For example, if the link negotiated a speed of mbps on a gigabit link, it will choose a cost of 19, not 4. Here we have altered the diagram slightly to introduce the RPC values. We now know the root ports for the switches and need to determine the designating and blocking ports. SW1 will have all ports set to designated as no ports will enter blocking mode. One will be designated, but the other will have to go blocking; this is because there can only be one designated port on a collision domain, and the root port has already been selected on both switches.
With this information, we can quickly work out which will become the designated port, and which will go blocking. First, which has the lowest RPC? Second, which has the lowest BID? This basically ends the function of STP, however there is one last concept to cover which is helpful to understand why it takes a long time for STP to converge reach its final state.
When something changes to a port, e.
- Cisco CCNA – Spanning Tree Protocol(Root Bridge, Root/Designated/Blocked Ports);
- What is STP for?!
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It will first place the port that changed into blocking mode, which quickly moves into listening mode. PortFast should only be placed on access ports pointing at clients, as this mode will skip the states, so from blocking it would actually go straight to forwarding. Beaming worked with key security industry players to develop the ProtectNet service. For businesses like Corps Security, it protects their network, and their reputation. When your business is all about the internet, brilliant digital connectivity is less of a helping hand and more of a vital organ.
Zoonou needed us — but more than they knew. Asset What is STP for? Root bridge election STP has a few things to set up before it can decide what links will be lost.
What is STP & how does it work?
Find out more. Calculating port roles There are three port roles that we are going to address in this blog: Root Ports: A root port is a port that has the best path to reach the root switch, of course a root switch has the best path to itself so it will actually have no root ports. Every other switch has only one root port. Designated Ports: Since it has no root ports, a root switch will have all its ports set to designated.
How does Spanning Tree Protocol prevent loops?
Blocking Port: When a switch port is neither root nor designated, it will go blocking. Switch with the lowest RPC wins Switch with the lowest BID wins Switchport with the lowest priority value default is unless changed Switchport with the lowest number e. Port states There are 5 states that STP can be in, explained in the following table State Name Description Disabled This is a port which has both its physical and logical lines down. It will then stay in this state for 15 seconds Forward Delay default time before moving onto the learning state.
Spanning tree will go through the following checks to determine Root bridge. Checks priority per VLAN. It then selects the device with the lowest priority. If there is a tie that is, 2 devices having the same priority Spanning tree will then skip to step 2 2. It is honored by having all its ports in Designated forwarding state. Every other device will now decide which of its ports is closest to the root bridge. Once they elect that port, they will identify it as the Root Port, meaning, "This port is the closest to the Root Bridge and we must use this port to connect to the Root Bridge.
Spanning tree sends probes into the network to determine loops and it is also used in the election of a Root Bridge. It then freely submits to that switch. If it receives another BPDU from another device that has even a lower priority or MAC address than the previous switch it had assigned as its Root Bridge, it automatically replaces the old switch and makes the new switch the Root Bridge. This is done per VLAN. If however, a switch sees its own BPDU, it knows there is redundancy set up somewhere which potentially can cause a loop.
It will then determine which of the redundant connections it needs to block if it is the "Highest switch in the pile" based on priority or MAC address. LVL Networking 8. Networking 5. LVL 4. Networking 1. Level Author Comment by: Akinsd I plan to follow it up with another article.
Not too many people understand the underlying theory behind most device processes. Articles like this will eliminate trivial questions. Works for me Thanks.
Spanning Tree Election | R&S Blog
Administrative Comment by: Eric AKA Netminder Akinsd, Congratulations! Your article has now been published. Learn More. Level 4. Expert Comment by: Sandeep Udgirkar Great Article. Understood it in one reading. Cyclops Only two things I would note.
- Computer Network | Root Bridge Election in Spanning Tree Protocol?
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You write "The default rule for a root bridge if all devices have default priority is that the oldest device becomes the root bridge" You correctly say later, but just so there's not confusion I would correct that mistype. The closest is MST. PVST is Cisco specific. Yes, the was a typo Thanks for the observation.