Packet Handling - Cisco Catalyst 4500 series Administration Manual

Understanding Virtual Switching Systems
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.
VSS Active Switch Failure
VSS Active switch failure results in a stateful switchover (SSO). See the
page 5-11
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.
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 switches. All frames forwarded
over the VSL link are encapsulated with a special header (up to ten bytes for data traffic and 18 bytes
for control packets), which provides information for the VSS to forward the packet on the peer switch.
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)SG
5-16
for details about SSO on a VSS. After the switchover, the MEC is operational on the new VSS
Traffic on the VSL, page 5-16
Layer 2 Protocols, page 5-17
Layer 3 Protocols, page 5-18
Chapter 5 Configuring Virtual Switching Systems
"VSS Redundancy" section on
OL-30933-01