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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

eeRoot

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

dingdong123

ASKER

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.

Don Johnston

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).

ASKER CERTIFIED SOLUTION

Marius Gunnerud

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dingdong123

ASKER

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!