The Middle of Layer Two Redundancy

In the month of August I talked about the “beginnings” of layer two redundancy mainly looking at the basic foundations and fundamentals of having layer two redundancy. Let’s continue are discussion about redundancy in the layer two environment.

To review what STP does (Spanning Tree Protocol) it works at layer two of the OSI model. If more than one path is available in the network the STP algorithm basically shutdowns the redundant links until the active link is unavailable (shutdown, disabled, etc.) So although there are multiple links connected physically to offer redundancy, logically the switches in the network only see one path that Ethernet frame can travel. Remember without STP networks would be on the fritz when we introduce redundancy in the environment.

STP or IEEE 802.1D is an open standard so it is not just on Cisco equipment but others as well which helps in a multivendor situation. So how does looping occur without the use STP? Looking at the picture below we can see the broadcasts frames are flooding the network which is by design but the thing that changes is these broadcasts will continue to flood the network because there is on TTL (Time-To-Live) on the data link layer of the OSI model. As more packets are passed and created the network will become slower and slower and eventually devices on the network will become unavailable this called a broadcast storm.

An example of an STP loop
STP Loop Example (Broadcast Storm)

There are also more problems when not using STP besides broadcast storms like the MAC address table becoming unstable in each switch. This causes the MAC table to be unreliable because of the looping frames in the network. These frames are usually sent to the wrong locations in the network. Multiple frames can also be a problem because of looping frames multiple frames appear on the network and if they are delivered to the intended host it can confuse the host and more likly drop the frames.

So looking at the same picture but with STP enabled we can see that broadcast frame will travel the network without looping. On S1 one of the ports is in a blocking state and blocks all traffic coming into that port and traffic coming out of that port.

STP Enabled
STP Enabled

The magic about STP is when the link between S2 and S3 becomes disconnected it will bring the link between S2 and S1 online and continue to send packets! When the link becomes available again it switch back to blocking on S1. So let’s stop there for now and check back next week to see the last chapter of STP, how does it calculate which switch goes to blocking? What other advantages does STP offer? That’s all next week, I hope this information is helpful and if you have an idea of the next topic that deals with either ICND1 or ICND2 let me hear it!

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5 thoughts on “The Middle of Layer Two Redundancy

  1. Steve September 7, 2011 / 12:27 pm

    Awesome Site. Ryan can you please contact me.

    • Ryan September 7, 2011 / 6:51 pm

      Steve do you have any questions?

      Ryan

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