reliably detecting the presence of a bridge?

[anoop at alumni.duke.edu (Anoop Ghanwani)]

If LLDP (link layer discovery protocol) is enabled, you could try using
that.  There is a system capabilities TLV in the LLDPDU sent by a system,
but I'm not sure how reliably it is filled in, especially if a device is
capable of both switching and routing.  The way LLDP is supposed to work is
a device will receive LLDPDUs from other devices immediately adjacent to
it.  It can then read the LLDP database of those devices (via management)
and figure out what those devices are connected to, and so on.

Otherwise, bridges are supposed to be "transparent," so there is no way to
know they are present by using user data frames.

Anoop