[cisco-voip] General QOS question
Bell, Joe
Joe_Bell at adp.com
Wed Jun 7 13:49:00 EDT 2006
Andy is right, you don't need desireable. An oversight on my part.
His config below is what we use for our newer switches, while the older,
non-IPT switches still need to be configured as a trunk because their
stupid. There were a number of questions that came up from this string,
so I'll try to answer a couple of them.
First, there was a question as to what switch port commands
automatically form a trunk and which doesn't. Here is a list:
---
If you enter access mode, the interface goes into permanent nontrunking
mode and negotiates to convert the link into a nontrunk link even if the
neighboring interface does not agree to the change.
If you enter trunk mode, the interface goes into permanent trunking mode
and negotiates to convert the link into a trunk link even if the
neighboring interface does not agree to the change.
If you enter dynamic auto mode, the interface converts the link to a
trunk link if the neighboring interface is set to trunk or desirable
mode. <-- So two autos do not form a trunk.
If you enter dynamic desirable mode, the interface becomes a trunk
interface if the neighboring interface is set to trunk, desirable, or
auto mode.
---
Second, some info on the quasi-nature of the trunk in a voice enabled
switch. The big reason why you do not want this to be a real trunk, is
because you don't want to run spanning tree for every VLAN, which is
what trunks do. The phone really is only concerned about 2 VLAN's
(voice and data) and one of which is untagged (from the phones point of
view) with 802.1q frames. Keeping that processing off the phone is
apparently a big hit. Second, the CAM table on a phone has room for 32
addresses, not nearly enough for a true trunk. If it acted like a real
trunk, it would be deleting and adding addresses all the time. The main
reason for a switch in the phone is tagging = QoS to protect voice from
the PC behind it. To that end, it's a very limited switch set and so
the newer switches do not form a true trunk. If it were a true trunk,
you could not run a sniffer on your PC behind the phone, and see traffic
to and from the phone as they are on two separate VLAN's.
There is a good discussion on the capabilities of the switch in the
phone in the Cisco IPT Troubleshooting handbook.
Hope this helps,
Joe
-----Original Message-----
From: cisco-voip-bounces at puck.nether.net
[mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of Clements, Andy
(E.ON IS UK)
Sent: Wednesday, June 07, 2006 8:31 AM
To: Voll, Scott; Chris Ellington; cisco-voip at puck.nether.net
Subject: Re: [cisco-voip] General QOS question
Hi,
I do not think you need the dynamic desirable as I have configured edge
switch interfaces as switch mode access with the switch voice vlan
command and that still sends the voice vlan in a dot1q trunk and the
access vlan as the native untagged vlan.
[Example]
switchport access vlan 505
switchport mode access
switchport voice vlan 405
I believe this "newer" configuration for voice is to allow the use of
802.1x as this wont work on a "trunked interface". Although I haven't
had experience of this.
By the way the configuration above is working for both Cisco and Nortel
IPT phones.
Regards Andy
-----Original Message-----
From: cisco-voip-bounces at puck.nether.net
[mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of Voll, Scott
Sent: Wednesday, June 07, 2006 3:33 PM
To: Chris Ellington; cisco-voip at puck.nether.net
Subject: Re: [cisco-voip] General QOS question
Chris--
Quasi -- to some degree; having a likeness to something; resembling;
having some resemblance; etc.
It's not really a trunking mode.
The desirable automagically goes into the config when you have both
switchport access and voice on the same interface. You don't have to
manually set it.
QoS at layer two is basically done by trusting the CoS, ToS, DSCP and
passing it along. IE>
interface FastEthernet0/6
switchport access vlan 2
switchport mode dynamic desirable
switchport voice vlan 110
no ip address
srr-queue bandwidth share 10 10 60 20
srr-queue bandwidth shape 10 0 0 0
priority-queue out
mls qos trust device cisco-phone
mls qos trust cos
no mdix auto
auto qos voip cisco-phone
spanning-tree portfast
Hope that helps.
