If your switching infrastructure needs something bigger than a standalone LAN switch but not quite as big (and expensive) as a modular chassis, you might want to consider stacking your switches. Cisco offers their StackWise feature on the Cisco Catalyst 3750 series switches, and StackWise Plus on Catalyst 3750-E series.

What StackWise basically does is build one big switch by intelligently joining several individual switches together (with a maximum of nice switches total). The switches are united into a single logical unit using special stack interconnect cables that create a bidirectional closed-loop path. The bidirectional path acts as a switch fabric for all the connected switches, where network topology and routing information is updated continuously through the stack. All stack members have full access to the stack interconnect bandwidth (32 Gb).

The stack is managed as a single unit by a master switch, which is elected from one of the stack member switches. Any stack member switch can become master, and election is done my one of these methods:

1. User priority-The network manager can select a switch to be master.
2. Hardware and software priority-This will default to the unit with the most extensive feature set.
3. Default configuration-If a switch has preexisting configuration information, it will take precedence over switches that have not been configured.
4. Uptime-The switch that has been running the longest is selected.
5. MAC address-Each switch reports its MAC address to all its neighbors for comparison. The switch with the lowest MAC address is selected.

When one master switch becomes inactive and while a new master is elected, the stack continues to function.

To build StackWise connections you need (at least two) of either of these cables:

• CAB-STACK-50CM= (0.5-meter cable)
• CAB-STACK-1M= (1-meter cable)
• CAB-STACK-3M= (3-meter cable)

The reason you need at least two cables (or more precisely: X + 1, where X is the number of switches in the stack) is that the stacking cables must form a ring, as the connections are bidirectional, and without the ring a failure of one of the cables would break the stack with all manner of nasty things happening to your network. In a correct configuration a stack cable failure will halve the capacity but the stack continues functioning, while after the failed cable starts functioning again full capacity is automatically restored.

The main differences between StackWise (supported on 3750 and 3750-E series) and StackWise plus (only supported on 3750-E series) are:

• StackWise plus performs destination stripping, where StackWise only supports source stripping. What this means is that with source stripping a unicast packet that is sent on the stack ring goes around the whole ring and is removed (i.e.: stripped) by the sender of the packet (i.e.: the source switch). With destination stripping the target switch of the packet removes it from the ring, freeing up bandwidth on the ring sooner.
• StackWise Plus supports local switching, where StackWise doesn’t, forcing all packets on the stack ring. On a StackWise Plus stack any traffic destined for a node connected to the same member switch stays within that member switch, and doesn’t use the stack ring at all.
• StackWise Plus will support up to 2 line rate 10 Gb Ethernet ports per switch.

You can build a stack consisting of a mix of 3750 and 3750-E switches, in that case only StackWise features are available with the exception of the local switching, the 3750-E’s will still do that.

All in all the StackWise feature is quite nice. Just as with RAID arrays you can build a resilient and easy to manage system by using relatively inexpensive devices. Management is almost as easy as managing a single switch, and most of the features that make StackWise worthwhile and in some respects even great are automatic and do their thing quietly and without fuss. Good stuff.