From: ccie2be (ccie2be@nyc.rr.com)
Date: Thu Sep 04 2003 - 17:22:36 GMT-3
Hey boby,
Thanks for getting back to me. Your response helped me understand this
stuff much better. But, if you don't mind, something is still confusing me.
You point out that the port is an access port which in this case is in vlan
10 and therefore data traffic isn't tagged or encapsulated in any way - it's
just a normal frame. Therefore, when the switch receives a frame on that
port, I assume that it handles it the same way that any normal switch would
ie it looks at the mac addr, does a table look up and makes a forwarding
decision.
But, how does the switch know whether the frame is a normal data frame or a
frame carrying voice? It seems to me, based on what you said, that the
switch would have to look inside each frame it gets to see whether or not
there's an 802.1q tag in there after the mac address fields.
Also, the switch would have to tag frames leaving the port carying voice
traffic destined to the IP Phone unless those frames arrived at the switch
with the 802.1q tag already inside the frame.
Also, according to the 3550 config guide, by default, "the cos value is not
trusted for 802.1P or 802.1Q traffic. So, with just the command, switchport
voice vlan 110, does the switch overwrite the cos value set by the IP Phone
to the default for the port?
Thanks again. dt
----- Original Message -----
From: <boby2kusa@hotmail.com>
To: "ccie2be" <ccie2be@nyc.rr.com>; "Group Study" <ccielab@groupstudy.com>
Sent: Thursday, September 04, 2003 1:58 PM
Subject: Re: 3550 to IP Phone
>
> ----- Original Message -----
> From: "ccie2be" <ccie2be@nyc.rr.com>
> To: "Group Study" <ccielab@groupstudy.com>
> Sent: Thursday, September 04, 2003 8:35 AM
> Subject: 3550 to IP Phone
>
>
> > Hi all,
> >
> > I'm trying to understand what's going on between the 3550 and the IP
Phone
> > under different circumstances, but the more I look into this, the more
> > confused I get.
> >
> > Consider this example from the Cisco DQOS book, page 737 where a 3550 is
> > connected to an IP Phone which is connected to PC. It says, "Assuming
> that
> > the voice vlan has been configured for 110 and the data vlan has been
> > configured for 10, Example 10-34 shows the configurations necessary on
the
> > 3550."
> >
> > 10-34
> > CatIOS (config)# interface FastEthernet0/11
> > CatIOS (config)# switchport access vlan 10
> > CatIOS (config)# switchport voice vlan 110
> > CatIOS (config)# spanning-tree portfast
> >
> >
> > Then the book says, (I paraphrase) this configures a 802.1Q trunk from
the
> IP
> > phone which tags voice traffic in vlan 110 and untagged traffic in vlan
> 10.
> >
> > Question 1: Does the switchport voice vlan # command automatically
create
> the
> > 802.1q trunk? Or, did that have to be configured previously but that's
> shown
> > in this example?
> It says FROM the IP Phone, there is no mention that the port will do
> trunking, technically the IP Phone will use Dot1p for CoS and vlan tag.
The
> Ip Phones and the switch uses the CDP to id the voice vlan. The IP Phone
> will not encapsulate the data packet therefore when the data packet
reaches
> the port it will be assume by the switch that it's origin is from vlan 10
> since the port is an access port for vlan 10.
> >
> > Question 2: How can a port be both a trunk port and an access port at
the
> > same time? Aren't they mutually exclusive?
> It is not trunking - show int fa 0/1, you will see it's not trunking,
> however, it will know there is voice vlan and access vlan.
> >
> > Question 3: Isn't untagged traffic, by definition or by default, in the
> > native vlan? And, isn't the native vlan vlan 1 by default?
> True but the native vlan concept is only when the port is trunking.
> And, therefore,
> > traffic in vlan 10 is tagged as vlan 10 unless vlan 10 was previously
> > configured as the native vlan? Does it matter whether or not the data
> traffic
> > is carried in the native vlan?
> The port is an access port that belongs to vlan 10.
>
> >
> > Question 4: Would the above config work whether or not the native vlan
> was
> > manually set to 10?
> Yep, since it is not trunking it will not even care for the Native vlan.
> >
> > Question 5: Does any particular configuration need to be done on the IP
> Phone
> > to make it compatible with the above configuration on the switch? If
so,
> > what?
> The IP PHone uses CDP to id the voice vlan. No configuration needed other
> than what you configure on that CM such as extension, VM received LED,
> forwarding, etc.
> >
> > As you can tell from all the questions above, I'm really confused about
> all
> > this. I'm not even sure if I'm asking all the right questions, so I
would
> be
> > extremely appreciative to anyone who could clear out all the fog from my
> brain
> > on this topic.
> >
> > Thanks in advance, dt
> >
> >
> > _______________________________________________________________________
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