[afnog] QoS setting verification

[Brian Candler]

On Fri, Jul 21, 2006 at 09:02:28AM +0200, Antonio Godinho wrote:
> On the subject of QoS, I have setup a serial link between two sites of 128K 
> and using 1760 routers. I setup autoQoS for both sides and it seems to work 
> ok, the only problem I have noticed to which I have not found any solution 
> is that when in one of the sites the ethernet interface of the router is 
> bombarded with traffic, the voice channels drop the calls even though there 
> is no traffic going through the serial link. Any ideas anyone?

Under what circumstances does the router's ethernet port get bombarded with
traffic? Do you mean something like this?

              hub
              |||'----------router========>
         +----'|'----+
         |     |     |
        PC1   PC2   PC3

Clearly, here, if PC1 and PC2 are using up all the available ethernet
bandwidth, then PC3 will have to fight for bandwidth on the ethernet segment
to get to the router (and the router will have to fight for bandwidth when
sending data back to PC3). No QoS settings on the router will help here.
When the router has multiple packets waiting to go over the ethernet then
QoS lets it decide which is the most important packet to send first; but the
ethernet port still has to wait until the port is clear before sending.

Things are much better if the hub is replaced by a switch, because PC1 and
PC2 can exchange frames at the same time as PC3 sends frames to the router.
However it's still possible for the ethernet port of the router to be
'bombarded with traffic' as you describe it:

(1) if you have a broadcast storm

(2) if traffic has to hit the router for legitimate reasons, e.g. because it
has to pass through the router but not go down the serial link. Let's
imagine your network is divided into two ethernet subnets, where PC4 is on
the second subnet, and your router is forwarding traffic between the two
subnets as well as to the serial line:

            switch      *
              |||'----------router=========>
         +----'|'----+         |
         |     |     |         |
        PC1   PC2   PC3       PC4

Here, if PC1 is sending at top speed to PC4, the link between the switch and
the router (marked *) could be filled; so if PC3 then wants to send traffic
via the router to the serial line, it will find that link (*) is congested.

If this is the case, then you'll need to use a switch with QoS features too
(and note that it will be doing layer2 forwarding but probably needs to look
at layer3 info to decide which priority to choose)

If you have a multi-switch or multi-router network then this becomes
difficult to scale, as it gets difficult to deploy all the right ACLs
everywhere. At this point you should be thinking of ingress marking. That
is, each switch or router in the core of your network just looks at the IP
TOS bits and/or ethernet 802.1p bits to decide on priority. At the edges
where customers or users connect, you use ACLs to categorise the traffic and
replace the existing TOS bits in each frame with ones of your choice.

Regards,

Brian.