From: Jonathan V Hays (jhays@jtan.com)
Date: Wed Aug 27 2003 - 09:37:32 GMT-3
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