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RE: Native-VLAN vs. Switchport Access VLAN posted 09/30/2003
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Mmmm.... Good question.  I'm not at home at the moment, so I don't have
equipment accessible to test that at the moment.

My mind tells me that you may be correct in the logic IF the switch
keeps the trunk "thinking" and the access "thinking" separate.  If the
'native vlan' command combines the thinking of the two then no.

Bottom line...  It doesn't seem to be the most sane approach to it, I'd
do the dynamic mode command and configure access vlan and trunk encap
stuff....  But that doesn't necessarily make the other approach wrong,
just one I can't verify at the moment!

Anyone else around have equipment setup they can test with now?

Scott

-----Original Message-----
From: Kenneth Wygand [mailto:KWygand@xxxxxxxxxxxxxxxx] 
Sent: Tuesday, September 30, 2003 10:44 AM
To: swm@xxxxxxxxxx; ccielab@xxxxxxxxxxxxxx
Subject: RE: Native-VLAN vs. Switchport Access VLAN


Scott,

The phrasing that got me was "if the trunk encapsulation should
FAIL...".  As I understand it, the native vlan is only in effect when
the trunk is up and functioning properly.  If the trunk fails, I would
expect it to fall back to an access port (if its negotiating trunk
status).  If I simply set the native VLAN on the trunk to 568, but leave
the access-port config unconfigured (thus, vlan 1), if the trunk port
fails, wouldn't the port become VLAN 1 (if I haven't changed the vlan of
the access port configuration).  Then traffic from VLAN 568 would no
longer cross the link, correct?

Thanks in advance for the sanity check. :)

Kenneth E. Wygand 
Systems Engineer, Project Services
CISSP #37102, CCNP, CCDP, MCP 2000, CNA 5.1, Network+, A+ Custom
Computer Specialists, Inc. 
"It's not just about ending up where you want to be, it's about making
the most of the trip there." -Anonymous

-----Original Message-----
From: Scott Morris [mailto:swm@xxxxxxxxxx] 
Sent: Tuesday, September 30, 2003 10:40 AM
To: Kenneth Wygand; ccielab@xxxxxxxxxxxxxx
Subject: RE: Native-VLAN vs. Switchport Access VLAN

Assuming you mean dot1q.  :)

Otherwise, you're right.  If you do not specify a particular "switchport
mode" command, you can allow the desireable/auto/whatever configuration
to take effect.  And therefore move to trunk or move to access vlan will
be dependent on negotiation.

Use "switchport mode dynamic desireable" to allow negotiation.  Set your
trunk encapsulation then, and set your access vlan then.  If it can
trunk, it will, if not, it will use the access vlan specified.

If your trunk is set using the "switchport mode trunk" then it will move
everything in encapsulated dot1q trunk.  By changing the native vlan,
you are specifying which vlan frames will be sent untagged.  I haven't
actually specifically tried running this, but in theory (in my feeble
mind) it would work.  Because the trunk really just sends tagged or
untagged items.  And if something isn't tagged, it must be on that vlan.

I think your solution is the more sane of the two.  IMHO.

 
Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713,
CISSP, JNCIS, et al. IPExpert CCIE Program Manager IPExpert Sr.
Technical Instructor swm@xxxxxxxxxx/smorris@xxxxxxxxxxxx
http://www.ipexpert.net


-----Original Message-----
From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf Of
Kenneth Wygand
Sent: Tuesday, September 30, 2003 9:35 AM
To: ccielab@xxxxxxxxxxxxxx
Subject: Native-VLAN vs. Switchport Access VLAN


Hello group,



I have two (2) 3550's with an ISL trunk between them on FastEthernet0/10
(both sides).



I have a practice lab question that states "If the trunk encapsulation
should fail, ensure that VLAN 568 is still forwarded across the link".



My thought was to set the trunk mode to DESIRABLE on both sides and
include a "switchport access vlan 568" command on the interface in the
case when the trunk does not establish.  However, the answer key states
that the trunk is nailed down on both sides (switchport mode trunk) and
the native-vlan is set to 568 on both sides (switchport trunk native
vlan 568).



Which is right, or do both answers satisfy the requirements?



If these yield different results and a question like this is asked on
the lab exam, what question could we ask of the proctor to determine
which configuration is correct?



Thanks,



Kenneth E. Wygand
Systems Engineer, Project Services

CISSP #37102, CCNP, CCDP, MCP 2000, CNA 5.1, Network+, A+ Custom
Computer Specialists, Inc. 70 Suffolk Court Hauppauge, NY 11788
(631) 864-6699 x1755

"It's not just about ending up where you want to be, it's about making
the most of the trip there." -Anonymous

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