[CentOS] VLAN tagging problems

Les Mikesell lesmikesell at gmail.com
Fri Oct 28 16:49:50 UTC 2005

On Fri, 2005-10-28 at 11:14, Robin Mordasiewicz wrote:

> >> We are using Centos behind an F5 Bigip load balancer.
> >> The linux box is using bonding and tagged VLAN's
> >>
> >> Everything works fine except that when traffic is forwarded from the BigIP
> >> to the linux box on the VLAN where the web server is running the linux box
> >> returns the traffic on the wrong VLAN, It returns traffic on the lowest
> >> ordered VLAN.
> >>
> >> ie. here is a tcpdump on my load balancer showing traffic being sent on
> >> VLAN 911 to the linux box, but the linux box returns traffic on VLAN 902.
> >> The linux box is returning traffic on the same VLAN as its configured
> >> default gateway. If I change the default gateway to be on the VLAN 911
> >> then everytyhing works.
> >
> > It seems reasonable to require a route to the destination on the
> > VLAN used.  Why should it ever do otherwise?  What are you trying
> > to accomplish by using a VLAN interface with no route back?
> Is there any way to say that if traffic is recieved on VLAN#911 to be sure 
> that the return traffic is tagged with the same vlan id. Currently traffic 
> is tagged based on the routing table, and even if traffic comes in on 
> VLAN#911, when it returns the traffc it uses the VLAN tag from the network 
> that the default gateway is on(VLAN#902).

The BigIP will do this sort of magic itself to save the time looking
up the return route, but it really is black magic in terms of
standard networking where asymmetrical routes are permitted and
expected.  The reply packet doesn't have much to connect it to the
one that came in and it's path is determined by the route to the
destination address.   That said, there may be some black magic
you can do with iptables and the ip_conntrack info or some sort
of policy based routing.

  Les Mikesell
    lesmikesell at gmail.com

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