Flex Link + STP Features

francisco_1 · ‎02-09-2010

Is Portfast & BPDUGuard still needed on access ports when Flex Link is enable?

STP is disable when FlexLink is enable but i believe both can be active on the same switch but Flex Links do not participate in STP in all VLANs in which STP is configured. If the switch is running STP, Flex Links is not necessary because STP already provides Link-level redundancy or backup. With STP not running, there are no loops in the configured topology. So my conclusion is to better have either one of the other active not both on the same switch and if Flex link is enable, no need for Portfast or BGPDUGard, Loopguard or any STP machanisms active since STP is no longer active on the switch.

Francisco

Giuseppe Larosa · ‎02-09-2010

Hello Francisco,

flex link is enabled on a pair of links and only those pair of links have STP disabled:

>> STP is disabled on Flex Link ports. A Flex Link port does not participate in STP, even if the VLANs present on the port are configured for STP. When STP is not enabled, ensure that there are no loops in the configured topology.

see

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/46sg/configuration/guide/swflink.html

So my understanding is that unless you disable STP on a per vlan basis, STP is still running on standard ports including access ports.

If so, you can take advantage of using both portfast and BPDU guard on access ports even if uplinks are on flex links for the usual reasons: no TCN on port state change, safety in case of connection of user provided lan switches etc.

I would suggest that this option should be preferred over disabling STP for selected vlans

Hope to help

Giuseppe