Elect One/Multiple Designated Ports on each Non RB:

Criteria:

• Port with the lowest cost to reach RB

  Tie Breaker:

• Lowest Local BID

• Lowest Local Port Priority

• Lowest Local port Number

• If however it is connected through a HUB then election goes to the lowest interface STP/RSTP priority and, if that ties, the lowest internal interface number (this scenario is very unlikely)

• As mentioned earlier that to prevent loop STP puts an interface either in Forwarding or Blocking mode. To do that it elects a RB first then Elect Root Port on each Non RB & then finally Elect One/Multiple Designated Ports on each Non RB.

• RB will send it's cost as 0 in BPDU, & the receiving Switches will add their port's cost to it then calculate accordingly.

• The Designated port of each LAN segment is the port that advertises the lowest-cost Hello onto a LAN segment to reach the RB.

• One or Multiple ports of a Non-Root Bridge will be marked as Designated Ports & hence will be put in the forwarding state to keep the links between each Switch up for forwarding traffic onto a network segment & the remaining ports will be blocked by default.

• E.g. SW1 is a RB (with the lowest MAC 001). Hence the opposite ports of SW1 will automatically become Root Port (gig0/2 of SW2 & gig 0/1 of SW3). Now for Designated port election between gig 0/2 of SW3 & gig0/1 of SW2 both will send their cost in Hellos & the Switch which utilizes least cost to reach the RB it's port will be marked as Designated Port (Gig0/1 of SW2) & neighbor Switch's port will be marked as Non-Designated (Gig0/2 of SW1) & hence will be blocked.

Reference:

• CCNA 200-301 OCG, Volume 1, Pg. 222- Wendell Odom.