Nortel adds layers to its stackable switch line

Nortel's BayStack 5520 stackable switch offers better performance, increased link aggregation capacity, new Layer 3 forwarding and a 25% price drop compared with its predecessor (see here). However, this line of switches still has no routing protocol support and some management quirks that could detract from its overall usefulness.

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Nortel's BayStack 5520 stackable switch offers better performance, increased link aggregation capacity, new Layer 3 forwarding and a 25% price drop compared with its predecessor (see here). However, this line of switches still has no routing protocol support and some management quirks that could detract from its overall usefulness.

A single 5520 has 48 copper 10/100/1000M bit/sec ports and four small form-factor pluggable ports. The backplane supports up to eight switches in a stack with 40G bit/sec throughput into and out of each switch. The performance of both a single switch and a three-switch stack was acceptable, but not exceptional. (See How we did it .)

There was line rate performance at Layer 2 with a single switch with all packet sizes. But performance degraded somewhat as the packet size increased with Layer 3 switching. Here we saw line rate throughput for 64- to 512-byte packets, but throughput decreased to 82% of line rate as the packet size increased to 1,518 bytes.

Latency values for a single switch ranged from about 10 microsec in the best case (with 64-byte packets) to 200 microsec (with 1,518-byte packets) when the switch was near full capacity. This is most likely because of the increased overhead of the internal chipset that chops packets into smaller units and then switches them across the backplane.

With three switches, we pumped as much as 130G bit/sec (64-byte packets) and as little as 72G bit/sec (1,518-byte packets) through the stack in a full mesh configuration. Layer 3 forwarding performance decreased as the packet size increased, as well. This amount of stack bandwidth should be adequate for most enterprise network applications with the exception of a highly utilized core network. Latency measurements for the stack were similar to the single switch test results.

The benefit of Layer 3 switching - in spite of the slight performance hit - is that it allows an administrator to configure an IP subnet for each port and/or virtual LAN (VLAN ) in the switch. This is a great way to segregate network devices and minimize Ethernet broadcast traffic, but the only way to let the stack or other network devices know that other subnets exist is by configuring static routes. This can quickly become an administrative nightmare. Therefore, Layer 3 switching is best used when most routes are contiguous and can be summarized by a few addresses. Routing protocol support (which Nortel says is coming, but won't say when) would relieve this limitation.

The reboot times for the 5520 in stand-alone and stack configuration is acceptable at less than 90 seconds.

The 5520 has a rich set of QoS features such as 802.1p VLAN tagging priority and Differentiated Services . Each port can have as many as eight egress queues. The number of queues per port is configured globally, which requires a reboot. We tested 802.1p priority queuing on both a single switch and a three-switch stack. It prioritized both packet loss and latency of the various tagged flows through the switches.