Re: mac-level bridges and internet addresses
Tue, 12 Apr 88 15:57:11 PDT

>We have an Ethernet here with hosts on both
> and without (many) problems.

One must be very careful when running multiple IP networks on the same
cable. For example, it makes it much more likely that a misbehaving
gateway will cause a broadcast storm aka "meltdown." Typical example:
suppose that is 8-8 subnetted, and that subnet is
on the shared cable. Suppose also that both IP networks have gateways to
the arpanet or something.

Suppose some host on sends an IP broadcast (as an Ethernet
broadcast) to its subnet broadcast address, say to A
gateway between and the arpanet, say (but NOT also
configured to be on, receives the packet, notes "this is a
packet for host 3.255 on net" (please observe that since
this gateway does not itself have any interfaces on, it
doesn't know what the subnet mask of that network is and hence can't
recognize this as a subnet broadcast), sends it to a gateway on, who explodes it onto the cable. The first gateway gets
another copy, forwards that packet, etc. As Chuck Hedrick noted in a
recent message to tcp-ip, it is essential that gateways pay attention
to whether the packet arrived as a media broadcast; unfortunately, many
gateways (e.g. 4.3BSD) do not. A fortiori, unless you're willing to
also have gateways drop ALL packets sent as Ethernet broadcasts (which
I am not), or are willing to insist that any gateway on a physical
network know about all the nets on that cable (which I don't see how
you can do), you must also forbid topologies such as the one above.

This archive was generated by hypermail 2.0b3 on Thu Mar 09 2000 - 14:41:55 GMT