From: Roberts, Larry (Larry.Roberts@expanets.com)
Date: Wed Aug 27 2003 - 10:30:57 GMT-3
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
<snip>
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
This archive was generated by hypermail 2.1.4 : Tue Sep 02 2003 - 18:54:07 GMT-3