[c-nsp] Multiple inline devices

[Christopher.Marget at usc-bt.com]

Multiple inline devices gets sketchy very quickly.

During device failure, some boxes turn into crossover cables, others into straight cables.

If you've got a 100Mb/s device that doesn't do MDI/MDI-X, how do you cable it up correctly so that link can be established regardless of which devices are online?

Then there's the port-bounce issue:  Inline device vendors insist that they can't interrupt your network, but generally they mean that a hardware watchdog will close some relays when things go south.  The connected devices experience a link bounce.  How long will it take you to re-learn all of the routes that just got purged?

It's not pretty.

For WAN accelerators, WCCP is an attractive option.

For devices that only make sense in-line, Network Critical's V-Line tap series looks interesting.  I've never used it, but am contemplating it for an upcoming rollout.

The tap continually tests the inline device with artificial traffic, and routes around the inline device if the heartbeats fail.  No port bounce to the end systems, provided the V-Line tap stays up.  You can daisy-chain several in a single chassis.  Sadly, they can't do both the V-Line function *and* provide a copy of tapped data to the chassis backplane.

The website has a nice animation detailing the behavior.

/chris