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New Member

rapid-pvst mode AND spanning-tree portfast trunk

Ok, if my switches are configured for RSTP, do I still have to add "spanning-tree portfast trunk" under the dot1q interfaces??

4 REPLIES
Hall of Fame Super Blue

Re: rapid-pvst mode AND spanning-tree portfast trunk

Alex

Regardless of which STP version you are using you should only use "spanning-tree portfast trunk" on ports connected to end devices such as servers that run 802.1q on their NIC's.

You should never use this command on a trunk link between 2 switches.

Having said that yes you would still benefit from this command even with RSTP.

Jon

Bronze

Re: rapid-pvst mode AND spanning-tree portfast trunk

Yes, there is certain advantage of using portfast while still having Rapid convergence using RSTP.

If port does not have Portfast enabled, eventhough it will go into forwarding state quickly as per RSTP standard, it will reach that state after going through BLK,LRN. Whereas configuring portfast feature compel it to skip these states and be in forwarding state right away.

I hope that you aware of this guideline by Cisco: "You can use PortFast to connect a single end station or a switch port to a switch port. If you enable PortFast on a port that is connected to another Layer 2 device, such as a switch, you might create network loops."

New Member

Re: rapid-pvst mode AND spanning-tree portfast trunk

what do you mean "or a switch port to a switch port."?

that is a loop cause; unless its a uplink port, like a sfp-to-sfp.... then no portfast is needed.....

Cisco Employee

Re: rapid-pvst mode AND spanning-tree portfast trunk

Hi,

In addition to all responses of other friends here, and perhaps somewhat aside of what you asked about originally, configuring PortFast on ports that go to non-switching devices (servers, workstations, PCs, printers, routers, firewalls, ...) is very important with RSTP. RSTP utilizes an internal mechanism called Proposal/Agreement whose purpose is to move a link between two switches from Discarding into Forwarding state rapidly if it does not create a switching loop. The Proposal/Agreement applies to all non-edge (that is, non-PortFast) ports and relies on both switches mutually exchanging RSTP BPDUs. However, if the Proposal/Agreement mechanism moves to a port that is connected to a non-switching device, the non-switching device does not speak RSTP and won't be able to respond to the Proposals with an Agreement, and this can result into intermittently occurring outages in the connectivity of this device that will last for 30 seconds. It truly happens quite often that a network is migrated from PVST to Rapid-PVST, and suddenly users start complaining about connectivity drops just because the ports toward end hosts have not been configured with PortFast.

So, the bottom line is: If the port goes to an end host, or to a network device that is not a switch but rather a Layer3 or higher device (router, firewall, load balancer, etc.), it should be configured with PortFast. If that port happens to be a trunk, you will need spanning-tree portfast trunk, otherwise, spanning-tree portfast is sufficient. However, you should never, ever, use either of these commands on an interface that connects to another switch, access point, or wireless bridge.

Best regards,
Peter

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