Cisco Catalyst 4500 Series Software Configuration Manual page 243

Chapter 5 Configuring Virtual Switching Systems
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Spanning Tree Protocol
The VSS Active switch runs Spanning Tree Protocol (STP). The VSS Standby switch redirects STP
BPDUs across the VSL to the VSS Active switch.
The STP bridge ID is commonly derived from the chassis MAC address. To ensure that the bridge ID
does not change after a switchover, the VSS continues to use the original chassis MAC address for the
STP Bridge ID.
EtherChannel Control Protocols
Link Aggregation Control Protocol (LACP) and Port Aggregation Protocol (PAgP) packets contain a
device identifier. The VSS defines a common device identifier for both chassis. You should use PAgP or
LACP on MECs instead of mode ON, although all three modes are supported.
A new PAgP enhancement has been defined for assisting with dual-active scenario detection. For
additional information, see the
Jumbo frame size restriction
The maximum jumbo frame size supported on a VSS interface is 9188 bytes (MTU of 9170 bytes). This
accommodates the overhead of transporting packets between the two member switches over VSL.
Not all frames traverse VSL. So, packets confined to one of the member switches could have a size of
9216 bytes (MTU of 9198 bytes). Such frames may require diversion over VSL when a failure occurs.
This is why the max configured MTU on non-VSL front panel ports is 9170.
Note
For example, if we send traffic between two ports on the active switch, no overhead exists. However,
overhead exists when we send packets between ports of active to ports of standby. Even more overhead
exists when we send packets from standby ports to the active CPU. The higher limit accommodates the
worst case and guarantees consistent forwarding under all scenarios.
SPAN
VSS supports all SPAN features for non-VSL interfaces.
Note SPAN on VSL ports is not supported; VSL ports can be neither a SPAN source, nor a SPAN destination.
The number of SPAN sessions available on a VSS matches that on a single switch running in standalone
mode.
Private VLANs
Private VLANs on VSS work similarly in standalone mode. The only exception is that the native VLAN
on isolated trunk ports must be configured explicitly. Refer to
VLANs"
Private VLANs, page 5-17
The MTU CLI is unavailable on a VSL interface. It is set internally to 9198 (Max frame size of
9216), addressing the overhead of VSL.
for details on how to configure the native VLAN on isolated trunk ports.
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
"Dual-Active Detection" section on page
Understanding Virtual Switching Systems
5-23.
Chapter 47, "Configuring Private
5-17