Storage/data convergence: Cisco stands alone

Most vendors have at least some virtualization features, including support for virtual switches

Data center switches offer the promise of combining previously separate data and storage networks, thus reducing capital expense costs. However, the switches tested take different approaches to convergence.

Until the introduction of 10G Ethernet switches, data centers usually contained at least two separate networks: Fibre Channel for storage and Ethernet for data traffic. Faster Ethernet switches make it possible to combine traffic, perhaps by encapsulating storage traffic as FCoE or other means. But just shoving encapsulated Fibre Channel through an Ethernet port may not work well.

Fibre Channel and FCoE have stringent loss and delay requirements. Both use a credit-based system in which loss should never occur. Contrast that with Ethernet, which is stateless and doesn't attempt to retransmit lost frames. Further, Fibre Channel and FCoE require deterministic (predictable) latency and jitter; Ethernet doesn't.

To accommodate storage traffic on Ethernet, the IEEE has developed new mechanisms that aim to deliver Fibre Channel-like service levels over Ethernet. The most important of these is priority-based flow control (PFC), described in IEEE 802.1bb and supported by the Blade, Cisco and Dell switches. PFC extends Ethernet flow control to work on a per-VLAN priority basis instead of a per-port basis. One or more of those priorities can carry storage traffic.

A related mechanism is 802.1Qau congestion notification, which tells a given traffic class's transmitters to back off as queues fill. And yet another specification is DCBX, the data center bridging extensions to the logical link discovery protocol (LLDP), supported in the Blade and Cisco switches for exchanging capabilities information such as PFC support.

We didn't assess Fibre Channel or FCoE performance in this test because only Cisco's Nexus 5010 fully supports these protocols. If native support of Fibre Channel and/or FCoE are key requirements, the Nexus is the only choice in this test. (For the record, Brocade also makes a top-of-rack switch with full FCoE support, but the vendor declined to submit it for testing.)

Virtualization support

Virtualization is probably the biggest differentiator between data center and plain-vanilla Ethernet switches. Most vendors have at least some virtualization features, including support for virtual switches.

Arista's vEOS is a virtual extension of the vendor's Linux-based operating system that ties into VMware's Vsphere management platform. It allows any switch, physical or virtual, to be managed using the same image with the same capabilities.

The Blade, Cisco and Extreme switches all keep track of attached virtual machines and migrate associated policies – such as VLAN IDs and access control lists – as the VM moves to a new host.

Blade and Cisco offer a virtual network interface card (NIC) capability, though they work in different ways. Blade's approach divides a single 10G server Ethernet connection into four virtual channels that appear as separate NICs.

Cisco Nexus switches use virtual NICs together with VN-Link and VNtag technologies to map VM policies to VMs. For Fibre Channel and FCoE connections, the Cisco Nexus switches also support mechanisms called N-port virtualization and N-port identifier virtualization to reduce the number of Fibre Channel domain IDs and map multiple IDs to a single port.

See next part: Management and usability: Extreme goes its own way

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Copyright © 2010 IDG Communications, Inc.

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