Scott
-----Original Message-----
From: cisco-voip-bounces at puck.nether.net
[mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of Chris Ellington
Sent: Wednesday, June 07, 2006 4:11 AM
To: cisco-voip at puck.nether.net
Subject: Re: [cisco-voip] General QOS question
Do you have links on the quasi trunking mode with the phones? I've done
a
quick search and don't find it on the Cisco site (now that's a surprise
:D)
My understanding of the desirable command is that is will allow a trunk
to
form if the other side requests it (say two switches are connected). I
have
had trouble in the past with both sides wanting a desirable mode in that
neither will 'request' the trunk so it stays in an access mode (that's
been
a while ago though, so it's probably fixed).
Also, do you really save much in the way of overhead/processing by using
this quasi trunk? Let's say I have a Sup1A in a 6500 running 384 ports,
all
trunked to various voice vlans - does the extra processing really hurt?
I'm not trying to be difficult, rather just understand the options. I
also
have a team of people to convince; my next move is to go to the SRND and
take the recommendation from there since TAC seems to fall back to that
when
I have troubles.
Thanks!
chris
--
Christopher S. Ellington
CCIE #6814
Network Solutions, Inc.
(317) 566 8897
Chris.Ellington at nsi1.com
> Thanks for all of the replies. Joe has the answer to my question. I
> understand you can still do ToS without trunking but in order to have
> CoS then you need 802.1Q which has the 802.1p header. The 802.1p
header
> is where you find CoS. Without trunking, there is no CoS period. So
> basically, the newer switches/IOS look for it anyway even though the
> switch port is not set for trunking. The IP phone still does what we
> consider standard 802.1q trunking. The switch just sees the CoS
> regardless.
>
>
>
> I must say though that I have never seen the dynamic desirable command
> used in implementations I've seen. Maybe I just don't remember.
>
>
>
> Thanks again!
>
>
>
> Jason
>
>
>
> ________________________________
>
> From: Bell, Joe [mailto:Joe_Bell at adp.com]
> Sent: Tuesday, June 06, 2006 7:35 PM
> To: ash AD; Wydra, Jason; cisco-voip at puck.nether.net
> Subject: RE: [cisco-voip] General QOS question
>
>
>
> Actually, layer 2 QoS (a.k.a CoS settings) are in the 802.1q header.
> Layer 3, (DSCP or PHB bits), are the 5 in the IP Precedence field. If
> your not using 802.1q, your not using layer 2 CoS.
>
>
>
> The reason you see switch configs that are not set for full trunking
and
> instead have command sets like:
>
>
>
> Interface fa0/1
>
> switchport dynamic desirable <-- negotiates a quasi-trunk
>
> switchport voice vlan 100
>
> switchport access vlan 200
>
> spanning-tree portfast
>
>
>
>
>
> Is because the newer voice enabled switches are smart enough to
> understand the voice vlan information and respect the tags coming down
> from the phone, and there are still 802.1q tags, even though you have
> not configured a true trunk. As I said, the phone does insert an
802.1q
> header and tags appropriately, but the port is not in a true trunking
> mode. The reason for this, is the new config keeps a lot of trunking
> management protocols off the port and frees it up for voip traffic =
> efficiency.
>
>
>
> Older switches were configured as full trunks and you might still see
> that out there.
>
>
>
> interface FastEthernet0/1 <-- true trunking config
>
> switchport trunk encapsulation dot1q
>
> switchport trunk native vlan 100 <-- data and untagged vlan
>
> switchport mode trunk
>
> switchport voice vlan 200 <-- tagged 802.1q header with the phone
> inserting layer 2 CoS and layer 3 DSCP tags
>
> spanning-tree portfast
>
>
>
> These configs accomplish the same thing, but the "newer" switches
config
> is much more efficient.
>
>
>
> Joe
>
>
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