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Accelerating  Ethernet Connection Startup on Cisco Switches

4 features enabled by default on Cisco switches that cause delay when connecting port to a host:
   1. Spanning Tree Protocol (STP)
   2. EtherChannel negotiation
   3. Trunking negotiation
   4. Link speed/duplex negotiation between the switch and the host

1. Spanning Tree Protocol (STP)

A port running STP can have 1 of 5 states: blocking, listening, learning, forwarding, and disabled. STP dictates that the port starts out blocking, then immediately moves through the listening and learning phases. By default, it will spend approximately 15 seconds listening and 15 seconds learning. During the listening state, the switch is trying to determine where the port fits in the Spanning Tree topology. The switch especially wants to know whether this port is part of a physical loop. If it is part of a loop, this port may be chosen to go into blocking mode. If the port is not part of a loop, it will proceed to the learning state, which involves learning which MAC addresses live off of this port. This entire STP initialization process takes about 30 seconds.

Configure ports connected to the host to portfast.
Configuring edge ports as port fast not only waaaay speed up startup connection negotiation, but also speed up the RSTP convergence. Always set edge ports to portfast!!!

Switch(config)#interface fa0/2
Switch (config-if)#spanning-tree portfast

2. EtherChannel Negotiation

EtherChannel  allow multiple links between the same two devices to work as if they were one fast link, with traffic load balanced among the links. A switch can form these bundles automatically with a neighbor using Port Aggregation Protocol (PagP).

By default all interfaces have PagP in auto mode. Which means The interface is in a passive negotiating state; it responds to PAgP packets it receives but does not start PAgP packet negotiation. PAgP is enabled only if a PAgP device is detected.

Running the protocol in auto mode can cause a port to delay for up to 15 seconds before passing control to the Spanning Tree Algorithm (STA) (PAgP runs on a port before STP does).

There is no reason to have PAgP running on a port connected to a host. Setting the switch port PAgP mode to off will eliminate this delay (if  your switch allow you to turn PAgP off…)

Fortunately some switches have EtherChannel disabled by default.

3. Trunking Negotiation

From Cisco school of thought: A trunk is configured between two devices when they need to carry traffic from multiple VLANs... meaning = VLAN tagging.

Usually, a port connected to a workstation belongs to only one VLAN, and therefore does not need to trunk. If a port has the ability to negotiate the formation of a trunk, it will usually default to the auto mode. If the port is changed to a trunking mode of off, it will further reduce the delay of a switch port becoming active.

Turning of the damn trunking when not needed!

Switch(config)#interface fa0/2
Switch(config-if)#switchport mode access
Switch(config-if)#switchport nonegotiate (optional)
Switch#sh int fa0/2 trunk
Port      Mode         Encapsulation  Status        Native vlan
Fa0/2     off          negotiate      not-trunking  1

switchport nonegotiate : Prevents the interface from generating Dynamic Trunking Protocol (DTP) frames. You can use this command only when the interface switchport mode is access or trunk. You must manually configure the neighboring interface as a trunk interface to establish a trunk link.

4. Speed and Duplex Negotiation

Turning on PortFast, turning off PAgP (if present), and disabling trunking is usually enough to solve an initial connectivity delay. If you need to eliminate every possible micro-second, you could also set the port speed and duplex manually on the switch.


SwitchB(config)#interface fa0/2
SwitchB(config-if)# speed 100
SwitchB(config-if)# duplex full

If you set the speed and duplex on one side, you must set the speed and duplex on the connecting device as well to avoid these problems.

homepage $Date: Sun Jan 16 19:03:56 CET 2005 $ © 2003-2005 Omar Gani