Etherchannel Trunk and STP behavior

[david]

Hi all, have a quick question on etherchannel, trunking and spanning tree behavior.  In packet tracert I've setup 2 physical interfaces as etherchannel "po1" on each side of my switches.  I then configured po1 on each side to be a trunk.  This is working fine and if I shutdown a single port, the link stays active and I can still forward traffic.  Perfect so far! 

However, when I do a "no shut" on the downed interface, the entire etherchannel goes through a Spanning Tree initialization and takes the entire etherchannel down for approximately 30 seconds.  Is this expected behavior?  I thought that Spanning Tree treated the Port Channel as a single entity and as long as one physical interface were up on each side, there would be no reason for spanning tree to renegotiate?            

[Edison Ortiz]

When a port is added to a bundle, it needs to check for consistency.

Please read to this document for its behavior

http://www.cisco.com/en/US/tech/tk389/tk213/technologies_tech_note09186a008009448d.shtml

Regards,

[paul driver]

Hello

You are correct STP treats this as one instance, and the issue you have stated shouldn't really occur, can you post your L2 etherchannel config,

Also

sh spanning-tree interface port-channel x

sh etherchannel port-channel

res

Paul

Please don't forget to rate any posts that have been helpful.

Thanks.

[david]

Edison, thanks for the link... will read in a few.

pdriver, keep in mind that I'm doing this in packet tracer for proof of concept before I try it in production so maybe it's a quirk with packet tracer?  Config is really bare bones  >

SW1

!

spanning-tree mode pvst

!

interface FastEthernet0/1

switchport access vlan 3

switchport mode access

!

interface FastEthernet0/2

switchport access vlan 3

switchport mode access

!

interface FastEthernet0/3

switchport access vlan 3

switchport mode access

!

interface FastEthernet0/4

switchport access vlan 96

switchport mode access

!

interface FastEthernet0/5

switchport access vlan 96

switchport mode access

!

interface FastEthernet0/6

switchport access vlan 96

switchport mode access

!

interface FastEthernet0/7

switchport access vlan 13

switchport mode access

!

interface FastEthernet0/8

switchport access vlan 13

switchport mode access

!

interface FastEthernet0/9

switchport access vlan 13

switchport mode access

!

!

interface GigabitEthernet0/1

channel-group 1 mode on

switchport mode trunk

!

interface GigabitEthernet0/2

channel-group 1 mode on

switchport mode trunk

!

interface Port-channel 1

switchport trunk encapsulation dot1q

switchport mode trunk

!

interface Vlan1

no ip address

shutdown

!

interface Vlan3

description 10.3.0.0

ip address 10.3.0.1 255.255.255.0

!

interface Vlan13

description 10.10.13.0

ip address 10.10.13.1 255.255.255.0

!

interface Vlan96

description 10.10.96.0

ip address 10.10.96.1 255.255.255.0

!

ip classless

!

SW2

spanning-tree mode pvst
!
!
interface FastEthernet0/1
switchport access vlan 3
switchport mode access
!
interface FastEthernet0/2
switchport access vlan 3
switchport mode access
!
interface FastEthernet0/3
switchport access vlan 3
switchport mode access
!
interface FastEthernet0/4
switchport access vlan 96
switchport mode access
!
interface FastEthernet0/5
switchport access vlan 96
switchport mode access
!
interface FastEthernet0/6
switchport access vlan 96
switchport mode access
!
interface FastEthernet0/7
switchport access vlan 13
switchport mode access
!
interface FastEthernet0/8
switchport access vlan 13
switchport mode access
!
interface FastEthernet0/9
switchport access vlan 13
switchport mode access
!
!
interface GigabitEthernet0/1
channel-group 1 mode on
switchport mode trunk
!
interface GigabitEthernet0/2
channel-group 1 mode on
switchport mode trunk
!
interface Port-channel 1
switchport trunk encapsulation dot1q
switchport mode trunk
!
interface Vlan1
no ip address
shutdown
!
ip classless
!
!

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

SW1

Switch#sh etherchannel port-channel
                Channel-group listing:
                ----------------------

Group: 1
----------
                Port-channels in the group:
                ---------------------------

Port-channel: Po1
------------

Age of the Port-channel   = 00d:14h:01m:55s
Logical slot/port   = 2/1       Number of ports = 2
GC                  = 0x00000000      HotStandBy port = null
Port state          = Port-channel
Protocol            =   PAGP
Port Security       = Disabled

Ports in the Port-channel:

Index   Load   Port     EC state        No of bits
------+------+------+------------------+-----------
  0     00     Gig0/2   On                 0
  0     00     Gig0/1   On                 0
Time since last port bundled:    00d:06h:29m:45s    Gig0/1

Switch#sh spanning-tree interface port-channel 1
no spanning tree info available for Port-channel1

[paul driver]

Hello

Hum it seems you have created the port-channel first then confined the physical interfaces plus the interfaces looks like they are not are not active given the stp and ether-channel readouts

You say you need to aggregate physical port for server connection but at present the physical ports are trunked which indicates switch to switch connections -(would expect to see server porta in access mode)

Anyway please take a look at this post:

https://supportforums.cisco.com/message/4146391#4146391


Res
Paul

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