Virtualization, long a hot topic for servers, has entered the networking realm. With the introduction of a new management blade for its Catalyst 6500 switches, Ciscocan 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 layer-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 maximize uptime, 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). (Compare switch redundancy features in the Network World’s Switch Buyer’s Guide.) 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% 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 virtualize 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. (See Nortel CTO’s take on this comparison.) At the heart of VSS is the Virtual Switching Supervisor 720-10G, a management and switch fabric blade for Cisco Catalyst 6500 switches. VSS — which began shipping last month — 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% 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.

A big swath of fabric

We assessed VSS performance with tests focused of fabric bandwidth and delay, failover times, and unicast/multicast performance across a network backbone.

In the fabric tests we sought to answer two simple questions: How fast does VSS move frames, and how long does it hang on to each frame? The setup for this test was anything but simple. We attached Spirent TestCenter analyzer/generator modules to 130 10G Ethernet ports on two Catalyst 6509 chassis configured as one virtual switch.

These tests produced, by far, the highest throughput we’ve ever measured from a single (logical) device. When forwarding 64-byte frames, Cisco’s virtual switch moved traffic at more than 770 million frames per second. We then ran the same test on a single switch, without virtualization, and measured throughput of 385 million frames per second — exactly half the result of the two fabrics combined in the virtual switch. These results prove there’s no penalty for combining switch fabrics.

We also measured VSS throughput for 256-byte frames (close to the average Internet frame length) of 287 million frames per second and for 1,518-byte frames (until recently, the maximum in Ethernet, and still the top end on most production networks) of 53 million frames per second. With both frame sizes, throughput was exactly double that of the single-switch case.

The 1,518-byte frames per second number represents throughput of nearly 648Gbps. This is only around half the theoretical maximum rate possible with 130 10G Ethernet ports. The limiting factor is the Supervisor 720 switch fabric, which can’t send line-rate traffic to all 66 10G ports in each fully loaded chassis. VSS doubles fabric capacity by combining two switches, but it doesn’t extend the capacity of the fabric card in either physical switch.

We also measured delay for all three frame sizes. With a 10% intended load, Spirent TestCenter reported average delays ranging from 12 to 17 microseconds, both with and without virtual switching. These numbers are similar to those for other 10G switches we’ve tested, and far below the point where they’d affect performance of any application. Even the maximum delays of around 66 microsec with virtual switching again are too low to slow down any application, especially considering Internet round-trip delays often run into the tens of milliseconds.

Faster failovers

Our failover tests produced another record: The fastest recovery from an Layer 2/Layer 3 network failure we’ve ever measured.

We began these tests with a conventional setup: Rapid spanning tree at layer 2, HSRP at layer 3, and 16,000 hosts (emulated on Spirent TestCenter) sending traffic across redundant pairs of access, distribution and core switches. During the test, we cut off power to one of the distribution switches, forcing all redundancy mechanisms and routing protocols to reconverge. Recovery took 6.883 seconds in this setup.

Then we reran the same test two more times with VSS enabled. This time convergence occurred much faster. It took the network just 322 millisec to converge with virtual switching on the distribution switches, and 341 millisec to converge with virtual switching on the core and distribution switches. Both numbers represent better than 20fold improvements over the usual redundancy mechanisms.

A bigger backbone

Our final tests measured backbone performance using a complex enterprise traffic pattern involving 176,000 unicast routes, more than 10,000 multicast routes, and more than 5.6 billion flows. We ran these tests with unicast traffic alone and a combination of unicast and multicast flows, and again compared results with and without VSS in place.

Just to keep things interesting, we ran all tests with a 10,000-entry access control list in place, and also configured switches to re-mark all packets’ diff-serv code point (DSCP) fields. Re-marking DSCPs prevents users from unauthorized “promotion” of their packets to receive higher-priority treatment. In addition, we enabled NetFlow tracking for all test traffic.

Throughput in all the backbone cases was exactly double with virtual switching than without it. This was true for both unicast and mixed-class throughput tests, and also true regardless of whether we enabled virtual switching on distribution switches alone, or on both the core and distribution switches. These results clearly show the advantages of an “active/active” design over an “active/passive” one.

We measured delay as well as throughput in these tests. Ideally, we’d expect to see little difference between test cases with and without virtual switching, and between cases with virtual switching at one or two layers in the network. When it came to average delay, that’s pretty much how things looked. Delays across three pairs of physical switches ranged from around 26 to 90 microsec in all test cases, well below the point where applications would notice.

Maximum delays did vary somewhat with virtual switching enabled, but not by a margin that would affect application performance. Curiously, maximum delay increased the most for 256-byte frames, with fourfold increases over results without virtual switching. The actual amounts were always well less than 1 millisec, and also unlikely to affect application performance.

