What defines a designated port? in spanning tree

aueddonline

I’m doing some spanning tree diagrams at the moment but I can’t figure out what makes a port a designated port or what makes a port a root port?

any help, out there

EdTheLad

Hello Ed, wonderful name, each segment in spanning tree has a designated port which is the port that has the least cost to the root.Each port on the root bridge is a designated port as the cost to root is zero.All other non root bridges have a port that has the lowest cost to root, since the upstream port must be closer to the root, i.e. directly connected or an upstream port the local port cannot be designated and is called root.So each non-root device can have only one root port,while still abiding by the one designated port per segment.

Paul Boz

A helpful tip to keep in mind is that if two ports have the same cost to the root bridge, bandwidth is the deciding factor, which is a controllable attribute.

tech-airman

aueddonline wrote:

I’m doing some spanning tree diagrams at the moment but I can’t figure out what makes a port a designated port or what makes a port a root port?

any help, out there

aueddonline,

Here's how I remembered the various terms of:

1. Root Switch
2. Designated Switch
3. Root Port
4. Designated Port

First, the root switch election. For the Cisco 2950 switch, by default the switch priority is 32768. Cisco says...
[quote=Catalyst 2950 Desktop Switch Software Configuration Guide, 12.1(6)EA2c - Configuring STP [Cisco Catalyst 2950 Series Switches] - Cisco Systems]
Election of the Root Switch

For each VLAN, the switch with the highest switch priority (the lowest numerical priority value) is elected as the root switch. If all switches are configured with the default priority (3276, the switch with the lowest MAC address in the VLAN becomes the root switch.
[/quote]

So by default, the root switch is the one with the lowest MAC address. That is why you may want to change the default priority value so that, you effectively **** the election process so that a particular switch will most likely win the "root switch" election process.

Now, after the "root switch" election process, the designated switches are elected among the non-root switches. Basically, it's like re-running the election process for the "Vice President" position.

So, you've got a root switch and maybe at least one designated switch. Now to determine the designated ports and root ports. Here's how I remembered it.

Root Switch port POINTING towards a designated switch is called a "designated port."
Designated Switch port POINTING towards the root switch is called a "root port."

Because there is only one root switch, that means there may be more than one designated port. Because STP has chosen one port on the designated switch that is "closest" to the root switch, like they say in Highlander, "there can only be one [root port]."

I hope this helps.

Source:

1. Catalyst 2950 Desktop Switch Software Configuration Guide, 12.1(6)EA2c - Configuring STP [Cisco Catalyst 2950 Series Switches] - Cisco Systems - http://www.cisco.com/en/US/products/hw/switches/ps628/products_configuration_guide_chapter09186a008007e8d1.html

JCrouse

To **** the election process so to say just use ..

SW<config>#spanning-tree vlan ## root primary

JC

tech-airman

JCrouse wrote:

To **** the election process so to say just use ..

SW<config>#spanning-tree vlan ## root primary

JC

JCrouse,

That's not cheating the election process. That's just eliminating the election process entirely.

Question:

1. If you configure the switch as root the way you stated, how does STP deal with the failure of the root switch as configured?

EdTheLad

tech-airman wrote:

That's not cheating the election process. That's just eliminating the election process entirely.

Question:

1. If you configure the switch as root the way you stated, how does STP deal with the failure of the root switch as configured?

It's not eliminating the election process entirely, stp will check the network for the current root and if the current root has a MAC address higher than the local switch, stp will set the local bridge priority to equal the current root, which will mean the local switch will have the lowest BID in the network and become root.
If the current root had a MAC address lower than the local router,stp will set the local router with a
priority equal to the current root bridge minus 4096, again causing the local bridge to become root.The election process is alway going on in the background as a stp bridge is always monitoring bpdus.

To answer your question, STP will reelect the next best switch as root.