Help with VTP Simulation on CCNA Exam

Answered Question
Sep 2nd, 2007

On the CCNA 840-601 there is a 4 switch simulation to which you can only console to SW3. How do I tell which is the root bridge, when I only have access to SW3? How do I tell where SW3 got it's VTP updates? Thanks!!

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Correct Answer by Jon Marshall about 9 years 4 months ago

Hi

Execute the following command

sh vtp status

The last line should give you the switch from which the update came. Here's an example from our lab

=============================================

lab_sw1>en

Password:

lab_sw1#sh vtp status

VTP Version : 2

Configuration Revision : 44

Maximum VLANs supported locally : 1005

Number of existing VLANs : 31

VTP Operating Mode : Client

VTP Domain Name : LAB

VTP Pruning Mode : Disabled

VTP V2 Mode : Disabled

VTP Traps Generation : Disabled

MD5 digest : 0xF4 0xBE 0x39 0xC4 0x2F 0x30 0x3D 0x89

Configuration last modified by 10.15.1.3 at 8-28-07 07:43:17

=============================================

As for the root bridge you can tell the bridge ID from a "sh spanning-tree" eg.

=============================================

HQ-FTZ-F00-SW1#sh spanning-tree vlan 10

VLAN0010

Spanning tree enabled protocol ieee

Root ID Priority 32778

Address 000a.b8b3.a980 <-- mac-address of ROOT bridge

Cost 19

Port 24 (FastEthernet0/24)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)

Address 000e.84e1.f680

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- ------------------------------

Fa0/1 Desg FWD 19 128.1 Edge P2p

Fa0/2 Desg FWD 19 128.2 Edge P2p

Fa0/24 Root FWD 19 128.24 P2p

=============================================

HTH

Jon

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Correct Answer
Jon Marshall Sun, 09/02/2007 - 22:26

Hi

Execute the following command

sh vtp status

The last line should give you the switch from which the update came. Here's an example from our lab

=============================================

lab_sw1>en

Password:

lab_sw1#sh vtp status

VTP Version : 2

Configuration Revision : 44

Maximum VLANs supported locally : 1005

Number of existing VLANs : 31

VTP Operating Mode : Client

VTP Domain Name : LAB

VTP Pruning Mode : Disabled

VTP V2 Mode : Disabled

VTP Traps Generation : Disabled

MD5 digest : 0xF4 0xBE 0x39 0xC4 0x2F 0x30 0x3D 0x89

Configuration last modified by 10.15.1.3 at 8-28-07 07:43:17

=============================================

As for the root bridge you can tell the bridge ID from a "sh spanning-tree" eg.

=============================================

HQ-FTZ-F00-SW1#sh spanning-tree vlan 10

VLAN0010

Spanning tree enabled protocol ieee

Root ID Priority 32778

Address 000a.b8b3.a980 <-- mac-address of ROOT bridge

Cost 19

Port 24 (FastEthernet0/24)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)

Address 000e.84e1.f680

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- ------------------------------

Fa0/1 Desg FWD 19 128.1 Edge P2p

Fa0/2 Desg FWD 19 128.2 Edge P2p

Fa0/24 Root FWD 19 128.24 P2p

=============================================

HTH

Jon

rangelo001 Mon, 09/03/2007 - 16:52

Jon,

Thank you very much for your quick reply! So in order to find the root bridge, would I then do a Show mac-address-table to see where that mac address is coming from? The questions was to indicate which of the four switches was the root. Or is their another way to trace back that mac address to the root bridge/switch.

Ray

Edison Ortiz Mon, 09/03/2007 - 18:17

The Root bridge is listed as being source from port 24.

Just issue show cdp neighbor and see the device name being announced on port 24.

rossua994 Tue, 09/04/2007 - 05:34

Hi, I have encountered a similar problem with a 4 switch simulation, which gives access to only SW3, and asks which switch is the root bridge.

By running show spanning-tree command I can see the MAC address of the root bridge, and the root port of SW3. I can then do show cdp neighbors and see the device directedly connected to this root port.

The problem is that this directly connected device is not necessarily the root bridge. The lowest cost BPDU from the root bridge comes in VIA this device to SW3, but this doesn't necessarily mean the root bridge IS this device.

With the information available it seems to me not possible to deduce for sure which is the root bridge. Or is there something I am missing here ?

Edison Ortiz Tue, 09/04/2007 - 05:55

You aren't missing anything. You need to treat it as a hop-by-hop discovery.

If you see the root is being sourced via port x, issue a cdp on that port and hop onto that switch.

Repeat the process on that switch (perform the spanning-tree command & cdp combo).

HTH,

rossua994 Tue, 09/04/2007 - 06:44

Hi, thanks for your reply. In the question it does not allow you to telnet or console onto the next switch. Only one switch out of the four can be connected to. It is this reason I think it is not possible to determine the root switch for sure without further information.

Edison Ortiz Tue, 09/04/2007 - 07:03

You can provide the MAC address of the switch as displayed on the show spanning-tree command.

The actual switch-name will be unknown, though. That information can only be obtained via CDP.

rossua994 Tue, 09/04/2007 - 07:08

Thats what I thought. The multiple choice answer will only accept a switch name though. I attach a diagram of the topology. Only SW-AC3 can be connected to, via the console connection shown.

rayroyaleverest Tue, 09/04/2007 - 07:19

If the MAC address to host cannot be verified, then the fourth switch must be the source.

rangelo001 Thu, 09/06/2007 - 06:50

Yes,

The old if it didn't originate from my neighbors, then it must be from their neighbor. Good thinking, tough to fathom through during a test. I think this resolves the issue.

Thanks.

rangelo001 Thu, 09/06/2007 - 06:53

This resolved my issue. Not sure how to indicate it in this post.

Thanks, again.

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