Solved

What is a root switch and how is it chosen?

Posted on 2009-04-11
4
1,193 Views
Last Modified: 2012-08-13
I need some help with my homework, I can't find root switch anywhere in my text.

Thanks!
0
Comment
Question by:penywisexx
  • 2
4 Comments
 
LVL 23

Accepted Solution

by:
that1guy15 earned 250 total points
ID: 24124083
0
 
LVL 12

Expert Comment

by:jjmartineziii
ID: 24127497
The root bridge of the spanning tree is the bridge with the smallest bridge ID.

When you type "show spanning-tree root" on the root bridge, you will see "This is the root bridge" in the output.


In any Spanning Tree instance, there is only one Root Bridge, and it must be elected. The Root Bridge is elected in the initial exchange of BPDUs between bridging devices. But how does the Root Bridge get elected? Thats simple. In networks running STP, every bridge has a priority value associated with it. By default, the priority of all bridges is 32,768, unless changed by an administrator. The bridge with the highest priority gets to be the Root Bridge. But wait  youll need to remember that the highest priority is the bridge with the lowest priority value. That is, a bridge priority of 1000 would beat the default priority of 32,768.
0
 
LVL 32

Expert Comment

by:harbor235
ID: 24131069


The default bridge priority is 32768, all switches start here. When you connect switches together they begin a process called a root war. Every device advertises it's bridge priority in BPDUs, lowest root bid or bridge priority becomes root and backup root bridge. If all things are equal then there is a tie breaking process. The device with the lowest mac address becomes the root bridge and the device with the next lowest mac address becomes backup root. Problem is this process can elect a root bridge is a non-optimal location. Thats why there are ways to influece the process, you can change the bridge priority to force election of a particular device to root and backup root. This allows you to pick the best device for root bridge operation. You can also influence the path traffic takes to get to the root bridge by manipulating the path cost, and path vlan cost of the interfaces
in the switch mesh.

Here is a good doc

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/spantree.html#wp1032530

harbor235 ;}
0
 
LVL 23

Expert Comment

by:that1guy15
ID: 24196935
Did this help get your answer solved?
0

Featured Post

What Security Threats Are You Missing?

Enhance your security with threat intelligence from the web. Get trending threat insights on hackers, exploits, and suspicious IP addresses delivered to your inbox with our free Cyber Daily.

Join & Write a Comment

Suggested Solutions

Title # Comments Views Activity
Configuring switch:  PortFast Edge vs PortFast 4 63
Cisco iWAN 8 45
High Density Switches 8 23
EIGRP on point-to-point vlan 14 20
Microservice architecture adoption brings many advantages, but can add intricacy. Selecting the right orchestration tool is most important for business specific needs.
I recently attended Cisco Live! in Las Vegas, a conference that boasted over 28,000 techies in attendance, and a week of hands-on learning hosted by a solid partner with which Concerto goes to market.  Every year, Cisco displays cutting-edge technol…
After creating this article (http://www.experts-exchange.com/articles/23699/Setup-Mikrotik-routers-with-OSPF.html), I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…
After creating this article (http://www.experts-exchange.com/articles/23699/Setup-Mikrotik-routers-with-OSPF.html), I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…

708 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

14 Experts available now in Live!

Get 1:1 Help Now