back to back bridges

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Date: Wed, 1 Jun 2005 16:14:52 -0500
To: freebsd-isp@freebsd.org

I've run into a bridge problem that has got me stumped. Here's the basic
layout:

                   AP 2km 802.11a link client
local net <---sis0|--|ath0<---------------------->ath0|--|sis0---> remote net

The "local net" consists of a Soekris 4526 with a 802.11a card running in
hostap mode and other access points (non-FreeBSD) as well as a router out to
the Internet connected to an ethernet switch. The "remote net" consists of
nothing more than 2 other APs (non-FreeBSD) and a hub. The client box
(Soekris 4526 with 802.11a card) associates to the AP and passes traffic
back and forth just great, with the exception of occational duplicated packets
which doesn't seem to harm things. BOTH the AP and the client are running
the kernel bridge code. This seems to perform just great at the AP end,
however, this is not so at the client end. Through some experimentation, I
have discovered the following:

* On the client, the IP address *must* be on the ath0 interface, otherwise
  the client is unpingable and otherwise invisible to the AP and the "local
  net". Likewise, the IP address *must* be on the sis0 interface on the AP
  or no packets ever seem to make it to the wireless link.
* Packets never seem to cross the wireless link from the local net to the
  remote net, with the exception to the IP address bound to the ath0
  interface on the client. Similarly, packets originating from the remote
  net never seem to cross the wireless link unless they come from the IP
  address on the ath0 interface.

Doing some sniffing on both networks, it would seem that ARPs from one net
are never forwarded across the wireless link to the other net. Why would this
occur? Is there some sort of odd magic involved with two bridged ethernets
sharing a common wireless link?

-- 
Jason T. Nelson <jtn@jtn.cx>                     http://www.jtn.cx/~jtn/
GPG key fingerprint = 6272 5482 EDDD D0A3 FED2  262A FABB 599D FF67 6C9E
disclaimer: My opinions are my own. Don't bother my employer about them.

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