Hi All,
I'm having some
issues with my bridge reassembling fragmented packets, with disastrous
results.
I have a simple
bridge set up:
brctl addbr
br0
brctl addif br0 eth0
brctl addif br0
eth1
Simple enough. The
MTU on each interface is 1500, and so is the MTU on the bridge itself.
I have the bridge
connected something like this:
PC A----Switch
A---eth0 (bridge) eth1---Switch B---PC B.
The Switches are
adding VLAN headers and the like, but this seems to be irrelevant to the
problem.
If i ping from PC A
to PC B, all is OK. But, when I ping using a 1500-byte payload (or larger), the
ping doesn't get through. Removing the linux bridge and just going switch-switch
works OK.
Watching the
ethernet ports on the bridge indicate that the ping is entering the bridge on
eth0, but not coming out of eth1. A packet sniffer shows that the ping is being
fragmented by PC A, and two packets enter eth0. I then increased the MTU on
eth0, eth1 and br0 to 1600, to see if this was an MTU issue. I then see packets
coming out of eth1, but my switch is dropping them, because they are
oversized.
Connecting eth1
straight to a packet sniffer shows that when two packets enter eth0, the bridge
is reassembling them into a single, larger packet, which it then either drops
because it is larget than the MTU of eth1, or it passes a large packet (in this
case, 1560 bytes or so). Obviously, this packet is then dropped by other
equipment in the network, because it is too large for a proper ethernet
packet.
I am seeing this
behaviour with any IP packets, not just ICMP. The result is that anything which
has a large-ish payload is being dropped after it leaves the
bridge.
The question is,
then: How do I stop the bridge from reassembling fragmented
packets?
Regards,
Leigh
Leigh
Leigh Sharpe
Network Systems Engineer
Pacific Wireless
Ph +61 3 9584 8966
Mob 0408 009 502
Network Systems Engineer
Pacific Wireless
Ph +61 3 9584 8966
Mob 0408 009 502
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