Packet Handling - Cisco Catalyst 4500 Series Software Configuration Manual

Chapter 5 Configuring Virtual Switching Systems
Data traffic terminating on the VSS Active switch reaches the MEC by crossing the VSL to the VSS
Standby switch. Control protocols continue to run in the VSS Active switch. Protocol messages reach
the MEC by crossing the VSL.
All MEC Links to the VSS Standby Switch Fail
If all links fail to the VSS Standby switch, the MEC becomes a regular EtherChannel with operational
links to the VSS Active switch.
Control protocols continue to run in the VSS Active switch. All control and data traffic from the VSS
Standby switch reaches the MEC by crossing the VSL to the VSS Active switch.
All MEC Links Fail
If all links in an MEC fail, the logical interface for the EtherChannel is set to unavailable. Layer 2 control
protocols perform the same corrective action as for a link-down event on a regular EtherChannel.
On adjacent switches, routing protocols and Spanning Tree Protocol (STP) perform the same corrective
action as for a regular EtherChannel.
VSS Standby Switch Failure
If the VSS Standby switch fails, the MEC becomes a regular EtherChannel with operational links on the
VSS Active switch. Connected peer switches detect the link failures, and adjust their load-balancing
algorithms to use only the links to the VSS Active switch.
In Quad-Supervisor VSS mode, the in-chassis standby (ICS supervisor in the VSS Standby switch
becomes the VSS Standby supervisor and the former VSS Standby supervisor becomes the ICS.
VSS Active Switch Failure
VSS Active switch failure results in a stateful switchover (SSO). See the
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Active switch. Connected peer switches detect the link failures (to the failed switch), and adjust their
load-balancing algorithms to use only the links to the new VSS Active switch.

Packet Handling

In a VSS, the VSS Active supervisor engine runs the Layer 2 and Layer 3 protocols and features for the
VSS and manages the ports on both switches.
The VSS uses the VSL to communicate system and protocol information between the peer switches and
to carry data traffic between the two switches.
Both switches perform packet forwarding for ingress traffic on their local interfaces. The VSS minimizes
the amount of data traffic that must traverse the VSL.
for details about SSO on a VSS. After the switchover, the MEC is operational on the new VSS
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Understanding Virtual Switching Systems
"VSS Redundancy" section on
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