Traffic on the vsl, Layer 2 protocols – Cisco 6500 User Manual
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Cisco IOS Software Configuration Guide, Release 12.2SX
OL-13013-06
Chapter 4 Configuring Virtual Switching Systems
Understanding Virtual Switching Systems
The following sections describe packet handling in a VSS:
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Traffic on the VSL
The VSL carries data traffic and in-band control traffic between the two chassis. All frames forwarded
over the VSL link are encapsulated with a special 32-byte header, which provides information for the
VSS to forward the packet on the peer chassis.
The VSL transports control messages between the two chassis. Messages include protocol messages that
are processed by the VSS active supervisor engine, but received or transmitted by interfaces on the VSS
standby chassis. Control traffic also includes module programming between the VSS active supervisor
engine and switching modules on the VSS standby chassis.
The VSS needs to transmit data traffic over the VSL under the following circumstances:
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Layer 2 traffic flooded over a VLAN (even for dual-homed links).
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Packets processed by software on the VSS active supervisor engine where the ingress interface is on
the VSS standby chassis.
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The packet destination is on the peer chassis, such as the following examples:
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Traffic within a VLAN where the known destination interface is on the peer chassis.
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Traffic that is replicated for a multicast group and the multicast receivers are on the peer chassis.
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The known unicast destination MAC address is on the peer chassis.
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The packet is a MAC notification frame destined for a port on the peer chassis.
VSL also transports system data, such as NetFlow export data and SNMP data, from the VSS standby
chassis 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 chassis), the VSS transmits packets to the access switch using a link on
the same chassis 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 chassis. Protocol messages that are transmitted and received on the VSS standby chassis
switching modules must traverse the VSL to reach the VSS active supervisor engine.
The following sections describe Layer 2 protocols for a VSS:
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Spanning Tree Protocol, page 4-18
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Virtual Trunk Protocol, page 4-18
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EtherChannel Control Protocols, page 4-18
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