Virtualisation, long a hot topic for servers, has entered the networking realm. With the introduction of a new management blade for its Catalyst 6500 switches, Cisco can make two switches look like one while dramatically reducing failover times in the process.

In an exclusive Clear Choice test of Cisco's new Virtual Switching System (VSS), Network World conducted its largest-ever benchmarks to date, using a mammoth test bed with 130 10G Ethernet interfaces. The results were impressive: VSS not only delivers a 20-fold improvement in failover times but also eliminates Layer 2 and 3 redundancy protocols at the same time.

The performance numbers are even more startling: A VSS-enabled virtual switch moved a record 770 million frames per second in one test, and routed more than 5.6 billion unicast and multicast flows in another. Those numbers are exactly twice what a single physical Catalyst 6509 can do.

All links, all the time

To maximise up-time, network architects typically provision multiple links and devices at every layer of the network, using an alphabet soup of redundancy protocols to protect against downtime. These include rapid spanning tree protocol (RSTP), hot standby routing protocol (HSRP), and virtual router redundancy protocol (VRRP).

This approach works, but has multiple downsides. Chief among them is the "active-passive" model used by most redundancy protocols, where one path carries traffic while the other sits idle until a failure occurs. Active-passive models use only 50 percent of available capacity, adding considerable capital expense.

Further, both HSRP and VRRP require three IP addresses per subnet, even though routers use only one address at a time. And while rapid spanning tree recovers from failures much faster than the original spanning tree, convergence times can still vary by several seconds, leading to erratic application performance. Strictly speaking, spanning tree was intended only to prevent loops, but it's commonly used as a redundancy mechanism.

There's one more downside to current redundant network designs: It creates twice as many network elements to manage. Regardless of whether network managers use a command-line interface or an SNMP-based system for configuration management, any policy change needs to be made twice, once on each redundant component.

Introducing Virtual Switching

In contrast, Cisco's VSS uses an "active-active" model that retains the same amount of redundancy, but makes use of all available links and switch ports.

While many vendors support link aggregation (a means of combining multiple physical interfaces to appear as one logical interface), VSS is unique in its ability to virtualise the entire switch -- including the switch fabric and all interfaces. Link aggregation and variations such as Nortel's Split Multi-Link Trunk (SMLT) do not create virtual switches, nor do they eliminate the need for Layer 3 redundancy mechanisms such as HSRP or VRRP.

At the heart of VSS is the Virtual Switching Supervisor 720-10G, a management and switch fabric blade for Cisco Catalyst 6500 switches. VSS requires two new supervisor cards, one in each physical chassis. The management blades create a virtual switch link (VSL), making both devices appear as one to the outside world: There's just one media access control and one IP address used, and both systems share a common configuration file that covers all ports in both chassis.

On the access side of Cisco's virtual switch, downstream devices still connect to both physical chassis, but a bonding technology called Multichassis EtherChannel (MEC) presents the virtual switch as one logical device. MEC links can use industry-standard 802.1ad link aggregation or Cisco's proprietary port aggregation protocol. Either way, MEC eliminates the need for spanning tree. All links within a MEC are active until a circuit or switch failure occurs, and then traffic continues to flow over the remaining links in the MEC.

Servers also can use MEC's link aggregation support, with no additional software needed. Multiple connections was already possible using "NIC teaming," but that's usually a proprietary, active/passive approach.

On the core side of Cisco's virtual switch, devices also use MEC connections to attach to the virtual switch. This eliminates the need for redundancy protocols such as HSRP or VRRP, and also reduces the number of routes advertised. As on the access side, traffic flows through the MEC in an "active/active" pattern until a failure, after which the MEC continues to operate with fewer elements.

The previous examples focused on distribution-layer switches, but VSL links work between any two Catalyst 6500 chassis. For example, virtual switching can be used at both core and distribution layers, or at the core, distribution and access layers. All attached devices would see one logical device wherever a virtual switch exists.

A VSL works only between two chassis, but it can support up to eight physical links. Multiple VSL links can be established using any combination of interfaces on the new supervisor card or Cisco's WS-6708 10G Ethernet line card. VSS also requires line cards in Cisco's 67xx series, such as the 6724 and 6748 10/100/1000 modules or the 6704 or 6708 10G Ethernet modules. Cisco says VSL control traffic uses less than 5 percent of a 10G Ethernet link, but we did not verify this.

At least for now, VSL traffic is proprietary. It isn't possible to set up a VSL between, say, a Cisco and Foundry switch.