Friday, 13 February 2009

Spanning tree Protocol


Spanning tree protocol is Layer 2 protocol that ensures the loop free topology, it is based in angorithum invented by Radia Perlam.

Some of the main Points to cram about STP are as under (soon i will add some Cisco related troubleshooting commands and PVST stuff............)

  • STP 802.1d was created to prevent loops in the network, and it is Industry standard protocol.

  • All the switches running STP use to send Probes into the network, which are known as “BRIDGE PROTOCOL DATA UNIT” (BPDUS) to discover loops, however these BPDUs also help to elect the ROOT BRIDGE, which is the core switch in the network.

  • In redundant network running STP switches find the best path to reach the root bridge elected, and then block rest of the redundant links.

  • BPDU packets are sent every 2 seconds , from every active ports of switch to detect the redundancy and link failure , this BPDU packet contains 2 major fields Mac Address and Priority , Which helps to elect the root Bridge

  • Priority field in BPDU can contain value from 0 to 61440 , but the default value is 32768 and lower is better

In order to break a tie between switches having default priority value i.e 32768 , is the point where MAC address kicks in , and lower the MAC address the higher the priority , if lower the MAC address then older the switch will be ............. (hope not got confused ;) ... so we cannot make older switch the root bridge ... so we need to configure root bridge with smaller priority value so the election will be on the basis of Priority not MAC address .

Three Port types in STP

Root Port : this is used reach the root bridge .... remember Root bridge don’t have any root port

Designated port : forwarding port but other end should be locked

Blocking/ Non Designated Port: its the blocked port by STP

STP find the best path by selecting the cost of links i.e cost of 100 Mbps is 19 if link cost ties then bridge id will break the tie

Bridge ID = Priority + Mac address

Per VLAN Spanning Tree

All modern Cisco Switches Run PVST , which runs one instance of Spanning tree per vlan

One root bridge for each VLAN which helps to load balance more effectively

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