Come all yee STP/VTP gurus

Q

QHalo

2[H]4U
Joined
Sep 30, 2002
Messages
3,432
Here's the scenario:

2 switches - 2950-24
2 trunks - both running 802.1q/dynamic/desirable on ports FA0/1 and 0/2 respectively
2 VLANs - obviously VLAN1 and VLAN100 that I created.
2 Ports - Fa0/3 on both switches was set to VLAN100 to test ping connectivity between hosts on each side of the topology.

Switch1 - VTP domain CCNA /VTP mode SERVER /STP - PVST /Root bridge of VLAN1
Switch2 - VTP domain CCNA /VTP mode CLIENT /STP - PVST

Status - VTP appears to be working fine, VLAN100 is showing up with show vlan brief on both switches. The configuration registrations are incrementing like they should. However, show spanning-tree vlan 100 on both switches shows that both switches believe they are the Root bridge for VLAN100. I went to Switch1 and set it as the root. Switch2 still believes it's the root bridge as well as does Switch1 most assuredly after hard setting it. I hard set Switch2 priority to the highest value possible as well. All ports for VLAN100 show in the FWD DESG state after running show spanning-tree vlan 100. VLAN1 is behaving as it should in regards to STP. Referenced by the show output below.

Switch 1 - Show spanning-tree vlan 100
Code: 
Switch#show spanning-tree vlan 100
VLAN0100
  Spanning tree enabled protocol ieee
  Root ID    Priority    16484
             Address     0001.9767.B8E5
             This bridge is the root
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    16484  (priority 16384 sys-id-ext 100)
             Address     0001.9767.B8E5
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20
Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Desg FWD 19        128.1    P2p
Fa0/2            Desg FWD 19        128.2    P2p
Fa0/3            Desg FWD 19        128.3    P2p

Switch 2 - Show spanning-tree vlan 100
Code: 
Switch#show spanning-tree vlan 100
VLAN0100
  Spanning tree enabled protocol ieee
  Root ID    Priority    61540
             Address     00E0.B04E.84D8
             This bridge is the root
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    61540  (priority 61440 sys-id-ext 100)
             Address     00E0.B04E.84D8
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20
Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/2            Desg FWD 19        128.2    P2p
Fa0/3            Desg FWD 19        128.3    P2p
Fa0/1            Desg FWD 19        128.1    P2p

Switch1 - show spanning-tree vlan1
Code: 
Switch#show spanning-tree vlan 1
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     0001.9767.B8E5
             This bridge is the root
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0001.9767.B8E5
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20
Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Desg FWD 19        128.1    P2p
Fa0/2            Desg FWD 19        128.2    P2p

Switch2 - show spanning-tree vlan1
Code: 
Switch#show spanning-tree vlan 1
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    16484
             Address     0001.9767.B8E5
             Cost        19
             Port        1(FastEthernet0/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     00E0.B04E.84D8
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20
Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Root FWD 19        128.1    P2p
Fa0/2            Altn BLK 19        128.2    P2p

Question - How in the world can VLAN100 have all ports in FWD state and a loop not exist? I'm sure this is going to be a /facepalm moment when someone tells me what I've done wrong but I think I'm going to go insane if I don't know why. Or perhaps there's a simple concept that's just not registering for me.

Side notes - After adding a host device off the FA0/3 port of Switch1 and Switch2, I was unable to ping across the switches after setting the ports with:
- switchport mode access
- switchport access vlan 100

I then turned on debug ip packet on the host device connected to switch1( i was using a router for the host) and pinged the other router on switch2 and got an encapsulation error. Then I ran it again and the pings started to go through all of a sudden. :confused::confused:
 
Blow it all away and reconfigure. Hard set your trunk ports. Make sure that you're not in VTP transparent mode as you cannot form a trunk when using dynamic desirable mode.

sh int trunk
sh int f0/1 switchport

make sure everything looks as it should, Administrative mode, operational mode, trunking, etc....

Also, if you're seeing encapsulation errors, I would venture to say that you're trunks are not forming correctly(for VLAN 100). Also try assigning each trunk to a *different* native VLAN, if you see errors your trunk are forming correctly.
 
The joys of using a simulator. Well I had this same thing happen at work when I was doing the same setup and I don't remember exactly what happened but all of a sudden it started working properly. So I wanted to see if it was just something I was missing in my process of setting it up. I'll try again and see if I can get it correct. Thanks for the help.

Monday I get a pair of 2950's to see it happen on the real thing. F this simulation crap.
 
Funny thing, i was having the same issue yesterday.

I ended up blowing my entire config away and starting over. Worked fine afterwords. (This was with 2x 2960G's and 1x 3560G)
 
Had you ever had this happen to you before or seen this before? If this is a common issue that's all I really want to know. I'd love to understand why, but for the test I can configure it correctly as I have done it before and conceptually I understand it. So I'm not horribly worried about it, but if there's something fundamentally flawed in my approach to setting it up, I'd like to remedy it.

I'm going to start over and do the trunks like xphil3 said. I only put them in dynamic/desirable because I wanted to understand the trunking modes in practice. Normally I hard trunk.
 
I've never had this or seen it happen before. Really weird issue, generally it works right.


As for the trunks, I always hard code mine. I don't trust VTP to auto prune either (had problems with VTP pruning a vlan off a trunk that was needed on the other end of the trunk). Not a big fan of dynamic/desirable either. I like being in control of my trunks.

In practice, your config is correct. I don't see any problems.
 
Had I not seen that this was for learning purposes I'd have suggested that with two switches you don't really need spanning tree or vtp.

Things to try (based on having worked with 3500XLs and 3550s, not sure what you lose with a 2950):
Does it work with only one trunk connection between the switches?
Does it work with vlan 100 being native (sw tr na vl 100)
If you give the switches an interface in vlan 100 can they ping each other?
if you split your trunks and allow vlan 1 on fa0/1 and vlan 100 on fa0/2 does it work? (switchport trunk allowed vlan xxx)
 
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