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Basic Network Fundamental - BPDU (Bridge Protocol Data Unit) priciple, Bridge ID, Bridge Priority Calculation, Pre-Vlan Spanning Tree

Introduction:

Understanding network fundamentals is essential for anyone learning networking or preparing for certifications like CCNA. In this guide, we explain Broadcast Storms, MAC Address Flapping, Spanning Tree Protocol (STP), STP Working, and Bridge Protocol Data Units (BPDUs) in simple words with easy examples.


1) 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.


2) What is the Bridge ID field?



• There is a bridge priority field, which is 16 bits in length, and then there is the MAC address of the switch, which is 48 bits in length.

• The default bridge priority is 32768 on all switches, so by default the MAC address is used as the tie-breaker (lowest MAC address becomes the root bridges).

• Therefore, by default switch with the lowest MAC address becomes the root bridge.


3) Bridge priority calculations?



• Bridge priority + extended system ID and the default bridge priority is 32768 on all switches.

• 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.


4) What is the PVST (Pre-Vlan Spanning Tree)?



• PVST stands for Per-VLAN spanning Tree and Its Cisco Proprietary protocol.

• PVST runs a separate STP ‘instance’ in each VLAN, so in each VLAN different interfaces can be forwarding or blocking.

• Example: One interface could be forwarding in VLAN1 but blocking in VLAN2 for example.

• By adding the VLAN ID into the bridge priority, the switch will have a different bridge ID in each VLAN.

• The max age timer is 20 seconds, and the listening and learning states are 15 seconds each, so it can take up to 50 seconds to respond to changes in the network.

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