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Spanning-tree port fast

Posted on 2016-08-12
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Last Modified: 2016-08-16
Is it a good idea to have this enabled on the interface between a L3 switch and router?
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Question by:PeraHoman
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by:SIM50
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ID: 41753949
Yes if a router is not configured for bridging. Just to be safe, put the following command:
spanning-tree portfast bpduguard enable
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Jan Springer earned 1000 total points
ID: 41753968
If it's a layer 3 interface, it should not have spanning tree configured anyway -- at all.
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by:PeraHoman
ID: 41754037
True, I'm looking through old configs and saw it and wanted some opinions.
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LVL 14

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by:SIM50
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ID: 41754327
If it's a layer 3 interface, it should not have spanning tree configured anyway -- at all.

STP is automatically disabled once you change port from L2 to L3 on a switch.
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by:Michael Ortega
Michael Ortega earned 500 total points
ID: 41756221
You should never have STP portfast enabled on ports between switches or routers, whether they are operating as an L3 device (switches) or not. Portfast is intended for network endpoints, e.g. computers, servers, printers, etc.

MO
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by:SIM50
SIM50 earned 500 total points
ID: 41756329
You should never have STP portfast enabled on ports between switches or routers, whether they are operating as an L3 device (switches) or not. Portfast is intended for network endpoints, e.g. computers, servers, printers, etc.

I don't completely agree with this statement, specifically the bolded part.
According to Cisco best practices:

Configure STP PortFast only on ports that are connected to end host devices that terminate VLANs and from which the port should never receive STP BPDUs, such as:
–Workstations
–Servers
–Ports on routers that are not configured to support bridging

Link: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/best/practices/recommendations.html#wp1061957

Also, when you change a port on a switch from L2 to L3, portfast is enabled automatically and the only option you have is to disable it. You are welcome to test all of this btw.
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by:Michael Ortega
Michael Ortega earned 500 total points
ID: 41756653
@SIM50

I misspoke. Bridging is the key. I guess I didn't quite think of it in the way of a true router where you're essentially L3 on each interface. I was thinking more in the application where you're connecting your environment to a firewall, where in many cases SMB sized firewalls have all the inside interfaces bound together as a bridge/switch.

Good point, and thanks for clearing that up. Where you employ bridge/switch to bridge/switch communication you want portfast off.

MO
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