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Taking links out of a port channel

TGF_Cisco
Beginner
Beginner

Hi

I am in the process of recovering some ports on my core switch connectin to the server farm switches.

gvadc-cr01#show int po1

Members in this channel: Gi1/0/5 Gi1/0/6 Gi2/0/5 Gi2/0/6

i want to remove 2 links out of the port channel to be used .

all the four links carry traffic and i want to make sure that there is no interruption to traffic .

I was planning to take the links out of the port channel first and then to shut down these links.

Is that correct?

1 ACCEPTED SOLUTION

Accepted Solutions

Peter Paluch
Hall of Fame Cisco Employee Hall of Fame Cisco Employee
Hall of Fame Cisco Employee

Hello Eric,

You are generally correct here but note that the withdrawal of a link from a Port-channel will cause that its bandwidth and resulting STP cost will change, regardless of why the link was removed (either shutting it down or disconnecting it). The change to the STP cost may cause the STP to reconverge.

I therefore suggest configuring the STP cost on the Port-channel statically, using the spanning-tree cost command, before disconnecting the link. Use the same cost value the port currently has.

Best regards,

Peter

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7 REPLIES 7

Eric_Wu
Beginner
Beginner

I would actually do the opposite because once you take the port out of the port-Channel you create multiple connections to the same server. I would shut down the physical port first.

Sent from Cisco Technical Support iPhone App

thanks,, so  you mean to say the best approach would be to shut down the ports  as the initial step.

the  portchannel does not use LACP / PaGP. it is setup between two 3750 switches.

Eric_Wu
Beginner
Beginner

Oh sorry, I misread your original post. I thought it was a port channel to a server. If it is between switches then definitely shut down the port first before taking out of port channel. If you take it out of the port channel first you create another down link to the switch which will kick off spanning tree. Then it will take a few seconds to reconverge. By shutting down the link while in the port channel, you are just removing an additional link from the port-channel.

Sent from Cisco Technical Support iPhone App

Peter Paluch
Hall of Fame Cisco Employee Hall of Fame Cisco Employee
Hall of Fame Cisco Employee

Hello Eric,

You are generally correct here but note that the withdrawal of a link from a Port-channel will cause that its bandwidth and resulting STP cost will change, regardless of why the link was removed (either shutting it down or disconnecting it). The change to the STP cost may cause the STP to reconverge.

I therefore suggest configuring the STP cost on the Port-channel statically, using the spanning-tree cost command, before disconnecting the link. Use the same cost value the port currently has.

Best regards,

Peter

the spanning tree cost on port    channel  is 3 .

show spanning-tree interface portchannel

Vlan                Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLANXX          Desg FWD 3         128.488  P2p
VLANYY            Desg FWD 3         128.488  P2p
VLANZZ            Desg FWD 3         128.488  P2p

The connections are between two members of stack on both sides.

3750 stack with 3 members (core 01) ----> 3750 stack with 2 members(serverfarm)

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|

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3750 stack with 3 members (core 02)

If I configure the same cost statically does thsi mean that the link will not participate in the spanning treee convergenece since it already has a cost? my preference would be to carry out this without any interruption to any traffic in the network.

As you can see, the core01 has a cross link to another core02, i believe that this will not have a bigger impact .

what do you suggest I do? is it better to carry out this late working hours?? The server farm also hosts our sql databases and other critical applications that i donot want to affect with this intervention.

I am trying to recover some ports since i see that the server farm traffic is not carrying as much traffic as the bandwidth provided.

apologies i made a mistake in the diagram

server farm --- core 01

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|

core02

the server farm connects to both the core devices and I am looking at gaining one port from each connection.

now it is 4 + 4 and i want to have 2 + 2 on each core.

Could you please test this and let me know?

does it matter where I configure this command ( server or core)?

thanks for helping me out..

am i correct if i configure the cost as 3 ?

show spanning-tree interface portchannel

Vlan Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLANXX Desg FWD 3 128.488 P2p
VLANYY Desg FWD 3 128.488 P2p
VLANZZ Desg FWD 3 128.488 P2p

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