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In a large STP enabled network, why is it important to methodically place the primary root bridge ?
Speed up STP convergence
To minimize the path cost
Improve performance, by making system more deterministic, predictable, and less susceptible to failure caused by lower-end switches with less reliability, 'that just happen to win the STP-Root-Election'
To reduce network failover
"It is a best practice to configure the "main" (core) switches with lower STP priorities so that one will be the root bridge and any other core bridges will have a slightly higher value and take over should the primary core bridge fail. Having "tiered" STP priorities configured on the switches determines which switch should be root bridge in the event of a bridge failure. This makes the STP network behave in a more deterministic manner. - - - (2) Many organizations take spanning tree for granted and simply accept the default configuration settings. This leaves all switches in the environment using the default root bridge priority of 32768. If all switches have the same root bridge priority, the switch with the lowest MAC address will be elected as the root bridge. Many networks have not been configured with a single switch to have a lower root bridge priority which would force that core switch to be elected as the STP root for any or all VLANs. In this situation, it is possible that a small access-layer switch with a low MAC address could be the STP root. This situation would add some performance overhead and make for longer convergence times because of the root bridge reelection." - - - - { http://www.networkworld.com/article/2223757/cisco-subnet/cisco-subnet-9-common-spanning-tree-mistakes.html } - - - - - -(3)CISCO's TAKE: " The network operation, manageability, and application performance could become very complex with traditional Layer 2 technologies such as spanning-tree. The framework of the distribution-layer system must be designed with consideration of Cisco recommended best practices, which significantly reduce network complexities, increase reliability, and accelerate network performance. To build a strong campus network foundation with the three-tier model, the distribution layer has a vital role in consolidating networks and enforcing network edge policies. - - - - - - - - - - - - { http://www.cisco.com/c/en/us/products/collateral/switches/catalyst-6800-series-switches/guide-c07-733457.html#_Toc406542410 } - - - - - - - MY TAKE #1, If you are using standard default STP, then something like half of your uplink ports will be blocking, and passing no traffic. - - But, by careful design and placement you can configure (rig-STP-elections) and design a system where a trunk is blocking for one-vlan, but forwarding for another. A VERY critical issue here it to KISS,(Keep-It-Simple-Stupid), make the fewest modifications from STP defaults as possible, so that you don't create nightmares. - - - - -MY-TAKE#2: Better still, eliminate STP, and get more bandwidth and redundancy by using etherchannels, make all other ports static-access-mode, Portfast, as default, with BPDUguard as default, Maybe even RootGuard
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