router trunks and RSTP

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I've been working lately to update the network designs for our various field offices. The first effort is to upgrade spanning tree to 802.1w using cisco's rpvst+. I had not noticed it before, but due to the rapid convergence, i'm now observing that switches with router trunk uplinks which detect TC's are putting the router uplink in blocking mode for about 25 or 30 seconds. If I understand correctly, this is because RSTP is trying to send proposals out the uplinks, and then holding the port in blocking mode until either its forward timer (max age + listening) expires, or it receives an agreement. The routers LAN facing trunks are not running STP at all, and so the lan looses connectivity for much longer than RSTP takes to converge.

Any idea's on how to improve LAN to WAN convergence times? I'm working with about 5,000 switches, so the simpler the better. My router images dont seem to support RSTP, but I'll be double checking that today in the lab.

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Peter Paluch Mon, 08/03/2009 - 08:54

Hello,

As far as I know, routers do not run STP on their routed ports. If you are using RSTP, you should configure the ports on your switches leading to servers, routers etc. as edge ports using the command spanning-tree portfast in the port configuration.

As a small reminder, RSTP achieves fast convergence only in these two instances:

1.) On a point-to-point link between two switches running RSTP using the proposal/agreement mechanism

2.) On edge ports that are by definition connected to an end device and where it does not make sense to go over the Discarding/Learning/Forwarding sequence

Exactly as you have suggested, the topology changes can make your Forwarding ports go to Discarding state and you then have to wait for timers to expire until the ports are forwarding data again. Therefore, in RSTP, it is vital to configure edge ports in order to speed up the convergence.

RSTP does not detect edge ports automatically, therefore, it is necessary that you go over your switches and configure the necessary access ports as edge ports.

If there are far too many ports that would require adding the "spanning-tree portfast" command, you can use the command spanning-tree portfast default in the global configuration mode. This command causes all ports on a switch that are configured as access ports to be automatically considered as edge ports.

Best regards,

Peter

Peter Paluch Mon, 08/03/2009 - 09:11

Hello,

For trunk lines interconnecting two switches, you should not need to enable PortFast because it is the responsibility of the RSTP to unblock this trunk rapidly.

However, for trunks leading to routers or 802.1Q capable servers, you can use the command spanning-tree portfast trunk in the port configuration. This will allow the port to immediately enter the Forwarding state even in trunk mode.

Pay attention to never configure this on a trunk link leading to another switch, as it may cause transient (or even permanent) Layer2 loops as a result.

Best regards,

Peter

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