portfast trunk

[rsabapathee]

what does portfast trunk mean?

From my existing configuration, I can see that 3 cisco 2900XL switches are trunked to a Cisco 2970 gigabit switch and the command portfast trunk is enabled on each link?........I must add network is working fine with several vlans configured on a router on a stick model.

What are the likely consequences of this command enabled?

thanks

[gpulos]

the 'portfast trunk' command places that trunk port in the SpanningTree portfast state.

portfast allows the port to go from blocking state to forwarding state without having to go through the delay of the listening and learning states.

this provides for faster port forwarding of packets. the downside is that without the STP listening/learning states, the ability to induce a loop into the network is greatly increased.

your consequences of this is that you'll want to assure you do not make another trunk, that has portfast trunk, between the two devices already trunked with the original trunk that has portfast trunk enabled currently.

(this would have the effect of creating a loop in your topology. ethernet does not like loops and your network will more than likely grind to a halt.)

it's just like a regular portfast port, but works for trunks.

[rsabapathee]

Currently, the main switch is the gigabit switch 2670. The other 3 switches are linked directly to the 2670 with dot1Q trunk enabled and also portfast enabled.

From your explanation, the set-up above should not have worked?? Please clarify.

Also, if i understand you correctly portfast trunk is meant only between 2 switches. If a third switch is added, portfast trunk should not be enabled. Is that correct?

thx

[rsabapathee]

also what happens when an unmanaged switch is connected to a port which has portfast enabled??

[Edison Ortiz]

Your setup would work but it's not optimal. You can easily introduce loops in your ethernet network by enabling portfast on ports connected to other switches.

If you connect two links from switch to switch, and you have portfast disabled - one link will be down while the other one will be up. This is the optimal setting.

With portfast enabled - both links can potentially be up at the same time hence creating loops with traffic between the switches.

That's the main reason Cisco recommends never to enable portfast on links connected to other switches because there is a chance of having one more link going somewhere else.

[gpulos]

please let me clarify...

if you have the following topology:

2670 >> swtich1

>> switch2

>> switch3

this is OK as far as what i was talking about with your trunk portfast.

what is not OK is as follows:

2670 >> switch1

>> switch2

>> switch3

switch2 >> switch3

in the above scenario, there would not be a loop. if the connections between these devices were all trunk portfast, then this loop would cause a network outage.

i hope this clarifies my original response.

as originally stated, you will want to be sure you do not create such a connection where as to create a loop, while the connections have portfast enabled.