Etherchannel and Spanning tree

Unanswered Question
Mar 6th, 2009
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Hi, I recently ran into this configuration on a Cisco 6509 and I read that you are not supposed to enable spanning tree if you create a interface port-channel. Question? Does the configuration below is wrong?

interface Port-channel25

bandwidth 2000000

no ip address


switchport trunk encapsulation dot1q

switchport trunk allowed vlan1,4,8-10,15,16,25,27,35,36,46,47,51-54,60-64,88

switchport trunk allowed vlan add 95,98,99,101,102,110,131,322,621,622,960

switchport mode trunk

spanning-tree portfast trunk

spanning-tree port-priority 240

Please advice - I want to make sure I understood the cisco document correctly, if we have a port-channel interface, then we should not have spanning tree turn on..

Thanks in advance...


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Overall Rating: 5 (1 ratings)
Francois Tallet Fri, 03/06/2009 - 16:46
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Spanning tree is turned on by default. I guess that, if someone complained about your configuration, they must have been puzzled by the "portfast trunk" line.

Portfast is recommended on ports leading to a host (something that is not bridging traffic). Port channel are generally configured to some other bridges, in which case, portfast is not recommended. This could lead to temporary bridging loops in case this peer has a redundant connection to the root.

Now, if this is a port channel to a host, then it is perfectly valid to have portfast trunk. On the other hand, in that case, the port-priority is completely of no use;-)



dirtipacket Fri, 03/06/2009 - 17:22
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Im just posting so I can rate the person whom answered. Nicely said.

For the OP: Some examples of using spanning-tree portfast trunk would be when you are trunking to a server for aggregation or on 3020/3120 architecture and you are connecting to V(m) switches.

mguzman4158 Fri, 03/06/2009 - 18:19
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Thank you Francois, well explained. This is a core switch connected to another core switch and the po25 is actually connecting to the other core - so I guess by your exponation, the set up is wrong, I should turn off spanning tree portfast. Question If I take off portfast, would it disable the ports temporarily or I should have no problems taking it off during production time?



Francois Tallet Fri, 03/06/2009 - 20:27
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No problem to change the portfast configuration on a live network. It will not have any impact on STP or the traffic flow. In fact, your port has probably already switched to an operational "non-portfast" mode automatically, as it is likely to have received BPDU from its neighbor. So you are only changing the configuration, that is used when the port comes up to set the operational value.




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