One LAN switching best practice is to locate the STP root in a strategic place for each VLAN. In Scenario 4, we’ll need to configure the STP root location…
Three switches are connected to each other as shown in the network diagram. Switches D1 and D2 form the distribution layer, while switch A1 is in the access layer. The links between the switches are all Gigabit Ethernet. Each of the switches has been left to the default STP configuration, and a partial output from each is shown below.
D1#show spanning-tree vlan 2
VLAN0002
Spanning tree enabled protocol rstp
Bridge ID Priority 32770 (priority 32768 sys-id-ext 2)
Address 001c.570f.5e80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
D2#show spanning-tree vlan 2
VLAN0002
Spanning tree enabled protocol rstp
Bridge ID Priority 32770 (priority 32768 sys-id-ext 2)
Address 001c.570f.41f0
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
A1# show spanning-tree vlan 2
VLAN0002
Spanning tree enabled protocol rstp
Bridge ID Priority 32770 (priority 32768 sys-id-ext 2)
Address 001c.570f.4180
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Answer the following questions:
- Based on the console output, which switch will be elected as the STP root for VLAN 2?
- Is this the best choice to be the root switch? Why or why not?
- If the root should be relocated, what configuration commands should be entered on each of the switches to force the appropriate switch to become the root?
- After configuring the root switch, suppose that a fourth switch, A2, is introduced into the access layer of the network a month later. A2 has an uplink to each of the distribution switches. From switch A2, we can see this partial output:
A2#show spanning vlan 2
VLAN0002
Spanning tree enabled protocol rstp
Bridge ID Priority 16386 (priority 16384 sys-id-ext 2)
Address 001c.570f.f030
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Does the addition of switch A2 sway the election and change the location of the STP root?
Stop here if you don’t want to see the solution! Otherwise, go on to page 2…
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August 17, 2010 at 5:43 pm
Hi Dave,
A couple of questions which may be relevant:
With MST there is a command to set an interface to use Pre_standard BPDUs on an interface. This comes up in a simulated example from Boson, in one of their labs. It appears that the command does not actually appear in the configuration, so my question is, how does one tell if an interface has been set up for pre-standard BPDUs?
Second question relates to Port channel and native VLANs. I came across an error message relating to native vlan mismatch across a port channel, which at the time I could not get rid of even when native vlans were explicitly configured at each end. Can you perhaps come up with a good scenario relating to port channels and 802.1q trunking?
August 19, 2010 at 7:13 pm
A bit of topic but hope you don’t mind
I don’t know if any one here uses ANKI flashcard software. But if you do I have been creating a CCNP SWITCH deck over the last few weeks.
It is not yet complete, but there a fair few card in it now. If you have the software you can find it in the shared decks by looking up my user name “DEVILWAH” or simple searching for “CCNP SWITCH”.
One thing you may find usefull is that you can filter by the keyword “config”. this will put out a list of cards from different topic areas with an example config, “verify” will give you the same for some of the commong verify commands.
I hopefully taking my exam in the next few weeks, but I will try to complete the set to include as many topics as possible.
If you find and errors let me know and I will sort them out.
Hope you find them use full.
PS. I think the next topic I add after I finish switch security will be STP
September 2, 2010 at 5:39 pm
“If the current root priority is more than 24,576, the local switch sets its priority to 24,576. If the current priority is less than that, the local switch sets its priority to 4096 less than the current root”. Cisco Cert Guide.
The Vlan # is added on to the priority in extended system-id mode.
This is a one time macro, as Dave points out.
September 11, 2010 at 3:53 pm
Hi Dave,
Would you mind if I link your blog to my blog?