Cisco’s VSS is a significant advancement in the state of the switching art. It dramatically improves availability with much faster recovery times, while simultaneously providing a big boost in bandwidth.

How we tested Cisco’s VSS

We assessed VSS performance with three sets of tests measuring fabric bandwidth and delay, failover times, and unicast/multicast performance across a network backbone.

For all tests described here, we configured a 10,000-line access control list (ACL) covering layer-3 and layer-4 criteria and spot-checked that random entries in the ACL blocked traffic as intended. As a safeguard against users making unauthorized changes, Cisco engineers also configured access and core switches to re-mark the diff-serve code point (DSCP) in every packet, and we verified re-marking using counters in the Spirent TestCenter traffic generator/analyzer. Cisco also enabled NetFlow traffic monitoring for all test traffic. To assess the fabric bandwidth and delay, the system under test was one pair of Cisco Catalyst 6509-E switches. Cisco engineers set up a virtual switch link (VSL) between the switches, each equipped with eight WS6408 10G Ethernet line cards and one Virtual Switching Supervisor 720-10G management/switch fabric card. That left a total of 130 10G Ethernet test ports: Eight on each of the line cards, plus one on each of the management cards (we used the management card’s other 10G Ethernet port to set up the virtual link between switches).

Using the Spirent TestCenter traffic generator/analyzer, we offered 64-, 256- and 1518-byte IPv4 unicast frames on each of the 130 10G test ports to determine throughput and delay. We measured delay at 10% of line rate, consistent with our practice in previous 10G Ethernet switch tests. The Spirent TestCenter analyzer emulated 100 unique hosts on each port, making for 13,000 total hosts.

In the failover tests, the goal was to compare VSS recovery time upon loss of a switch with recovery using older redundancy mechanisms.

This test involved three pairs of Catalyst 6509 switches, representing the core, distribution and access layers of an enterprise network. We ran the failover tests in three configurations. In the first scenario, we used legacy redundancy mechanisms such as rapid spanning tree and hot standby routing protocol (HSRP). Then we ran two failover scenarios using VSS, first with a virtual link on the distribution switches alone, and again with VSS links on both the distribution and core switches.

For each test, we began by offering traffic to each of 16 interfaces on the core and access sides of the test bed. We began the failover tests with a baseline event to verify no frame loss existed. While Spirent TestCenter offered test traffic for 300 seconds, we cut off power to one of the distribution switches. Because we offered traffic to each interface at a rate of 100,000 frames per second, each dropped frame represented 10 microsec of recovery time. So, for example, if Spirent TestCenter reported 32,000 lost frames, then failover time was 320 millisec.

The backbone performance tests used a setup similar to the VSS configurations in the failover tests. Here again, there were three pairs of Catalyst 6509 switches, representing core, distribution and access layers of an enterprise network. Here again, we also conducted separate tests with a virtual link on the distribution switches, and again with virtual links on the distribution and core switches.

To represent enterprise conditions, we set up very large numbers of routes, hosts and flows in these tests. From the core side, we configured Open Shortest Path First to advertise 176,000 unique routes. On the access side, we set up four virtual LANs (VLAN), each with 250 hosts, on each of 16 ports, for 16,000 hosts total. In terms of multicast traffic setup, one host in each access-side VLAN joined each of 40 groups, each of which had 16 transmitters; with 16 core-side interfaces. In all, this test represented more than 10,000 multicast routes, and more than 5.6 billion unique unicast flows.

In the backbone tests, we used a partially meshed traffic pattern to measure system throughput and delay. As defined in RFC 2285, a partial mesh pattern is one in which ports on both sides of the test bed exchange traffic with one another, but not among themselves. In this case, that meant all access ports exchanged traffic with all core ports, and vice-versa.

We tested all four combinations of unicast, mixed multicast/unicast, and virtual switching enabled and disabled on the core switches (virtual switching was always enabled on the distribution switches and always disabled on the access switches). In all four backbone test setups, we measured throughput and delay.

We conducted these tests in an engineering lab at Cisco’s campus in San Jose. This is a departure from our normal procedure of testing in our own labs or at a neutral third-party facility. The change was borne of logistical necessity: Cisco’s lab was the only one available within the allotted timeframe with sufficient 10G Ethernet test ports and electrical power to conduct this test. Network Test and Spirent engineers conducted all tests and verified configurations of both switches and test instruments, just as we would in any test. The results presented here would be the same regardless of where the test was conducted.

Newman is president of Network Test, an independent test lab in Westlake Village, Calif. He can be reached at

Newman is also a member of the Network World Lab Alliance, a cooperative of the premier reviewers in the network industry each bringing to bear years of practical experience on every review. For more Lab Alliance information, including what it takes to become a member, go to

Thanks to Spirent Communications for its support of this project. Spirent test engineer Brooks Hickman provided on-site configuration and troubleshooting assistance for these tests.


Share on LinkedIn Share with Google+