Hm. Oddly enough I'm getting different results than I have in the past with
this, IOS version maybe?
In the past I've seen CDP scream about the native VLAN mismatch but the
traffic still flowed over the trunks just fine. Tonight however I'm seeing
ports going into an STP inconsistent state. Here is my setup:
CAT2 is the root bridge for VLAN's 1,3,12,34,56
CAT1 has a single dot1q trunk back to CAT2 (Gig0/19)
CAT2
SW2(config-if)#do sh span | incl VLAN|root
VLAN0001
This bridge is the root
VLAN0003
This bridge is the root
VLAN0012
This bridge is the root
VLAN0034
This bridge is the root
VLAN0056
This bridge is the root
CAT1
SW1(config-if)#do sh run int giga0/19 | beg inter
interface GigabitEthernet0/19
switchport trunk encapsulation dot1q
switchport mode trunk
SW1(config-if)#do sh int trunk
Port Mode Encapsulation Status Native vlan
Gi0/19 on 802.1q trunking 1
Port Vlans allowed on trunk
Gi0/19 1-4094
Port Vlans allowed and active in management domain
Gi0/19 1,12,34,56
Port Vlans in spanning tree forwarding state and not pruned
Gi0/19 1,12,34,56
As of right now both are using a native VLAN of 1 but we will change SW2 to use
VLAN 3 and SW1 to use VLAN 12
SW2
SW2(config)#int gig0/19
SW2(config-if)#sw tr nat vla 3
SW1
SW1(config-if)#int giga0/19
SW1(config-if)#sw tr na vl 12
01:48:48: %SPANTREE-2-RECV_PVID_ERR: Received BPDU with inconsistent peer vlan
id 3 on GigabitEthernet0/19 VLAN12.
01:48:48: %SPANTREE-2-BLOCK_PVID_PEER: Blocking GigabitEthernet0/19 on
VLAN0003. Inconsistent peer vlan.
01:48:48: %SPANTREE-2-BLOCK_PVID_LOCAL: Blocking GigabitEthernet0/19 on
VLAN0012. Inconsistent local vlan.
01:49:08: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on
GigabitEthernet0/19 (12), with SW2 GigabitEthernet0/19 (3).
So now what do we see?
SW1(config)#do sh int trunk
Port Mode Encapsulation Status Native vlan
Gi0/19 on 802.1q trunking 12
Port Vlans allowed on trunk
Gi0/19 1-4094
Port Vlans allowed and active in management domain
Gi0/19 1,3,12,34,56
Port Vlans in spanning tree forwarding state and not pruned
Gi0/19 1,34,56
Notice that VLAN 3 and VLAN 12 has now been pruned off the trunk, since they
are now in a STP blocking state
SW1(config-if)#do sh span blocked
Name Blocked Interfaces List
-------------------- ------------------------------------
VLAN0003 Gi0/19
VLAN0012 Gi0/19
02:11:55: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on
GigabitEthernet0/19 (3), with SW1 GigabitEthernet0/19 (12).
--More--
02:12:55: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on
GigabitEthernet0/19 (3), with SW1 GigabitEthernet0/19 (12).
--More--
02:13:55: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on
GigabitEthernet0/19 (3), with SW1 GigabitEthernet0/19 (12).
I'm pretty sure in the past I wasn't having this issue and the two native
VLAN's communicated and passed the traffic just fine however that doesn't seem
to be happening now.