September 22, 2010 at 7:13 pm
Hi Dave,
Further to my question above related to port channel and native vlan mismatches…I have been experimenting with packet tracer and have reproduced the problem. It seems that to get rid of the native vlan mismatch issue, I have to configure the native vlan explicitly on both the port channel interface, po1 in this case, and on each of the individual ports that make up the etherchannel, at both ends of course. Is this right? I haven’t found any documentation on this anywhere yet.
regards
John
September 22, 2010 at 11:43 pm
Ok, a little more on the above port channel question. I have set this up with one end being a simulated 2950, the other is a simulated 3560. I have found a cisco document with a configuration example for the 2950 at http://www.cisco.com/en/US/products/hw/switches/ps628/products_configuration_example09186a00800ef797.shtml
When I follow this example to set up a port channel on the 2950, I get the error message about native vlan mismatch. (I am setting the native vlan to 99, a vlan that exists on both switches. ) When I explicitly set the native vlan on both the port-channel AND on all the ethernet interfaces that make up the port channel it stops sending the message. When I set the port-channel up on the 3560, it does not seem need the native vlan configured on the interfaces as well as on the port-channel.
So the simulated 3560 is behaving as per the configuration example (for a 2950)in the link above, but the simulated 2950 is not. Could somebody with access to a range of real devices try this and tell me what should actually work? Then maybe document it since without revealing too much can I say that you might regret not knowing about this.
regards
John
September 23, 2010 at 12:54 pm
John / Mjolnir,
I just tested this on a real 2950 and 3560. You only need to apply the native vlan command to the port-channel interface. However, the 2950 will apply that 1 command to all the interfaces in the bundle. In general, 2950′s are pretty ancient and have lots of little quirks like that. Here’s another one: 2950′s don’t support ISL, thus there is no switchport trunk encapsulation command. Your best bet is to work with something newer like a 2960.
Best of luck!
Walker
September 23, 2010 at 3:45 pm
Well, I can only work with what is available on the two simulators that I have available. (Three if you count Dynamips, but that does not do switches.)
However, the real concern to me is that the Cisco site says that you only need to apply the native vlan comamnd to the port-channel interface, but their own simulator does not behave unless it is applied to all the ethernet interfaces in the channel as well. This appears to apply not only to Packet Tracer, but also to whatever simulator they are using in the current examination. Not knowing about this little quirk cost me a lot of time in my exam, which I missed by not too big a margin.
Possibly this is also IOS version dependent? But shouldn’t the exam be using the most up to date IOS?
Also Dave I think we could use something about all this in the updates to the book since it is in the exam but does not seem to be covered in the book. Although I get the impression that Dave is no longer looking at the updates on this blog.
regards John
September 23, 2010 at 1:26 pm
Hi Dave,
Not really sure this is the correct place to be posting a general comment…
Just wanted to say a big thanks to you for your book and the video mentor dvd – passed the CCNP SWITCH today (Second attempt) and both of these study aids seriously helped get me a PASS!
Thanks
October 6, 2010 at 5:33 pm
Hi CiscoKid,
Where did you get the video mentor for the CCNP SWITCH exam?
Thanks,
D
October 12, 2010 at 5:31 am
Hi Darren,
I just got it online, somewhere like amazon or play. Comes with a reference book and free assessment style questions which you can do online.
Cheers
October 11, 2010 at 2:29 pm
1. Switch A1, because it has a lower MAC address
2. No, the root switch should be as close to the center of the network as possible, which in this network which appears to be a collapsed-core topology, would be the distribution switch.
3. On switch D1
spanning-tree vlan 2 root primary
On switch D2
spanning-tree vlan 2 root secondary
4. Yes, despite D1 and D2 being configured as root primary and secondary, the new switch will enter the network with a lower priority. This will cause the new switch to become the root for VLAN 2.
October 24, 2010 at 2:42 pm
Hi Dave,
I just wanted to get some clarifications on the snmp config. We have the configuration step right, but are we configuring ALL the switches involved or are we dealing with only a particular switch. If all, are there any differences between management switches and agent switch configurations?
November 12, 2010 at 12:22 pm
Dave,
Thanks for writing the scenario’s. They are refreshing to go through.
-Blake
November 19, 2010 at 10:38 pm
Hi Blake,
Thanks for your nice comments. I’ve been absent from the blog for quite a while, but am hoping to get back in the saddle and get some new scenarios posted.
Best regards,
Dave H
April 17, 2011 at 4:47 pm
Great clue ful examples. Thanks Dave!!
June 8, 2011 at 8:08 pm
Nice tutorial, i will bookmark this link on this site http://cisco.tutbook.net . Thanks