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We have two catalyst 2960s and four linux servers that we would like to connect to both switches (each server having one link to each switch.) Ideally, we are looking for seamless failover in case a single switch goes offline.

We have already presued etherchannels as a possible solution, but apparently the 2960s don't support cross-switch etherchannels.

Is there something here that we've completely overlooked, or is something like this just beyond the capabilities of the 2960s?

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up vote 1 down vote accepted

Presumably your 4 Linux servers have at least two nics? Do they actually need the combined bandwidth of both nics to handle their normal load? If not then some of the bonding modes provide for redundancy without using etherchannel.

Anway, see the bonding document it provides lots of several example HA situations depending on what you need, and what your network hardware is capable of.

Excerpt from bonding.txt

11.2 High Availability in a Multiple Switch Topology

    Below is a sample network, configured to maximize the
availability of the network:

                |                                     |
                |port3                           port3|
          +-----+----+                          +-----+----+
          |          |port2       ISL      port2|          |
          | switch A +--------------------------+ switch B |
          |          |                          |          |
          +-----+----+                          +-----++---+
                |port1                           port1|
                |             +-------+               |
                +-------------+ host1 +---------------+
                         eth0 +-------+ eth1

    In this configuration, there is a link between the two
switches (ISL, or inter switch link), and multiple ports connecting to
the outside world ("port3" on each switch).  There is no technical
reason that this could not be extended to a third switch.

11.2.1 HA Bonding Mode Selection for Multiple Switch Topology

    In a topology such as the example above, the active-backup and
broadcast modes are the only useful bonding modes when optimizing for
availability; the other modes require all links to terminate on the
same peer for them to behave rationally.

active-backup: This is generally the preferred mode, particularly if
    the switches have an ISL and play together well.  If the
    network configuration is such that one switch is specifically
    a backup switch (e.g., has lower capacity, higher cost, etc),
    then the primary option can be used to insure that the
    preferred link is always used when it is available.

broadcast: This mode is really a special purpose mode, and is suitable
    only for very specific needs.  For example, if the two
    switches are not connected (no ISL), and the networks beyond
    them are totally independent.  In this case, if it is
    necessary for some specific one-way traffic to reach both
    independent networks, then the broadcast mode may be suitable.
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Yes, each server has 2 nic cards. And no, we're really just looking for failover instead of combined bandwidth. I'll definitely take a look at the bonding document though, see if i missed anything. – user24171 Oct 27 '09 at 18:23
updated the link to a more recent version. Please see section 11.2. It seems like you probably just wand a active-backup mode. – Zoredache Oct 27 '09 at 18:28

If I'm not mistaken, this is really a function of the NIC\driver in the server and not the switch. It sounds like you're wanting to set up a SFT (switch fault tolerance) configuration on each server. I've never seen this done at the switch level, only at the server level. what NIC's do the servers have and do they support teaming?

As far as I'm aware etherchannel (and link aggregation) have to do with aggregating the bandwidth provided via binding multiple adapters together, but provide no fault tolerance in the sense that you're looking for.

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etherchannel provides fault tolerance. At least in my experiments, if I disconnect on of the network in a channel group the other interfaces continue to operate. – Zoredache Oct 27 '09 at 18:34
Etherchannel (more generically link aggregation) does provide fault tolerance, but its primary function is to increase throughput by combining multiple physical links into one logical link. – Murali Suriar Oct 27 '09 at 23:48

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