Datasheet

55
Release Notes for Catalyst 4500 Series Software Release 7.x
OL-1983-28
Usage Guidelines, Restrictions, and Troubleshooting
Spanning Tree
This section contains usage guidelines, restrictions, and troubleshooting information that apply to
spanning tree:
• The Spanning Tree Protocol (STP) blocks certain ports to prevent physical loops in a redundant
topology. On a blocked port, the Catalyst 4500 series switch receives spanning tree bridge protocol
data units (BPDUs) periodically from the neighboring device. To configure the frequency with
which BPDUs are received, enter the set spantree hello command (the default frequency is set to
two seconds). If a Catalyst 4500 series switch does not receive a BPDU in the time 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 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.
• If the STP parameters are reduced in value, be sure that the number of STP instances is also reduced
proportionally in order to avoid spanning tree loops in the network.
• You should configure a Catalyst series switch as the root for every VLAN, especially VLAN 1. In
order to recover from an extended broadcast storm, Catalyst series switches must reset blocked
ports. To ensure recovery, all Catalyst series switches in the network should reset blocked ports at
the same time by sending synchronization packets on VLAN 1. A Catalyst series switch will not
send synchronization packets unless it is the root bridge.
• Disabling spanning tree on the native VLAN of an IEEE 802.1Q trunk might cause spanning tree
loops. We recommend that you leave spanning tree enabled on the native VLAN of an 802.1Q trunk.
If you plan to disable spanning tree in an 802.1Q environment, disable it on every VLAN in the
network to ensure that a loop-free topology exists.
• On your Catalyst 4500 series switch, be sure that the total number of logical ports across all
instances of spanning tree for different VLANs does not exceed the number allowed for your
supervisor engine.
You can use the show spantree summary command and this formula to compute the sum of logical
ports on the switch:
(number of trunks on the switch
× number of active VLANs on those trunks) + number of nontrunking
ports on the switch
The sum of all logical ports, as calculated with the formula above, should be less than or equal to
the following:
– 600 instances in PVST+ mode for the Catalyst 4500 series Supervisor Engine I and II
– 9000 instances in MSTP mode for the Catalyst 4500 series Supervisor Engine I and II
Caution If you enable numerous memory-intensive features concurrently (such as VTP pruning, VMPS,
EtherChannel, and RMON), or if there is switched data traffic on the management VLAN, the maximum
number of supported logical ports is reduced.
Note Count each port in an EtherChannel port bundle independently (do not count the bundle as a single port).
• On a blocked spanning tree port, check the duplex configuration to ensure that the port duplex is set
to the same type as the port of the neighboring device.