Spanning Tree Protocol: Root Bridge or not, Path Costs and STP Port States

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Root Bridge only has desg/fwd ports, root ports are only for other switches to reach the root bridge.

Path Cost is local to a port, Root path Cost is overall cost to get to the root switch.

Root Path Cost increments as BPDUs are RECEIVED not SENT, as BPDU passes from switch to switch, the RPC is calculated.

1. Superior BPDU, 2. Lowest RPC, 3. Lowest Sender BID 4. Lowest sender Port ID.

Path Costs Review

  • 10Mbps:100
  • 100Mbps: 19
  • 1Gbps: 4
  • 10Gbps: 1

Physical shortest path is not always the logical shortest path. Always verify link speeds.

Changing a ports path cost

Under physical interface: ‘spanning-tree cost VALUE’

PVST Path Costs modify:

Under physical interface: ‘spanning-tree vlan NUMBER cost VALUE’

DIS/Disabled port state is a port that is admin down. Not officially taking place in STP.


STP Timers

Hello Time = 2 seconds – How often the RB will originate config BPDU.

Forward Delay = 15 seconds – Length of both listening and learning stages.

Maximum Age = 20 seconds – How often switch will retain the superior BPDU before discarding it.

Should always be changed on RB.


Spanning Tree Protocol: BPDU and Root Bridge Election Fundamentals

BPDU and Root Bridge Election Fundamentals

The Source MAC address is the first thing that the switch looks at on incoming frames.

Unknown unicast, broadcast and multicast frames are forwarded out all ports excluding the one that came in on.

Known unicast frames are unicast via the port listed in the MAC address table

Broadcasts traditionally result in more broadcasts, hence a broadcast storm!

Switching loops cause 3 issues, frames don’t reach destination – strain on CPU, wasted bandwidth

BPDUs are sent every 2 seconds to multicast MAC address 01-80-c2-00-00-00


Once root bridge is elected, only RB will originate a CONFIGURATION BPDU, all non root bridge switches will send COPIES only.

BID = 2 byte Priority value and 6 byte MAC address

If a tie in the priority value, then the mac address breaks the tie (Priority and MAC making up the entire BID)

CCNP Revision

Ok so as previously stated I am going to go back to the past and revise all of the routing and switching concepts to NP level. I made a smart a decision a year or so to purchase the excellent CCNP training material from Chris Bryant at Chris teaches with incredible depth and  offers a good prerequisite of knowledge required for the IE. I will try and fit as much as this in as possible over the next few weeks. I am also going to document notes from the training videos so this website will grow into a full on repository of NP/IE concepts.

Time to go back..

Ok so I reviewed the check list and left a lot of blanks. The problem is I am scared to commit to being an ‘expert’ at any concept, let alone ones I am good at. I have decided to go back to my CCNP R&S Videos to recap a lot of concepts, then I hopefully can begin to tick those boxes.

Materials Used:

Chris Bryant in my opinion is the best instructor I have come across for video based traning. Jeremy Cioara of CBT Nuggets does enough to open the door on a concept and show you enough to probably get away with it, but Chris Bryant offers far more depth and even quotes a lot of time ‘if you want to get your IE, you want to know this!’

My intention is to catch up on a lot of CCNP concepts, then begin the training for the written exam. I am not sure at this point what materials I will use. We will cross that bridge in time..