VLAN Trunking 1

| Saturday, June 12th, 2010 | 3 Comments »
VLAN 1 Trunking by Dave Hucaby

VLAN 1 Trunking by Dave Hucaby

In another lab created by Dave Hucaby, we look at playing with VLAN trunking and mixing and matching native vlan’s and what the effects of doing so are. As always, you can view his original lab here:

http://dhucaby.wordpress.com/2010/06/11/new-scenario-vlan-trunking-1/

Here is his original question:

Here’s a new scenario to think about. I haven’t built it in a lab yet, but I think it might be interesting and it just might work.

Switches A and B are connected by a single link. The configurations for each end of the link are shown in the figure below. PCs A, B, and C are connected to access interfaces assigned to VLANs 10, 20, and 30, respectively, on Switch A. PC X is connected to an interface on Switch B, which has the interface configuration shown.

And here are the available options:

Assuming that the switches are configured for Layer 2 traffic only, which one of the following can PC X reach?

A. PC A

B. PC B

C. PC C

D. None of the above

Again, since I don’t have the exact equipment in his scenario, I’ve modified the lab just a tad bit. Here’s what we’ll be working with:

The first thing that I did was ping PC A, B and C from PC X with everyone on the same VLAN(and everyone with the same IP Addressing scheme). Which by default is VLAN 1. This is to simply test connectivity before moving onto the actual configuration of the switches. Ping tests confirmed that PC X is able to communicate with PC A, B and C.

Since the diagram already includes the configuration info, we won’t repeat it again here.

As soon as I started configuring the switchports on both switches (2950 and 2960) for trunking or access I began receiving native VLAN mismatch errors.

SW-2960-24
*Mar  1 02:56:38.309: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet0/1 (20), with SW-2950-24 FastEthernet0/1 (10).

SW-2950-24
02:50:34: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEthernet0/1 (10), with SW-2960G-24 GigabitEthernet0/1 (20).

07:25:31: %SPANTREE-2-RECV_PVID_ERR: Received BPDU with inconsistent peer vlan id 20 on FastEthernet0/1 VLAN10.
07:25:31: %SPANTREE-2-BLOCK_PVID_PEER: Blocking FastEthernet0/1 on VLAN0020. Inconsistent peer vlan.
07:25:31: %SPANTREE-2-BLOCK_PVID_LOCAL: Blocking FastEthernet0/1 on VLAN0010. Inconsistent local vlan.

Once all the switchports and trunking have been confgured. I attempted to ping PC’s A, B and C with no success from PC X.

Conclusion

I’m not entirely convinced that “D: None of the Above” is suppose to be the correct answer. I do know that you can have native vlan mismatches and still have traffic forwarded across the switches(if you’re using 802.1Q, not ISL). If this was the intended purpose, then I was unable to configure this lab properly.

I am open to suggestions!

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  • AdAmAn

    Hi. I my opinion answer A is correct. Why? PC-X makes ping. What does Switch B do? It sees frame on interface from vlan 20 so it can tag frame to put in trunk. But. vlan 20 is native vlan. So switch put untagged frame in trunk. We have native vlan mismatch. So Switch A receives this frame untagged and put in vlan 10 because it's configured that vlan 10 is native.

  • brandontek

    Ahhh or so the theory goes! I do agree with you, untagged frames on both ends should be able to communicate while getting mismatch messages. It actually didn't work in this lab though. And the creator of this lab Dave Hucaby himself has mentioned that “D” is in fact the correct answer.

    Go figure eh? If you get a chance to lab this up, please let me know your results.