Task 1.3 MST Spanning Tree Instance 1 - P2p Dispute

Hi All,


Whilst working on Task 1.3 i noted that on SW1 the connecttions to SW2 displayed a 'Dispute' and were in a blocking state.

As Below:



Rack1SW1#sh spanning-tree mst 1

##### MST1    vlans mapped:   3-7

Bridge        address 001f.9d65.d580  priority      24577 (24576 sysid 1)

Root          this switch for MST1


Interface        Role Sts Cost      Prio.Nbr Type

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

Fa1/0/1          Desg FWD 200000    128.3    P2p 

Fa1/0/3          Desg FWD 200000    128.5    P2p 

Fa1/0/13         Desg BLK 200000    128.15   P2p Dispute 

Fa1/0/14         Desg BLK 200000     32.16   P2p Dispute 

Fa1/0/15         Desg BLK 200000     16.17   P2p Dispute 

Fa1/0/16         Desg FWD 200000    128.18   P2p 

Fa1/0/17         Desg FWD 200000     32.19   P2p 

Fa1/0/18         Desg FWD 200000     16.20   P2p 

Fa1/0/19         Desg FWD 200000    128.21   P2p 

Fa1/0/20         Desg FWD 200000     32.22   P2p 

Fa1/0/21         Desg FWD 200000     16.23   P2p 




Eventually i tracked this to that fact that i had not added the follwoing command on SW1 and SW3:

vlan dotiq tag native


This was stated in the solution guide for task 1.2, but I hadnt relalised that this was required on SWs 1 and 3 at the time..

Note from SG talks specifically about MTU not so much the 'vlan dotiq tag native' command:



system mtu 1504

system mtu routing 1500

vlan dot1q tag native



? Quick


Although not needed on SW3 for this task, SW3 will

be running OSPF with SW4 later in the lab. By

ensuring that the MTUs match, it will alleviate any

potential problems. Tagging native VLAN is needed

to hide the innermost VLAN header for VLAN26,

which is not configured in any switch. Notice that the

3550s (SW3, SW4) don’t support the system

mtu routing command.



This linked below help with the solution, but for me without copying the solution guide and having internet access, i'd never have tracked this.


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