[Bridge] linux bridge does not forward arp reply back packets in a vmware vm
Stephen Hemminger
stephen at networkplumber.org
Fri Dec 15 15:55:12 UTC 2017
On Fri, 15 Dec 2017 15:37:39 +0200
Adrian P <adrian27oradea at gmail.com> wrote:
> 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
Does each tap instance and the ens160 have a different and valid Ethernet
address? Also make sure the these are in the bridge forwarding table.
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