2.7 Configure, verify and troubleshoot STP related optional features : Portfast and BPDU guard

Good Morning! I can now feel the breeze of Christmas. Anyways, let’s go straight forward to these STP features.

Portfast

Portfast resolves the following scenario:

You implement STP to all devices in the network including access devices. The issue is that STP runs for 50 seconds and since STP runs for 50 seconds, access devices will get a response after 50 seconds. Remember, access devices receive the response after 50 sec0nds, once DHCP is implemented on that device—> the access device will get an impression that there is no DHCP server and instead dynamically assign an IP address from a range of private IP address, it will self-configure itself with APIPA range of IP address:169.254.0.1 through 169.254.255.254 with a default subnet mask of 255.255.0.0.The setting of the network may go to limited or no connectivity at all.

APIPA IP address configuration will stop once it detected the prescence of DHCP server. Portfast resolves that issue and from the word itself, “fast” means it transition the port from a blocking state into forwarding state. Always remember that portfast should be configured only to those switch ports wherein the other end is access device because STP function to resolve the switching loops may not be operating. Also, let me highlight that a connection of end device and switch port will not introduce switching loops and that’s why immediate transition from blocking state into forwarding state will be the best option for this end device to switch connection.

 

BPDU Guard

Follow-up question Mate? what if there is another switch connected to the end device, does Portfast still applicable? Does it cause switching loops also? To sum the question all up, we have to answer the question if a switch is connected to a portfast enabled interface. Since switches response as soon as you connect them together by sending BPDU request claiming to be the root bridge, a portfast enabled interface will have like a shield called BPDU guard which turn the portfast enabled interface to err-disabled status.So that portfast enabled interface will still be on immediate transition from blocking to forwarding state. So these STP related features should work hand by hand to answer the following:

  1. To prevent limited or no network connectivity when an access device got an impression of no DHCP server present because of the long wait time of STP or RSTP/
  2. To transition a switch interface to portfast enabled interface since it is not causing switching loop , access device doesn’t need to wait for 50 seconds.
  3. BPDU guard act like a shield to portfast enabled interface, shield from receiving BPDU request that may lead to 50 second transition of STP process but what is actually happening is that the switch will go to err-disabled status.
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