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Basic Network Fundamental ( Change bridge priority without change VLAN ID, Root Bridge, Root port election, Root cost measurement)

How to change the bridge priority without change the VLAN ID? If you want to change the switch’s bridge priority you can only change the total bridge priority (bridge priority + extended system ID) in units of 4096. The STP bridge priority can only be changed in units of 4096. The valid values you can configure are: 0, 4096, 8192, 12288, 16384, 20480, 24576, 28762, 32768, 36864, 40960, 45056, 49152, 53248, 57344 or 61440. The extended system ID will then be added to this number to make the total bridge priority value. Why 32768 default bridge priority? It’s because this field is 16 bits in length, and the most significant bit is set 1 by default. Therefore, the default bridge priority was 32768 With the addition of the extended-system ID, adding the VLAN ID number to the bridge priority, that changed. So, the default VLAN ID is 1, therefore the bridge priority in total isn’t 32768, its 32769. In the default VLAN of 1, the default bridge priority is 32769, which is 32768+1. What is the...
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Basic Network Fundamental (Broadcast Strom, MAC Address Flapping, Spanning Tree Protocol (STP), STP Working principle, Details of Bridge Protocol Data Units - BPDU)

What is the Broadcast Storm?



• The network will be too congested for legitimate traffic to use the network. This is called a Broadcast Storm.


What is the MAC Address Flapping?



• When frames with the same source MAC address repeatedly arrive on different interfaces, the switch is continuously updating the interface in its MAC address table. This called MAC address Flapping.


What is the STP (Spanning Tree Protocol)?



• STP stands for Spanning Tree Protocol (IEEE 802.1D) and it is a Layer 2 Protocol.

• It prevents Layer 2 loops (broadcast storm and MAC address flapping) by placing redundant ports in a blocking state, essentially disabling the interface.

• Interfaces in a forwarding state behave normally. They send and receive all normal traffic.

• Interfaces in a blocking state only send or receive STP message, this is called BPDUs = Bridge Protocol Data units.


How STP (Spanning Tree Protocol) works?


• By selecting which ports are forwarding and which ports are blocking, STP creates a single path to/from each point in the network. The prevents Layer 2 loops.

• STP-enabled switches and hello BPDUs out of all interfaces, the default timer is 2 seconds. (the switch will send a Hello BPDU out of every interface, once every 2 seconds)

• If a switch receives a Hello BPDU on an interface it knows that interface is connected to another switch (because routers PCs etc. do not use STP, so they do not send Hello BPDUs.


What exactly are these BPDUs used for?


• Bridge Protocol Data Units (BPDUs) are the messages that are transmitted across LAN networks to enable switches to participate in Spanning Tree Protocol (STP) by gathering information about each other.

• It contains information regarding switch ports such as port ID, port priority, port cost, and MAC addresses.

• A switch sends BPDUs from their origin port to a multicast address with a destination MAC address.

• Switches use one field in the STP BPDU, the Bridge ID field, to elect a root bridge for the network.

• The switch with the lowest Bridge ID becomes the root bridge.

• All ports on the root bridge are put in a forwarding state.


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