Layer 2 Protocols - Cisco Catalyst 4500 series Administration Manual

Chapter 5 Configuring Virtual Switching Systems
The VSL transports control messages between the two switches. Messages include protocol messages
that are processed by the VSS Active supervisor engine, but received or transmitted by interfaces on the
VSS Standby switch. Control traffic also includes module programming between the VSS Active
supervisor engine and switching modules on the VSS Standby switch.
The VSS needs to transmit data traffic over the VSL under the following circumstances:
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VSL also transports system data, such as NetFlow export data and SNMP data, from the VSS Standby
switch to the VSS Active supervisor engine.
To preserve the VSL bandwidth for critical functions, the VSS uses strategies to minimize user data
traffic that must traverse the VSL. For example, if an access switch is dual-homed (attached with an
MEC terminating on both VSS switches), the VSS transmits packets to the access switch using a link on
the same switch as the ingress link.
Traffic on the VSL is load-balanced with the same global hashing algorithms available for
EtherChannels (the default algorithm is source-destination IP).

Layer 2 Protocols

The VSS Active supervisor engine runs the Layer 2 protocols (such as STP and VTP) for the switching
modules on both switches. Protocol messages that are transmitted and received on the VSS Standby
switch switching modules must traverse the VSL to reach the VSS Active supervisor engine.
All Layer 2 protocols in VSS work similarly in standalone mode. The following sections describe the
difference in behavior for some protocols in VSS:
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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.
OL-30933-01
Layer 2 traffic flooded over a VLAN (even for dual-homed links).
Packets processed by software on the VSS Active supervisor engine where the ingress interface is
on the VSS Standby switch.
The packet destination is on the peer switch, such as the following examples:
Traffic within a VLAN where the known destination interface is on the peer switch.
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Traffic that is replicated for a multicast group and the multicast receivers are on the peer switch.
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– The known unicast destination MAC address is on the peer switch.
– The packet is a MAC notification frame destined for a port on the peer switch.
Spanning Tree Protocol, page 5-17
EtherChannel Control Protocols, page 5-18
Jumbo frame size restriction, page 5-18
SPAN, page 5-18
Private VLANs, page 5-18
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)SG
Understanding Virtual Switching Systems
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