Redundant Link & Spanning-Tree

• Redundant Links between Switches are good to prevent network failure if one link stops working, But have Disadvantages like :

1. If no loop avoidance schemes were put it will lead to flood broadcasts endlessly (Broadcast Storm), It means that data will go on endless circle unless something or someone stops it.

   
   
   
   

2. Device can receive multiple copies of the same frame from different segments at the same time.

3. Confuse the MAC table about the device location (Trashing the MAC table), means the SW will receive the frame from more than 1 interface and SW is know to record the source address in the MAC table so it will have multiple entries for 1 or more MAC addresses.

4. Due to all of that the SW can’t perform frame switching and the speed will decrease by time till all the network fails.

HERE COMES THE HELP OF THE 802.1d (SPANNING-TREE PROTOCOL)

• Cisco runs 802.1d by default.

• The main task for Spanning-Tree is to stop loops by shutting down redundant links, but in the mean time the link is still available if other link goes down (BACKUP LINK)

  
  
  
  

Spanning-Tree Protocol Terms:

• Root Bridge(means root SW):

  The Bridge(SW) with the best Bridge ID

• Bridge ID: An ID determined by

1. The Bridge priority (Default 32,768 in all SWs) ( the VLAN number will be added to the priority so VLAN 1 will have priority of 32,769 and so on.

2. The Base MAC Address (Of the SW)

   Ex of Bridge ID (ROOT):

   priority of 32,768 and a MAC of 11-22-33-44-55-66, the BID(Bridge ID) would be 32768:11-22-33-44-55-66

   
   
   
   

   
   

• The LOWEST Bridge ID will become the ROOT

• Priority Possible values range from 0 to 65535, a SW with the priority of 0 means always become the ROOT

• All the decisions in this network’s SWs are made from the view of the ROOT. (will have the big role in deciding which port will be shut)

• BPDU: Bridge Protocol Data Unit: A frame that all SWs send that includes the ID of the SW, helps in the selection of the root, every SW compares it’s ID with the ID it receives from the other SW and see which ID is the lowest to choose the ROOT, After choosing the ROOT all SWs including the ROOT continue to send periodic BPDUs.

• None-Root Bridges (SWs which are not the ROOT) exchange BPDUs to update the STP topology.

• Port-Cost: A number that helps in choosing which port to be closed in a SW that has redundant links in STP, it is the cost of using the port to reach the root bridge, that number is determined by the Bandwidth of the Port, The more bandwidth The less the number is, the better the port is.

• Port Cost numbers according to the port’s Bandwidth are:

  10Mpbs costs 100

  100Mpbs costs 19

  1000Mpbs (1Gpbs) costs 4

  10000Mpbs (10Gpbs) costs 2

…. to be continues isa

~ by Network Techs. on September 6, 2009.