WIll a spanning-tree convergence happen in this scenario

Hi,

I need to allow a new vlan (144) across a trunk between a Cisco 4500 and a Cisco 2950 both running 12.1 IOS (full details below). They are both have vlan 144 configured in isolation. My question is, given this config, will a spanning tree convergence happen if I set the 2950 to a lower priority to ensure it stays the root then add vlan 144 to the trunk? I am not bothered if the 4500 does a spanning-tree re-calulation but I cannot have an outage on the 2950 (without lots of grief!). There is only one link between the 2 switches.

2950
=====

2950#sh ver
Cisco Internetwork Operating System Software
IOS (tm) C2950 Software (C2950-I6Q4L2-M), Version 12.1(11)EA1, RELEASE SOFTWARE (fc1)

2950#sh run
(only spanning-tree commands shown)
!
spanning-tree extend system-id
!

2950#sh spanning-tree sum
Root bridge for: VLAN0001, VLAN0xxx, VLAN0xxx, VLAN0xxx, VLAN0144, VLAN0xxx.
Extended system ID is enabled.
PortFast BPDU Guard is disabled
EtherChannel misconfiguration guard is enabled
UplinkFast is disabled
BackboneFast is disabled
Default pathcost method used is short

Name                   Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
......
VLAN0144                  0        0         0        4          4      
......
---------------------- -------- --------- -------- ---------- ----------
6 vlans                   0        0         0        26         26    


2950#sh spanning-tree vlan 144 detail

 VLAN0144 is executing the ieee compatible Spanning Tree protocol
  Bridge Identifier has priority 32768, sysid 144, address 000b.5fdd.4640
  Configured hello time 2, max age 20, forward delay 15
  We are the root of the spanning tree
  Topology change flag not set, detected flag not set
  Number of topology changes 23 last change occurred 27w2d ago
          from FastEthernet0/20
  Times:  hold 1, topology change 35, notification 2
          hello 2, max age 20, forward delay 15
  Timers: hello 0, topology change 0, notification 0, aging 300

2950#sh run interface fa0/20
Building configuration...

Current configuration : 224 bytes
!
interface FastEthernet0/20
 description ***trunk link to 4500***
 switchport access vlan 145
 switchport trunk allowed vlan 145,1002-1005
 switchport mode trunk
 no ip address
 speed 100
 no cdp enable
end


4500#
=======

sh run (only spanning-tree commands shown)

!
spanning-tree extend system-id
!

4500#sh ver
Cisco Internetwork Operating System Software
IOS (tm) Catalyst 4000 L3 Switch Software (cat4000-IS-M), Version 12.1(14)E1, EARLY DEPLOYMENT RELEASE SOFTWARE (fc1)

4500#sh spanning-tree sum
Switch is in pvst mode
Root bridge for: VLAN0xxx-VLAN0xxx, VLAN0144, VLAN0xxx-VLAN0xxx, VLAN0xxx
  VLAN0xxx, VLAN0xxx-VLAN0xxx
Extended system ID   is enabled
Portfast             is disabled by default
PortFast BPDU Guard  is disabled by default
Portfast BPDU Filter is disabled by default
Loopguard            is disabled by default
EtherChannel misconfiguration guard is enabled
UplinkFast           is disabled
BackboneFast         is disabled
Pathcost method used is short

Name                   Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
.........
VLAN0144                     0         0        0          6          6
.........
---------------------- -------- --------- -------- ---------- ----------
26 vlans                     0         0        0        156        156


4500#sh spanning-tree vlan 144 detail

 VLAN0144 is executing the ieee compatible Spanning Tree protocol
  Bridge Identifier has priority 32768, sysid 144, address 000c.30b0.c400
  Configured hello time 2, max age 20, forward delay 15
  We are the root of the spanning tree
  Topology change flag not set, detected flag not set
  Number of topology changes 32 last change occurred 3d03h ago
          from FastEthernet6/47
  Times:  hold 1, topology change 35, notification 2
          hello 2, max age 20, forward delay 15
  Timers: hello 0, topology change 0, notification 0, aging 300
..........................................................................................................

 Port 321 (FastEthernet6/1) of VLAN0144 is forwarding
   Port path cost 19, Port priority 128, Port Identifier 128.321.
   Designated root has priority 32912, address 000c.30b0.c400
   Designated bridge has priority 32912, address 000c.30b0.c400
   Designated port id is 128.321, designated path cost 0
   Timers: message age 0, forward delay 0, hold 0
   Number of transitions to forwarding state: 1
   Link type is point-to-point by default
   BPDU: sent 8255152, received 33
...................................................................


4500#sh run int fa6/1
Building configuration...

Current configuration : 151 bytes
!
interface FastEthernet6/1
 description Link-to-2950
 switchport trunk encapsulation dot1q
 switchport mode trunk
 duplex full
 speed 100
end


dingdong123Asked:
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eeRootCommented:
Since there's only one connection your don't have to worry about a major spanning-tree related outage, but it will take the switches a few seconds to negotiate the new connections.  You do need to verify that only one switch will try to be the root for VLAN 144.

Since both devices currenlty see themselves as the root brdige for VLAN144, one will have to have a lower value then the other when they connected.  The 2950 can be made to see itself as the root of VLAN 144  with these commands.

spanning-tree vlan 144 priority 1
spanning-tree vlan 144 root primary

0
dingdong123Author Commented:
Thanks for this. With regard to "it will take the switches a few seconds to negotiate the new connections"  Could you clarify whether there will be any dropped packets? Do you just mean that the trunk link will take a few seconds to start forwarding vlan 144 packets or do you mean all traffic will be halted during these couple of seconds?

Thanks again.
0
Don JohnstonInstructorCommented:
Once you allow VLAN144 on the trunk, it will take 30 seconds before the trunk starts moving the VLAN 144 traffic. After that, the only spanning-tree events will be the normal 30 second delay when a new port comes online (unless portfast is enabled).
0
Marius GunnerudSenior Systems EngineerCommented:
As there is only one link this would mean Spanning Tree convergence will not need to take place. Therefore the one link will always be the root and forward traffic. I don't see any outage happening in this scenario
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dingdong123Author Commented:
Thanks for this MAG03 and everyone else. I ended up setting up a lab with 2 pc's connected to a 2950 and then added vlan 144 to the trunk connected to a 4900 (didnt have a 4500 available but no difference I figured). This was whilst a ran a continous PING - and hey presto no dropped packets!
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