Installation guide

Release Notes for Catalyst 6000 Family Software Release 5.x 35

VLAN Troubleshooting

This section contains troubleshooting guidelines for VLAN problems:

Note Catalyst 6000 and 6500 series switches do not support ISL-encapsulated Token Ring frames.

To support trunked Token Ring trafﬁc in your network, make trunk connections directly between

switches that support ISL-encapsulated Token Ring frames. When a Catalyst 6000 or 6500 series

switch is conﬁgured as a VTP server, you can conﬁgure Token Ring VLANs from the switch.

Catalyst 6000 family switches ship with trunking-capable ports in a nontrunking state and the

Dynamic Trunking Protocol (DTP) feature in the auto mode. In this mode, if a port sees a DTP on

or DTP desired frame, it transitions into trunking state. Although DTP is a point-to-point protocol,

some internetworking devices might forward DTP frames. To avoid connectivity problems that

might be caused by a Catalyst 6000 or 6500 series switch acting on these forwarded DTP frames, do

the following:

• For ports connected to non-Catalyst 6000 or 6500 series devices in which trunking is not

currently being used, conﬁgure trunk-capable Catalyst 6000 and 6500 ports to off by entering the

set trunk mod_num/port_num off command.

• When manually enabling trunking on a link to a Cisco router, use the set trunk

mod_num/port_num nonegotiate command. The nonegotiate keyword is available in

Catalyst 6000 and 6500 supervisor engine software release 5.1(1)CSX and later that transitions

a link into trunking mode without sending DTP frames.

Spanning-Tree Troubleshooting

This section contains troubleshooting guidelines for spanning-tree problems:

The Spanning-Tree Protocol (STP) blocks certain ports to prevent physical loops in a redundant

topology. On a blocked port, the Catalyst 6000 or 6500 series switch receives spanning-tree bridge

protocol data units (BPDUs) periodically from its neighboring Catalyst 6000 or 6500 series switch.

You can conﬁgure the frequency with which BPDUs are received by entering the set spantree hello

command (the default frequency is set to 2 seconds). If a Catalyst 6000 or 6500 series switch does

not receive a BPDU in the time period deﬁned by the set spantree maxage command (20 seconds

by default), the blocked port transitions to the listening state, the learning state, and to the forwarding

state. As it transitions, the Catalyst 6000 or 6500 series switch waits for the time period speciﬁed by

the set spantree fwddelay command (15 seconds by default) in each of these intermediate states.

Therefore, a blocked spanning-tree port moves into the forwarding state if it does not receive BPDUs

from its neighbor within approximately 50 seconds.

Note We do not recommend using the uplinkfast feature (also known as spanning-tree Uplink

FastSwitchover) on switches with more than 20 active VLANs. The convergence time might be

unacceptably long for a switch with more than 20 active VLANs.

Use the following guidelines to debug STP problems:

• After a switchover from the active to the standby supervisor engine, the uplink ports on the

standby supervisor engine take longer to come up than other switch ports.

• Keep track of all blocked spanning-tree ports in each Catalyst 6000 or 6500 series switch in your

network. For each of the blocked spanning-tree ports, keep track of the output of the following

commands: