3Com  is targeting the enterprise backbone with its new Expandable Resilient Networking architecture, and our tests support 3Com's claim that its XRN interconnect technology combines high availability with excellent performance.

3Com, long a player in workgroup switching, is adding redundancy features to top-end models to compete with established enterprise backbone vendors, including Cisco, Extreme Networks and Foundry Networks. By using the XRN Interconnect Kit to aggregate multiple switches  as one virtual unit, 3Com says it is matching competitors' availability and performance - at a significantly lower price. For example, 3Com says two of its 4060 switches connected in an XRN stack would cost $1,061 per Gigabit Ethernet port, vs. $1,859 per port for Cisco's Catalyst 4500 and $1,914 per port for Extreme's Alpine 3800 in equivalent configurations.

Furthermore, 3Com says the stackable nature of the XRN approach lets users take a pay-as-you-go approach, purchasing new capacity as needed.

The basic idea behind XRN is to interconnect backbone switches so they offer full redundancy at Layer 2 and Layer 3. Our tests, in which we tested a 3Com SuperStack 4050 and SuperStack 4060 connected via the XRN Interconnect Kit, showed that this redundancy worked well, with subsecond failover in all cases. To create redundancy, users first connect core switches to create an XRN stack, and then dual-attach workgroup switches or computers to the stack. In a Layer 3 environment, the XRN stack appears as one device with a single IP and media access control (MAC) address, even though each element in the stack contains its own routing table. In testing, this design worked as intended. The XRN stack offered line-rate throughput for Layer 2 traffic and throughput equivalent to around 95% of line rate for Layer 3 traffic.

If one element in the XRN stack fails, the routing table of another element in the stack takes over. 3Com says it keeps the different routing tables synchronized through periodic triggered updates using a feature called Distributed Resilient Routing (DRR).

A major differentiator of the XRN approach is its use of active-active load sharing. Competitors' switches allow Layer 2 redundancy via spanning-tree bridging, but this is an active-passive approach. With spanning tree, one switch sits idle until a link, interface or switch fails. With XRN, more bandwidth is available because all switches in the XRN stack share the load until a failure occurs.