Solved: Should an entire Port-Channel be blocking?

JoshGarmonCiscoCCNA · ‎10-22-2015

I got a Topology.

Everything was configured correctly, I've veried that all Port-Channels are working on all Switches...all hosts can talk to each other in their respective VLANS.

I got Trunks on every link between all the Switches.

My show etherchannel summary indicates the Port Channel is GOOD though:

Sw1#show etherchannel summary

Flags: D - down P - in port-channel

I - stand-alone s - suspended

H - Hot-standby (LACP only)

R - Layer3 S - Layer2

U - in use f - failed to allocate aggregator

u - unsuitable for bundling

w - waiting to be aggregated

d - default port

Number of channel-groups in use: 2

Number of aggregators: 2

Group Port-channel Protocol Ports

------+-------------+-----------+----------------------------------------------

1 Po1(SU) LACP Gig0/1(P) Gig0/2(P)

However, you'll notice that two Gigabit ports (Port-Channel 1 on that Switch) are ORANGE...meaning BLOCKED...and this was verified when I ran show spanning-tree on that Switch

Is this supposed to happen?

jhager001 · ‎10-23-2015

Yes, even if one of those links were green in your LACP portchannel you would still be performing a loop, therefor it has blocked due to the fact you have more links (aggregated bandwidth) between the other switches so it's allowing more transmission between.

View solution in original post

Rolf Fischer · ‎10-22-2015

Hi,

spanning-tree treats a port channel like a single link (just with lower costs than the physical member-ports).

So from a spanning-tree perspective, the three switches are interconnected to each other and STP has to break the loop at some point. The sequence to determine the roles and states follows the same rules as it does when switches are interconnected with single (physical) links.

HTH

Rolf

Joseph W. Doherty · ‎10-23-2015

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As Rolf says. A port-channel is seen as a single link. If the port-channel is L2, and its existence would create a L2 loop, then STP might block it. What link will actually get block follows ordinary STP rules.

Ganesh Hariharan · ‎10-22-2015

Hi,

Considering the topology the blocked ports are two link ether channel compare to other 4 port ether channel.

Port cost for the ether channel is updated immediately in order to reflect the new available bandwidth.

Check out the below table for more detail..

Hope it Helps..

-GI

Rate if it Helpss

glen.grant · ‎10-23-2015

With that design it is definetly correct. You have a built in loop so spanning tree chose to block the link (port-channel) with the lowest bandwidth . It did it's job and is working as designed. If you do a show spanning-tree blockedports if will show that entire channel as blocked.

jhager001 · ‎10-23-2015

Yes, even if one of those links were green in your LACP portchannel you would still be performing a loop, therefor it has blocked due to the fact you have more links (aggregated bandwidth) between the other switches so it's allowing more transmission between.

JoshGarmonCiscoCCNA · ‎10-23-2015

hey thanks everybody, that was helpful