[cisco-voip] General QOS question

[Chris Ellington]

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
> 
>