[Bridge] linux bridge does not forward arp reply back packets in a vmware vm

Adrian P adrian27oradea at gmail.com
Fri Dec 15 13:37:39 UTC 2017


Hello,

I have a strange issue with a linux bridge created by
openstack-neutron (pike release). This linux bridge is hosted in a
vmware VM running latest CentOS 7, with a single network interface in
promiscuous mode.

>From openstack neutron perspective, the networking setup is simple: a
single flat external provider network, with a single cirros VM
instance connected to it.

Therefore, in the linux bridge running in the vmware host, I have 3 interfaces:

# brctl show
bridge name     bridge id               STP enabled     interfaces
brq025a9a94-58          8000.005056a6b378       no              ens160
                                                        tap2eb4cad6-cd
   <----- neutron DHCP agent tap interface
                                                        tap6d31a191-9f
   <----- cirros VM instance tap interface

The ens160 is the "physical" CentOS 7 host interface, that is in
promiscuous mode.

The  tap2eb4cad6-cd tap interface is the neutron DHCP agent interface,
and the tap6d31a191-9f tap interface is used by the cirros VM
instance.

The problem is the following:

With a tcpdump, I am able to see the arp request (ARP, Request who-has
10.20.21.1 tell 10.20.21.233) going out from the cirros VM instance on
tap interface tap6d31a191-9f, and well as on the bridge itself
(brq025a9a94-58). However, the reply back to the arp request (Reply
10.20.21.1 is-at 00:17:08:c4:52:80) does not reach the cirros VM
instance anymore. With tcpdump, I am able to see the arp reply back
packets in the bridge (brq025a9a94-58), however they do not show up
anymore on the cirros VM instance tap interface tap6d31a191-9f.

To me it seems that for whatever reason, the bridge does not forward
the arp reply back packets to the cirros VM tap interface, and I do
not understand why. The strange thing is that after a while, for
apparently no reason, a single arp reply back packet gets through the
bridge and the tap interface, and the arp table gets updated with
correct IP address in the cirros VM instance. However, if I clean up
the arp table in the cirros VM instance, it takes again 10 to 15
minutes of continuously sending arp requests, until a single arp reply
back packets gets through.

I was banging my head to the table for a few days with this issue, and
finally, for apparent no reason, I manually configured the bridge max
aging time to 0, to convert it in a hub, and from that moment
everything started to work without any issue. Still, I do no
understand why is this happening, and obviously I cannot manually set
up the bridge aging time to 0 all the time in all the bridges
openstack neutron creates automatically.

Any thoughts?

Many thanks in advance.

Best regards,
Adrian


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