Saturday, 16 January 2010

VSS Introduction

VSS stands for Virtual Switching System and is a technology for use on 6500 series switches.

It works in a similar way to stackwise on the 3750s, you have two physical devices that end up with a single logical management plane.

In a large Cisco-style network you would have two core/distribution devices for redundancy. The network topology will typically need an active/standby topology. With Spanning-tree, only one uplink is forwarding for a given VLAN. To avoid unicast flooding the network should then have HSRP/VRRP and the IGP configured so that all traffic for that VLAN is routed via the same distribution (or core) switch.

Traffic is load balanced by alternating VLANs between the two distribution switches.

This topology is shown below for a single VLAN:

The basic VSS topology is shown below, the access device still has redundant uplinks but traffic flows over all links in etherchannels:

This topology it produces is similar to a network built around Nortel kit and the Split Multi-Link Trunking (SMLT) technology. The difference being that SMLT still has two separate management planes but VSS only has one.

There are a few benefits to this system:

Simplified Network Architecture.
  • Single logical device at the core/distribution layer.
  • Behaves as a single unit for management purposes.
  • No need to fudge STP, HSRP and IGP per VLAN.
  • No need of first hop redundancy protocols (FHRP).

Simpler Routing
  • Half as many IGP neighbors.
  • Half as many IGP routes.
  • No IGP load balancing/weighting required.

  • Software upgrades carried out without risky failovers and associated downtimes during convergence.
  • Easier to roll back.

Faster Convergence.
  • No STP convergence delays.
  • Loss of redundant switch does not change logical topology, avoiding convergence entirely.

Reduced function of Spanning-Tree.
  • No blocked ports to distribution/core from access layer.
  • Reduces greatly the chance of bridging loops.
  • Can use portfast trunk on access device uplinks.

The Cisco VSS Design Guide is well worth a read for more detailed information.

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