AGIS Route Flaps Interrupting its Peering?

Sean Doran smd at
Fri Jul 5 15:41:36 UTC 1996

| These NetEdges seem to have three different possible operating states:
| completely working (which doesn't happen often enough); broken (often, right
| out of the box); and kind of working (which happens all too often).  

That these things work at all under load is nothing short of miraculous.

There are plenty of mixed-media-bridging devices that have been tried
and which have failed miserably in the past, most notably:

	-- Magnum 100s (ethernet<->funny framing<->ethernet),
		which don't meet the standard IFG specification;
		jam too many back-to-back packets at them, and it will lose
		most of a burst of traffic.  This hurt the old MAE-EAST.
	-- various ADSUs (FR<->ATM<->FR),
		which lack buffering and perform SAR too slowly,
		which delayed the PAC*Bell and Ameritech NAPs for months
		and some of which still exhibit flakiness under load
	-- a horrible idea (ethernet<->MFS ATM<->ethernet)
		which didn't work under load; perhaps a veteran
		user of these neato little things could explain
		the failure mode to anyone interested.  I remember
		one service provider who had a national "10Mbps" 
		ethernet backbone who had various horrible problems
		including the breakdown of the LIS (such that some
		routers couldn't talk to others), connections
		going simplex, frame loss and other wonderful things
	-- another horrible idea (FR<->MFS ATM<->FR)
		this was pretty neat; some of the problems above
		plus a brand new problem: the ADSU would strip the
		FR frame checksum, perform SAR, send the cells, and
		the ADSU on the opposite end would reassemble the
		frame and produce a correct FR checksum.  All fine
		and dandy, unless cells arrived out of order, SAR
		was done wrong, or there was data corruption under
		load in the DSU or in the network.   This interesting
		technology advanced the state of IS:IS in one vendor's
		software rather considerably
	-- mixed-media bridging (NetEdges, FDDI/Ethernet bridges)
		these break in all sorts of interesting ways.
		In particular, NetEdges have an annoying habit of
		confusing FDDI stations in particularly toxic ways,
		and some FDDI/Ethernet bridges resemble roach motels:
		frames check in but they don't check out.   

Essentially, most of these things worked mostly perfectly under
low load, but when faced with the kind of traffic one sees at a
busy exchange point, most bridging technology has failed in really
awkward ways.

My advice is that if you can avoid talking to something across
a bridge at an exchange point, you should do so.   The keep-it-simple
principle is a bunch more expensive, but probably not as expensive 
as a very public failure.

Finally, why is it that most vendors never test their products in
a serious battlefield environment like an ISP of size medium to huge?
These places tend to be excellent worst-case testing grounds.


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