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Level 10
Level 10

Core issue

The EtherChannel feature allows multiple links between the same two devices to work as if they were one fast link, with the traffic load balanced among the links. A switch can form these bundles automatically with a neighbor through the Port Aggregation Protocol (PAgP). Switch ports that can run PAgP usually default to a passive mode called auto mode. This means that they are able to form a bundle if the neighbor device across the link makes a request. If the protocol is run in auto mode, this can cause a port to delay for up to 15 seconds before it passes control to the Spanning Tree Algorithm (STA).

Note: PAgP runs on a port before STP does. There is no reason to have PAgP run on a port connected to a workstation. In order to eliminate this delay, set the switch port PAgP mode to off.

If a workstation or a server is connected with a single Network Interface Card (NIC) to a switch port, this connection cannot create a physical loop. These connections are considered leaf nodes. There is no reason to make the workstation wait 30 seconds while the switch checks for loops when the workstation cannot cause a loop. With the addition of the PortFast or fast-start feature, the STP for this port assumes that the port is not part of a loop. In this case, the port immediately moves to the forwarding state, and skips the blocking, listening, or learning states. This command does not turn STP off. This command makes STP skip a few steps in the beginning on the selected port, although unnecessary in this circumstance.

Note: The PortFast feature must never be used on switch ports that connect to other switches, hubs, or routers. These connections can cause physical loops, and it is very important that Spanning Tree go through the full initialization procedure in these situations. A Spanning Tree loop can bring the network down. If the PortFast feature is turned on for a port that is part of a physical loop, it can cause packets to be continuously forwarded and even multiply in such a way that the network cannot recover.

While teaming is performed, the PortFast does not work if EtherChannel is not enabled. This can be due to the shutdown command in the portchannel id, which applies to all of the ports in that EtherChannel group.


When multiple ports are connected to a server, enable and configure EtherChannel for load balancing and redundant link on the switch.

After EtherChannel is configured with the server NIC team, issue the no shutdown command under the portchannel ID.

Refer to Configuring EtherChannel Between a Catalyst Switch Running CatOS and a Workstation or Server for more information on how to enable and configure EtherChannel.

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