Re: danger of bridges


tsuchiya@mitre-gateway.arpa
Thu, 26 Mar 87 08:54:54 EST


> Date: Tue, 24 Mar 87 22:45:50 est
> From: ddp#@andrew.cmu.edu (Drew Daniel Perkins)
> To: tcp-ip@sri-nic.arpa
> Subject: danger of bridges
> Status: R
>
>
>
> And then there's DECnet. I won't claim to be a DECnet expert, but from my
> observations it appears to me that all Phase IV DECnet hosts connected to an
> ethernet transmit HELLO multicast messages every 15 seconds. These of course
> all pass through the bridge or else intra-area routing wouldn't work. We
> have somewhere around 100 DECnet hosts connected to our backbone ethernet
> system. Dividing these two numbers I expect to see about 6 HELLO's a second
> on the net. Using PCIP NETWATCH I indeed measured 5 per second. Of course,
> this is with only 100 hosts. Doing the same calculation with 1000 hosts one
> would see 66 HELLO's/sec. 2000 hosts would yield 133/sec, 4000 hosts would
> give 266/sec. Can you imagine EVERY DECnet machine on a network processing
> 266 routing packets/sec? I sure wouldn't want to try to get work done on
> such a machine.
>
> To summarize, level 2 bridges are very useful, but you have realize that they
> are not the perfect solution. You have to keep their limitations in mind.
> There are very good reasons for having level 3 routing.
>
> Drew
>
>
>

I should hope that the situation is not this bad, especially since x3s33
ES-IS (host-gateway) routing protocol uses a similar HELLO scheme. Although
I am also not a DECNET expert (or even novice, for that matter), I'm would
be EXTREMELY suprized if the HELLO timer was not settable, meaning that
for crowded networks, it could and I assume normally would be set
to greater than 15 seconds. Second, host HELLO's typically go to gateways,
not other hosts, so every host doesn't need to process every host
HELLO, just gateway HELLO's. There should be much fewer gateways than
hosts.

Paul (no I don't work for DEC) Tsuchiya



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