should port fast enabled on uplink port or not?

Answered Question
Nov 18th, 2008

Hi all,

I have question on Portfast .

i have layer 3 switch that connects to another layer 3 switch.

here is switch 1 port config

interface GigabitEthernet1/2

description uplink bmanysw02

switchport trunk encapsulation isl

switchport trunk allowed vlan 1,5,9

switchport mode trunk

logging event link-status

speed 100

duplex full

udld port aggressive

channel-protocol pagp

channel-group 1 mode desirable

spanning-tree portfast disable*******

as seen above port fast is disabled on this uplink port.

i want to know why we do not use port fast on uplink ports or ports which connect to other switches.i know that port fast is normally used on user access port.

I have this problem too.
0 votes
Correct Answer by Jon Marshall about 8 years 1 week ago

Mahesh

A loop is not formed by enabling portfast on trunk ports. But if you do have a loop and your switch interconnects have portfast enabled then the switch will not be able to detect the loop in time.

Enabling portfast on a port does not disable STP but it allows the port to forward immediately.

A loop is formed when you physically connect up the switches in a certain way. There is nothing wrong with having loops, in fact a lot of L2 networks have loops for redundancy but if there is a loop STP needs to block one of the ports to break that loop. If portfast is enabled on a switch interconnect forming a loop then because portfast allows a port to immediately forward it would be too late for STP to try and break the loop because the L2 network would be overwhelmed.

Jon

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Jon Marshall Tue, 11/18/2008 - 09:44

Mahesh

When you configure portfast on a switchport this allows that port to begin forwarding traffic immediately without having to go through the normal STP process of listening/learning/forwarding. With end devices such as PC's this is fine because they will not be forming a loop in the L2 network.

However when you connect one switch to another there is the possibility that you could then have created a loop. Loops are fine in that STP would then block one of the connections to stop that loop but if you have enabled portfast on the connections then by the time STP realises what is happening it is already too late and your network is unuseable.

Jon

mahesh18 Tue, 11/18/2008 - 11:29

Hi Jon,

thanks for reply.

Want to know in detail or more explaination of how loop is caused by enabling port fasst

on trunk ports?

many thanks

mahesh

Correct Answer
Jon Marshall Tue, 11/18/2008 - 12:05

Mahesh

A loop is not formed by enabling portfast on trunk ports. But if you do have a loop and your switch interconnects have portfast enabled then the switch will not be able to detect the loop in time.

Enabling portfast on a port does not disable STP but it allows the port to forward immediately.

A loop is formed when you physically connect up the switches in a certain way. There is nothing wrong with having loops, in fact a lot of L2 networks have loops for redundancy but if there is a loop STP needs to block one of the ports to break that loop. If portfast is enabled on a switch interconnect forming a loop then because portfast allows a port to immediately forward it would be too late for STP to try and break the loop because the L2 network would be overwhelmed.

Jon

Giuseppe Larosa Tue, 11/18/2008 - 12:15

Hello Mahesh,

just a little add to Jon's posts.

for uplinks there is another feature to speed up convergence called uplinkfast in classic STP that makes some changes in order to be able to use the alternate uplink port (alternate port in Rapid STP terminology).

Enabling it in global config it makes the following changes:

it add 3000 to each port cost

it increases the STP bridge priority to something in the order of 45000 or more if using extended system-id.

The switch must be a leaf not in the path to root bridge for another switch (like access layer switches in campus design)

When the root port fails the switch can use the alternate port without waiting for all the usual timers.

In addition to refresh the CAM table of other devices it will send out multicast frames each with a source learned on its access ports.

Hope to help

Giuseppe

ju_mobile Tue, 12/16/2008 - 06:48

spanning-tree portfast trunk

can you elaborate on this one ?

Jon Marshall Tue, 12/16/2008 - 08:05

"spanning-tree portfast trunk" has the same effect for trunk ports that "spanning-tree portfast" has for access ports ie. it allows the port to being forwarding immediately rather than having to go through the listening/learning stages of STP but STP is still run on that port.

You wouldn't ordinarily want to run "spanning-tree portfast trunk" on switch uplinks but it can be useful on server connections where the server NIC is runnng 802.1q.

Jon

cisco24x7 Tue, 12/16/2008 - 08:13

"spanning-tree portfast trunk" are extremely

useful for switchports that are connect to

VMWare ESX servers that use 802.1Q.

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