Layer 2 LOOP
Hi all,
Does anyone know how a native Vlan mismatch in Cisco switch creates bridging loops?
Thank you for your Help
Regards
Nabs
Hi all,
Does anyone know how a native Vlan mismatch in Cisco switch creates bridging loops?
Thank you for your Help
Regards
Nabs
Comments
Native vlan carries administrative protocols like cdp, vtp and stp.
if you send spanning-tree in the navitve vlan 1 but the peer switch sends you spanning-tree bpdus in vlan 2 you wont expect those bpdus to come in that vlan
and then a bloking port could transition to forwarding creating the loop.
HTH
Santiago E
I tried labbing this up and could not confirm my theory... so I removed it to avoid confusion :-)
Hi Santiago and Nikola,
Thank you very much for your reply.
I have 2 Distrib Switches that connect Access Switch and sometimes the administrator of access switches do bad configuration of native vlan and It do a loop . So my question is: Do you know if is it possible to avoid this ?
Nabs
HTH
Santiago E
http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_50_se/configuration/guide/swstpopt.html
--
--------------------------
Santiago Enciso
CCNA/CCDA/CCNP
Hi,
I know all of these technics but they don't talk about how to avoid mismatch native vlan.
Nabs
As much as the problem is no longer theoretical but practical
, can you post some more info on the topology?
Are you positive the cause of the loop is a native vlan mismatch on the other side?
Have a look at this if you haven't already:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml
If
you use ISL trunk you wont have this problem because isl doesn’t implement the
concept of native vlan
:ccie[email protected]] En nombre de nikola
Enviado el: Viernes, 29 de Mayo de
2009 14:44
Para: [email protected]
Asunto: Re: [CCIE R&S] Layer 2
LOOP
As much
, can you post some
as the problem is no longer theoretical but practical
more info on the topology?
Are you
positive the cause of the loop is a native vlan mismatch on the other side?
Have a
look at this if you haven't already:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml
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Wouldn't tagging the native in dot1q have the same effect?
I only want to understand the theory of how a mismatch native vlan can cause a loop. But it's not my problem.
I have a real loop problem and I expose it to you:
spanning-tree on vlan 1 is not activate on SW1, SW2, SW3 and SW4.
SW1---------Trunk vlan 500--------------SW2
| |
Trunk vlan 500 Trunk vlan 500
| |
| |
SW3--------trunk vlan 100,500----------SW4
Fa0/1(access vlan 100) Fa0/1(access vlan 100)
| |
| |
| |
| |
SW5--------------------------------------SW6
The fa0/1 interface of sw3 and sw4 is in access vlan 100.
When I plug SW5 and SW6 with default cisco configuration and servers on them, i have a loop on sw1, sw2, sw3 and sw4. (I wait at about 2 hours and the CPU climb to 100 %)
I don't understant why???
When I configure the correct sw5 and sw6 with spanning-tree in vlan 100 (and trunk between sw5 and sw6) there is no problem.
Hi all,
I have find something about mismacth native vlan on the BCMSN cisco book:
page 2-92 "if there is a native VLAN mismatch on either side of an 802.1q link, Layer 2 loop may occur because VLAN 1 STP BPDUs are sent to the IEEE STP mac address 0180.c200.0000 untagged."
But I haven' find anything yet for my real problem.
Nabs
Hey all,
I have find the problem with a default stp configuration that can make a loop layer 2:
The problem is on duplex mismatch configuration. Look at this:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml#duplex
So becareful with cisco default configuration and your particular switch configuration.
Regards
Nabs