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Virtual Switching System (VSS)
The VSS combines a pair of physical switches into a single logical network element. VSS was developed to
address the ever-increasing adoption of delay-sensitive applications, for example, voice, 2video, and
collaboration, that are appearing in enterprise networks. Traditional network topologies relied protocols such as
Spanning Tree Protocol and HSRP to manage loops and first-hop gateway implementation, but those protocols
were proving to be unable to handle the delay sensitivity of these newer applications. With its support for MEC and
quad-supervisor SSO, VSS with Supervisor Engine 2T provides the most highly available and reliable solution to
address these requirements in the backbone of a unified access campus architecture.
Multichassis EtherChannel
MEC is an EtherChannel with ports that terminate on both chassis of the VSS. A VSS MEC can connect to any
network element that supports EtherChannel by using manual ON mode, Link Aggregation Control Protocol
(LACP) or Port Aggregation Protocol (PAgP). At the VSS, an MEC is an EtherChannel with additional capability:
the VSS balances the load across ports in each chassis independently. For example, if traffic enters the active
chassis, the VSS will select an MEC link from the active chassis. This MEC capability makes sure that data traffic
does not unnecessarily traverse the VSL. Figure 26 shows how an MEC changes the traditional campus design.
Figure 26.
Changing Traditional Campus Design with VSS MEC
In the traditional campus design, the use of Spanning Tree Protocol results in one of the uplinks from the access
layer being blocked to prevent a network loop. If the active link fails, then Spanning Tree Protocol has to go
through a multisecond process to unblock the blocked link. With VSS MEC, both of the uplinks from the access
layer are actively forwarding traffic even though the two links are still physically connected to two separate
chassis. The access layer switch sees just a single switch in the backbone above it because VSS makes the two
physical chassis appear as one from a protocol standpoint. As a result, the access layer can form what it thinks is
a regular EtherChannel with the backbone switch.
© 2013 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information.
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