The Evil Packet Sniffer
A "Packet Sniffer" is a utility that sniffs without modifying the network's packets in any way. By comparison, a firewall sees all of a computer's packet traffic as well, but it has the ability to block and drop any packets that its programming dictates. Packet sniffers merely watch...
Losing Data Over VPN
Sometimes change is good, sometimes it's bad, and sometimes it's a mixed bag. Such was the case recently when a customer switched a remote office from a 56K line to using a Kerio VPN over Internet connections. The 56K line was slow and expensive, so the switch had...
ColdFusion And Server Monitoring
Unlocking the CF Server Black Box. How to answer questions about your server. How is it doing? What templates/queries are slow? New tool: CF Server Monitor. Monitors requests via all paths (template, CFC, web service, gateway, Flash Remoting) Has minimal...
ISIS Router Types
To pass the BSCI exam and earn your CCNP, you´ve got to know ISIS inside and out. There are many similarities between ISIS and OSPF, but one major difference is that ISIS has three different types of routers - Level 1 (L1), Level 2 (L2), and L1/L2. L1 routers are...
Configuring An Access Server
As your CCNA / CCNP home lab expands, an access server such as the Cisco 2509 or 2511 is one of the best investments you can make. In this article, we´ll look at the basic configuration for an access server and discuss how to connect to the other routers and switches...
Trunking And Trunking Protocols
To earn your CCNA or CCNP certification, you´ve got to understand the basics of trunking. This isn´t just a CCNA topic - you must have an advanced understanding of trunking and etherchannels to pass the BCMSN exam and earn your CCNP as well.
OSPF Hub-And-Spoke Details You Must Know!
CCNA exam success depends greatly on knowing the details, and if there´s one protocol that has a lot of details, it´s OSPF! This is true particularly of hub-and-spoke networks, so in this CCNA OSPF tutorial we´ll take a look at some of the more important...
Changing Root Bridge Election Results
By Chris Bryant
Your BCMSN and CCNP studies will include mastering the details of Spanning Tree Protocol (STP).
While you learned some of these details in your CCNA studies, quite a bit of it may be new to you. Before going on to the intermediate and advanced STP features, lets review the root bridge election process and learn how to change these results.
Each switch will have a Bridge ID Priority value, more commonly referred to as a BID. This BID is a combination of a default priority value and the switchs MAC address, with the priority value listed first. For example, if a Cisco switch has the default priority value of 32,768 and a MAC address of 11-22-33-44-55-66, the BID would be 32768:11-22-33-44-55-66. Therefore, if the switch priority is left at the default, the MAC address is the deciding factor.
Switches are a lot like people - when they first arrive, they announce that they are the center of the universe! Unlike some people, the switches will soon get over it. BPDUs will be exchanged until one switch is elected Root Bridge, and its the switch with the lowest BPDU that will end up being the Root Bridge.
If STP is left totally alone, a single switch is going to be the root bridge for every single VLAN in your network. Worse, that single switch is going to be selected because it has a lower MAC address than every other switch, which isnt exactly the criteria you want to use to select a single root bridge.
The time will definitely come when you want to determine a particular switch to be the root bridge for your VLANs, or when you will want to spread the root bridge workload. For instance, if you have 50 VLANs and five switches, you may want each switch to act as the root bridge for 10 VLANs each. You can make this happen with the spanning-tree vlan root command.
In this example, weve got two switches, and SW1 has been elected the root bridge for VLANs 10, 20, and 30. Well use the spanning-tree vlan root command on SW2 to make it the root bridge for VLANs 20 and 30.
SW 2 is now the root bridge for both VLAN 20 and 30. Notice that the priority value has changed from the default of 32768.
In the next CCNP / BCMSN tutorial, well take a look at more STP features.
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.
For a FREE copy of his latest e-books, "How To Pass The CCNA" or "How To Pass The CCNP", and for free daily exam question, visit the website and download your copies!