Layer 3 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
Spanning Tree Protocol
The VSS active chassis runs Spanning Tree Protocol (STP). The VSS standby chassis redirects STP
BPDUs across the VSL to the VSS active chassis.
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.
Virtual Trunk Protocol
Virtual Trunk Protocol (VTP) uses the IP address of the switch and local current time for version control
in advertisements. After a switchover, VTP uses the IP address of the newly VSS active chassis.
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 to use.
A new PAgP enhancement has been defined for assisting with dual-active scenario detection. For
additional information, see the
“Dual-Active Detection” section on page 4-22
Multicast Protocols
In Release 12.2(33)SXI4 and later releases, fast-redirect optimization makes multicast traffic redirection
between inter-chassis or intra-chassis line cards faster for Layer 2 trunk multichassis EtherChannel or
distributed EtherChannel in case of member port link failure and recovery. This operation occurs mainly
when a member port link goes down (port leaves the EtherChannel) and when the member port link goes
up (port joins or rejoins the EtherChannel). Fast-redirect does not take effect when you add or remove a
member port due to a configuration change or during system boot up.
Layer 3 Protocols
The MSFC on the VSS active supervisor engine runs the Layer 3 protocols and features for the VSS.
Both chassis perform packet forwarding for ingress traffic on their interfaces. If possible, ingress traffic
is forwarded to an outgoing interface on the same chassis, to minimize data traffic that must traverse the
VSL.
Because the VSS standby chassis is actively forwarding traffic, the VSS active supervisor engine
distributes updates to the VSS standby supervisor engine PFC and all VSS standby chassis DFCs.
The following sections describe Layer 3 protocols for a VSS:
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IPv4
The supervisor engine on the VSS active chassis runs the IPv4 routing protocols and performs any
required software forwarding.
Routing updates received on the VSS standby chassis are redirected to the VSS active chassis across the
VSL.