one path would exist, no loop
is created. However, there are advantages to installing multiple switches or
bridges. By installing multiple switches/bridges, the network can incorporate
some degree of fault tolerance.

Troubleshooting Spanning Tree Problems

There are several troubleshooting targets for isolating and resolving spanning
tree protocol problems in a switched network. The most essential aspect of trou-
bleshooting spanning tree problems is to understand the spanning tree protocol.
It is also important to pay attention to indicators that there may be loops in the
network. A simple indicator is the LED on the Supervisor engine: If the LED
shows around 80% load, this may be a signal that loops are occurring.

Proper spanning tree functionality requires that there is only one unique bridge

ID for each VLAN. You must also be aware that trunk ports on the Catalyst 5000
may belong to multiple spanning trees. This can cause the problem that if loops
occur on one, the other spanning trees may be adversely affected. The show
spantree

command will display this information.

When the Cisco port-fast and uplink-fast modes are enabled on ports, some

of the transitions of the spanning tree protocol are skipped. This could add to the
potential of loops in the network. The show spantree command also shows
whether the fast-start option has been enabled on a port-by-port basis.

The Catalyst 5000 Internal Structure

here are four significant components of the Catalyst 5000 system that

manage the switching functions: SAMBA, SAINT, SAGE, and EARL. Each of
these components is described in detail below. A clear understanding of these com-
ponents is useful for overall troubleshooting of the Catalyst system in switched
networks. This is especially true when evaluating different switches and options,

www.sybex.com