CCNP 3 Implementing Spanning Tree

Describing STP

Identifying Traffic Loops

Switching loop

Prevent briding loops

Describing the 802.1D Spanning Tree Protocol

802.1D

Describing the Root Bridge

Root Bridge

Extended System ID

Prority Field

Describing Port Roles

Port Roles

Port Roles

Root Port Election

STP Path Cost

STP Topology

Explaining Enhancements to STP

STP Enhancments

Implementing RSTP

Describing the Rapid Spanning Tree Protocol

RSTP

Describing RSTP Port States

RSTP Port States

RSTP Port States

RSTP Port States

Describing RSTP Port Roles

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Explaining Edge Ports

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Examining the RSTP BPDU

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Identifying the RSTP Proposal and Agreement Process

RSTP Proposals

Switch A has a path to the root via switch B and switch C. A new link is then created between the root and switch A, and both ports are in designated blocking state until they receive a BPDU from their counterpart. When a designated port is in a discarding or learning state (and only in this case), it sets the proposal bit on the BPDUs it sends out. This is what happens for port P0 of the root bridge. Switch A sees that the proposal BPDU has a superior path cost. It blocks all non-edge designated ports other than the one over which the proposal-agreement process are occurring. This operation is called sync and prevents switches below A from causing a loop during the proposal-agreement process. Edge ports do not have to be blocked and remain unchanged during sync. Bridge A sends an agreement that allows the root bridge to put root port P0 in forwarding state. Port P1 becomes the root port for A.