From: Richard Foltz (ccie2b@rfoltz.com)
Date: Tue Sep 16 2003 - 21:15:12 GMT-3
No, thats one of the differences between ISL and dot1q. ISL doesnt use a
"native vlan" and dot1q does, basically, untagged. With an IP Phone the
switch tells the phone via cdp which vlan to tag voice frames with. the data
vlan is untagged, i.e. it is in the native vlan. Usually you set a switch
port thats on a voice enabled network into trunking mode "switchport mode
trunk" and set you native vlan to your data vlan. Thats why you can hook a
PC to a switchport set for trunking and it will be in the data vlan, becuase
it is sending untagged frames.
Richard Foltz, CCIE#8339
----- Original Message -----
From: "Hossam" <sam6626@yahoo.com>
To: "Roberts, Larry" <Larry.Roberts@expanets.com>; "'Jonathan V Hays'"
<jhays@jtan.com>; <ccielab@groupstudy.com>
Sent: Friday, August 29, 2003 9:47 AM
Subject: Forcing 802.1Q Trunk without using dot1q keyword ?
> Group,
> Can we force two switches to use the 802.1Q trunking without using the
dot1q keyword on any of them??
>
> As far as i know and as the output in the below message shows that ISL is
the default.
>
> So how can we change it?
>
> Thanks
>
> "Roberts, Larry" <Larry.Roberts@expanets.com> wrote:
> N-isl is "negotiated isl"
>
> If you think about it all ports have a native VLAN, its just the VLAN the
> port is a member of. If you have 2 switch's that are connected via a
> cross-over and one switch is all in VLAN10 and one switch is all in
VLAN20,
> and they were both configured with CDP disabled and switchport mode
access,
> they wouldn't realize that they are in different VLAN's and traffic would
> flow back and forth just fine. Now, turn on CDP, and suddenly, the
> configuration that worked fine starts generating errors.
>
> I don't recall whether ISL can pass untagged traffic or not. ISL is Cisco
> specific, and since that would imply that its cisco to cisco connections
> they could therefore dictate no traffic can be untagged.
>
> What would be interesting to see is if you can run ISL to a Cisco Phone? I
> would be curious as to how the non-voice vlan traffic is passed. That
might
> be a good way to test if isl supports untagged traffic. The native VLAN is
> what port the ethernet jack on the phone is a member of.
>
>
>
> Thanks
>
> Larry
>
> -----Original Message-----
> From: Jonathan V Hays [mailto:jhays@jtan.com]
> Sent: Wednesday, August 27, 2003 7:38 AM
> To: Roberts, Larry; ccielab@groupstudy.com
> Subject: RE: Cat3550: ISL native VLAN mismatch?
>
>
> Larry,
>
> Thanks for the clarification.
>
> No, you didn't miss my question but I implied (but was not clear) that the
> other side of the cross-connected FA0/20 link was unconfigured (in default
> mode of dynamic desirable). Thanks for the additional insight.
>
> *** ISL and N-ISL
>
> Here's another related question. After erasing the switch, here are the
> unconfigured lines on one side:
>
> S1_3550#sh in trunk
>
> Port Mode Encapsulation Status Native vlan
> Fa0/19 desirable n-isl trunking 1
> Fa0/20 desirable n-isl trunking 1
>
>
> Note that the encapsulation type is 'n-isl.'
>
> S1_3550(config)#int fa0/20
> S1_3550(config-if)#switchport trunk encapsulation isl
S1_3550(config-if)#^Z
> 15:39:12: %SYS-5-CONFIG_I: Configured from console by console S1_3550#sh
in
> trun
>
> Port Mode Encapsulation Status Native vlan
> Fa0/19 desirable n-isl trunking 1
> Fa0/20 desirable isl trunking 1
>
> Note that after specifically configuring the port for ISL, the 'n-isl'
label
> under the Encapsulation column changes to 'isl'.
>
> Q. What is 'n-isl' anyway? <====
>
>
> *** UNTAGGED FRAMES
>
> In my original question I wondered whether 'Native vlan' applied only to
> dot1q trunking. The 'sh interfaces trunk' implies that Native vlan applies
> to ISL also, although the Cisco documentation discusses Native vlan only
in
> a dot1q context (or did I miss a page?). Perhaps the Native vlan column in
> the 'sh in trunk' output does not apply to isl?
>
> I seem to recall that isl trunks do not allow untagged frames although I
> can't find a specific statement to that effect after a quick search of the
> Doc CD. But this excerpt from the 3550 Software Configuration Guide
implies
> that only dot1q allows untagged frames.
>
> http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12114ea1/3550s
> cg/swvlan.htm#1101186
>
> Configuring the Native VLAN for Untagged Traffic
> A trunk port configured with 802.1Q tagging can receive both tagged and
> untagged traffic. By default, the switch forwards untagged traffic in the
> native VLAN configured for the port. The native VLAN is VLAN 1 by default.
>
>
>
> Thanks,
>
> Jonathan
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Roberts, Larry
> Sent: Tuesday, August 26, 2003 11:32 PM
> To: 'Jonathan V Hays'; ccielab@groupstudy.com
> Subject: RE: Cat3550: ISL native VLAN mismatch?
>
>
> Just to add my .02
>
> First, the Native VLAN is 1 on all ports, unless otherwise specified.
(e.q.
> switchport access vlan 2)
>
> I also don't see Switch 1's port 20 configuration in your configs? Im
going
> to assume it looks like such
>
> interface FastEthernet0/20
> switchport mode access
> no ip address
> --> Notice the lack of "switchport access vlan 2" meaning it defaults to
> vlan 1
>
> With Trunk ports:
> The Native VLAN is what VLAN the switch will put any untagged packets it
> recieves into. While I don't remember where I read it, I also believe that
> any packets that are in that VLAN are sent between two trunk ports without
> being tagged. I suspect as a way to save BW, but that's a guess.
>
> This error is just the switch saying that " Hey, were are going to cross
> talk on VLAN's because your sending me untagged packets from your VLAN X,
> and I put them into my VLAN Y"
>
> Considering plain old access ports:
> On F0/20 on both routers, change the native vlan ( switchport access vlan
2
> ) so that they both match and the error will go away.
> Or , just turn off CDP. Since these are access ports, no VLAN info is
> carried in the packets. And the switchs wouldn't know that they each had a
> different native VLAN
>
> Make sense, or did I miss your question?
>
>
> Thanks
>
> Larry
>
>
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