The trouble with trunking

By David Newman, Network World |  Networking

Gigabit Ethernet switch/routers offer blazing performance in basic configurations, but enabling a feature called link aggregation causes some devices' throughput to tank, latency to leap, failover to falter and quality of service to quit.

the major finding of an evaluation of these devices conducted by Network World Global Test Alliance member Network Test. Teaming with test equipment maker Spirent Communications, we pounded high-end switch/routers from Cisco, Extreme Networks and Nortel Networks with traffic representing more than 14,000 hosts - just the kind of loads these boxes might handle at the core of corporate backbone networks.

Foundry Networks and Avaya originally agreed to participate in our test, but failed to show up in the lab. Alcatel and Enterasys refused our invitation.

All these devices adequately move packets through one Gigabit Ethernet interface at a time. Gigabit Ethernet offers a fat pipe, but even that isn't enough for some enterprise networks.

Vendors say the way to burn through these bandwidth bottlenecks is link aggregation or "trunking," a means of combining up to eight interfaces to form a single, larger virtual pipe.

It's a good idea, but our results suggest there's still work to complete. In assessing basic trunking, failover and QoS capabilities, we found that link aggregation introduced as many problems as it solved. In the worst cases, configurations that worked flawlessly with single interfaces wouldn't work at all when we tried to repeat tests using trunking.

That's not to say our results were all bad news. Cisco's Catalyst 6509 handled most link aggregation tests far better than the other boxes. Some of the 6509's results with two trunks defined were better than with one. Our results suggest Cisco's device is up to the task of delivering vast amounts of bandwidth on corporate backbones, even with advanced features such as QoS enabled. While not all the 6509's numbers were perfect, the Cisco device easily earned the Network World Class award in this round of testing.

It wasn't as pretty a picture for the Extreme and Nortel switch/routers, which were plagued by performance gremlins in many trunking tests. The line-rate throughput normally seen in Extreme's devices plunged as low as 10% of line rate in some tests.

Nortel's boxes delivered generally satisfactory results in the tests completed, but couldn't complete other tests because features such as multiple concurrent trunks and Open Shortest Path First (OSPF) failover aren't supported.

First things first

We conducted four sets of tests: baselines, link aggregation, failover and QoS capabilities.

Baseline tests tell us about a device's basic packet-handling capabilities. In these baselines (see how we did it, page 58), we measured throughput, latency, jitter and packet sequencing.

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