Packet handling – Cisco 6500 User Manual
Page 16
4-16
Cisco IOS Software Configuration Guide, Release 12.2SX
OL-13013-06
Chapter 4 Configuring Virtual Switching Systems
Understanding Virtual Switching Systems
All MEC Links to the VSS Standby Chassis Fail
If all links fail to the VSS standby chassis, the MEC becomes a regular EtherChannel with operational
links to the VSS active chassis.
Control protocols continue to run in the VSS active chassis. All control and data traffic from the VSS
standby chassis reaches the MEC by crossing the VSL to the VSS active chassis.
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 Chassis Failure
If the VSS standby chassis fails, the MEC becomes a regular EtherChannel with operational links on the
VSS active chassis. Connected peer switches detect the link failures, and adjust their load-balancing
algorithms to use only the links to the VSS active chassis.
VSS Active Chassis Failure
VSS active chassis failure results in a stateful switchover (SSO). See the
for details about SSO on a VSS. After the switchover, the MEC is operational on the new VSS
active chassis. Connected peer switches detect the link failures (to the failed chassis), and adjust their
load-balancing algorithms to use only the links to the new VSS active chassis.
Failed Chassis MEC Recovery
When a failed chassis returns to service as the new VSS standby chassis, protocol messages reestablish
the MEC links between the recovered chassis and connected peer switches.
Although the recovered chassis’ MEC links are immediately ready to receive unicast traffic from the peer
switch, received multicast traffic may be lost for a period of several seconds to several minutes. To
reduce this loss, you can configure the port load share deferral feature on MEC port channels of the peer
switch. When load share deferral is configured, the peer’s deferred MEC port channels will establish
with an initial load share of 0. During the configured deferral interval, the peer’s deferred port channels
are capable of receiving data and control traffic, and of sending control traffic, but are unable to forward
data traffic to the VSS. See the
“Configuring Port Load Share Deferral on the Peer Switch” section on
for details about configuring port load share deferral.
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 DFC modules for both chassis.
The VSS uses the VSL to communicate system and protocol information between the peer chassis and
to carry data traffic between the two chassis.
Both chassis perform packet forwarding for ingress traffic on their local interfaces. The VSS minimizes
the amount of data traffic that must traverse the VSL.