Here are some simple question we have to understand, and remember about spanning tree protocol, i wrote this little note from last year on my oneNote and now i think that it would be better if rewrite on my blog so i can read it anytime and help me remember some of the details i may forget about the process of STP. I am preparing my CCNP Switch right now, so this will also be my first CCNP switch post edition....
this post may contain some of my awkward imagination heheh
What is Spanning Tree Protocol ?
- Creating loop free L2 topology, it's all started when the backup links are pluged on the world of switches network to obtain the power of redundancy, as the drawback of the power, redundancy can rise the frightening evil that can bring down the entire network, called The Loop.
WHY
It Could Be Happen, the Loop ?
- ARP Request, yes A Host will generate broadcast arp request as the ritual to seacrh MAC Address of the Local Destination Host.
- Mysterious or Unknown Unicast Traffic, will be flooded by the switch to all His Interfaces except one where it originated from.
So
What is the Horror Effect of this Loop ?
- Ethernet frame is immortal, it doesn't have TTL , So the frame will loop forever and take Down your switch because they are overburdened with the evil traffic .
What
can This Amazing STP do?
- Before all the disaster come, It will block One or More of the interface, he believes that the larger the network the more interface that will be blocked. It's all to reach the All Hail Loop Free Network.
Explain
to me more, so I can believe the true power of STP
- In the switches Network, There is a messanger called BPDU ( Bridge Protocol Data Unit ). He Brings information that used to create and maintain the Spanning-Tree . One of information he has is Bridge Identifier that contain Three important things that is Priority, Extended System ID, and Mac Address.
- First, STP will elect a Root Bridge. Switches that have lower priority will be choosen and if the priority is same it's MAC address will be the Tie breaker.
Hold
on a second, why should you choose one switch as the Root bridge ?
- Root bridge is like a King, Root Bridge will send down their BPDU downwards to all switches.
Ok,
after the king is elected what will you do
- Second, All the non Root Bridge will have to find the shortest path to the King (Root Bridge) called Root Port. the shortest path determined by the path cost, STP used cost to determine the shortest path. The slower the interface the higher the cost is.
- Third, choose the Designated port on each segment, All Root Bridge port are designated and forwarding traffic.
- And the Last is Block the remaining interface to break the loop. a port of the switch with the higher bridge ID are prefered to be blocked. if there is more than one port, the port priority will be the Tie breaker.
Wow,
so now the STP kingdom is done right?
- Yes Sir. but there STP has some weakness too, The oldest version of Spanning tree, it's only calculate single STP instance for All VLANs. it's lead to some unfair traffic allocation to the link, but you don't have to worry sir, the new stp successor able to calculate single STP topology for each VLAN. It's called PVST ( Per VLAN spanning Tree ).
What
is the benefit of Per Vlan spanning Tree?
- The former STP, it will create only one STP topology for all vlan. doesn't matter how many the vlan are, there will be one or more links that have no traffic to be forwarded for.
- If we use PVST, we can create different Root Bridge for each VLAN, so that the blocked traffic may be vary for each VLAN, and do Load Balancing/Sharing.