SW2(config-if)#do sh span vl 12
VLAN0012
Spanning tree enabled protocol ieee
Root ID Priority 12
Address 001c.0e4b.4500
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 12 (priority 0 sys-id-ext 12)
Address 001c.0e4b.4500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 600
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/19 Desg BKN*4 128.19 P2p *PVID_Inc
SW2(config-if)#do sh span vl 3
VLAN0003
Spanning tree enabled protocol ieee
Root ID Priority 3
Address 001c.0e4b.4500
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 3 (priority 0 sys-id-ext 3)
Address 001c.0e4b.4500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 600
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/19 Desg BKN*4 128.19 P2p *PVID_Inc
Thank you,
Steve E. Di Bias | Network Engineer
CCNP (R&S), CCNA (R&S/Security), FNCNE, BCNE,
CE|H, CCA, MCSE, MCSA, MCTS, MCITP, A+, Net+
Valley Health System | www.valleyhealthsys.com
Direct: 702-369-7594 | Mobile: 702-241-1801
Email: [email protected]
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Di Bias, Steve
Sent: Monday, October 03, 2011 8:35 PM
To: Douglas Koobs; [email protected]
Subject: Re: [OSL | CCIE_RS] How does a native VLAN mismatch create a loop?
Hey Doug!
First you would need to have more than one link going between your switches to
even have the possibility of a loop. Secondly, while I can't guarantee it, I've
never seen a loop form when using mismatched native VLAN's on my trunks (and no
I don't design bad networks so stop looking at me like that ;) ;))
I like to think of this as the poor man's method for layer 2 bridging between
two VLAN's, also known as "VLAN leaking". If you are really bored and have a
few switches lying around you can lab this up and tell us what you find.
While It's possible that some IOS versions will err-disable the ports the
majority will just give you an error message. The error is generated via CDP
since, since CDPv2 will pass the native VLAN information between the switches.
Lab it up and see what you find!
Thank you,
Steve E. Di Bias | Network Engineer
CCNP (R&S), CCNA (R&S/Security), FNCNE, BCNE,
CE|H, CCA, MCSE, MCSA, MCTS, MCITP, A+, Net+
Valley Health System | www.valleyhealthsys.com
Direct: 702-369-7594 | Mobile: 702-241-1801
Email: [email protected]
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Douglas Koobs
Sent: Monday, October 03, 2011 6:27 PM
To: [email protected]
Subject: Re: [OSL | CCIE_RS] How does a native VLAN mismatch create a loop?
Thinking about this a bit more, wouldn't the two VLANs just re-converge and
create a loop-free topology around one bridge? Certainly not optimal, but not a
loop...
On Mon, 2011-10-03 at 20:39 -0400, Douglas Koobs wrote:
> I'm reading the SWITCH foundation learning guide, and it states that a
> trunk with mismatched native VLANs can lead to a loop because BPDUs
> are sent to the IEEE MAC address 0180.c200.0000
>
> I think I understand at a very high level how this could happen: The
> root bridge of one of the VLANs would be superior to the root bridge
> of the other VLAN, and some of the switches would begin to converge
> around the root bridge from the wrong VLAN.
>
> However, this situation is still fuzzy around the edges to me, which
> usually means I'm missing something fundamental. Am I?
>
> Thanks!
>
> Doug
_______________________________________________
For more information regarding industry leading CCIE Lab training, please visit
www.ipexpert.com
Are you a CCNP or CCIE and looking for a job? Check out
www.PlatinumPlacement.com
UHS Confidentiality Notice: This e-mail message, including any attachments, is
for the sole use of the intended recipient (s) and may contain confidential and
privileged information. Any unauthorized review, use, disclosure or
distribution of this information is prohibited. If this was sent to you in
error, please notify the sender by reply e-mail and destroy all copies of the
original message.
_______________________________________________
For more information regarding industry leading CCIE Lab training, please visit
www.ipexpert.com
Are you a CCNP or CCIE and looking for a job? Check out
www.PlatinumPlacement.com
UHS Confidentiality Notice: This e-mail message, including any attachments, is
for the sole use of the intended recipient (s) and may contain confidential and
privileged information. Any unauthorized review, use, disclosure or
distribution of this information is prohibited. If this was sent to you in
error, please notify the sender by reply e-mail and destroy all copies of the
original message.
_______________________________________________
For more information regarding industry leading CCIE Lab training, please visit
www.ipexpert.com
Are you a CCNP or CCIE and looking for a job? Check out
www.PlatinumPlacement.com
