Spanning tree portfast issues

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Jul 28th, 2009
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1. Is it required to enabling portfast for all access port on catalyst switch to avoid initial response delay? What happens if not enabled or vice versa?

2. Access ports are typically connected to host/pc, if portfast enabled on that port, everytime that switch port is in forwarding mode if its up or down.

3. If yes then,Can i enable the portfast for all access ports for L2 switches to avoid initial delay?

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Jon Marshall Tue, 07/28/2009 - 06:45
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1) Not required but recommended. If portfast is not enabled as you say there is an initial delay and this could create problems with DHCP address requests etc.

In addition hosts are often switched off and on and without portfast enabled this will create a TCN (Topology Change Notification) within STP.

2) Not sure i understand question. If portfast is enabled on a port that port will immediately transition to forwarding.

3) You should enable portfast for all end devices such as hosts/servers/routers/firewalls etc.


pradip.narkhedkar Tue, 07/28/2009 - 22:17
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Thanks Jon

One small query,

If the host/servers connected switch port up or down, then creates TCN, Can TCN affect the LAN traffic?

Amit Singh Tue, 07/28/2009 - 23:04
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If the host/server switchport is constantly going up/down, then it might effect the LAN traffic. When a link on a switch port goes up or down, there is eventually a TC, once the STP state of the port is changing to or from forwarding. When the port is flapping, this would cause repetitive TCs and flooding.

Ports with the STP portfast feature enabled will not cause TCs when going to or from the forwarding state. The configuration of portfast on all end-device ports (such as printers, PCs, and servers) should limit TCs to a low amount and is highly recommended.

The way TCN BPDU's are generated and are being handled varies depending upon the STP standard being used and it directly shortens the convergence time on the LAN. IEEE 802.1w RSTP /IEEE 802.1S MSTP provides you faster convergence in case of a TCN generation in comparison to IEEE802.1D STP.


-amit singh


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