Chapter_05_3

Concepts & Keywords of Chapter 05:

Port Roles:

Root ports - Switch ports closest to the root bridge. In the example, the root port on switch S2 is F0/1 configured for the trunk link between switch S2 and switch S1. The root port on switch S3 is F0/1, configured for the trunk link between switch S3 and switch S1.

The root port exists on non-root bridges and is the switch port with the best path to the root bridge. Root ports forward traffic toward the root bridge. The source MAC address of frames received on the root port are capable of populating the MAC table. Only one root port is allowed per bridge.

Designated ports - All non-root ports that are still permitted to forward traffic on the network. In the example, switch ports F0/1 and F0/2 on switch S1 are designated ports. Switch S2 also has its port F0/2 configured as a designated port.

The designated port exists on root and non-root bridges. For root bridges, all switch ports are designated ports. For non-root bridges, a designated port is the switch port that receives and forwards frames toward the root bridge as needed. Only one designated port is allowed per segment. If multiple switches exist on the same segment, an election process determines the designated switch, and the corresponding switch port begins forwarding frames for the segment. Designated ports are capable of populating the MAC table.

Non-designated ports - All ports configured to be in a blocking state to prevent loops. In the example, the STA configured port F0/2 on switch S3 in the non-designated role. Port F0/2 on switch S3 is in the blocking state.

The non-designated port is a switch port that is blocked, so it is not forwarding data frames and not populating the MAC address table with source addresses. A non-designated port is not a root port or a designated port. For some variants of STP, the non-designated port is called an alternate port.

Disabled Port

The disabled port is a switch port that is administratively shut down. A disabled port does not function in the spanning-tree process. There are no disabled ports in the example.

How to configure Bridge Priority:

Method A:

switchA(config)#spanning-tree vlan vlan-id root primary

switchA(config)#spanning-tree vlan vlan-id root secondary

Method B:

switchA(config)#spanning-tree vlan vlan-id priority <0-61440>

How to configure Port Priority:

switchA#(config-f)#spanning tree port-priority <0-240>

!!!/ It is recommended that the BPDU timers not be adjusted directly because the values have been optimized for the seven-switch diameter

Cisco PortFast Technology

PortFast is a Cisco technology. When a switch port configured with PortFast is configured as an access port, that port transitions from blocking to forwarding state immediately, bypassing the typical STP listening and learning states. You can use PortFast on access ports, which are connected to a single workstation or to a server, to allow those devices to connect to the network immediately rather than waiting for spanning tree to converge. If an interface configured with PortFast receives a BPDU frame, spanning tree can put the port into the blocking state using a feature called BPDU guard.

switchA(config)#spanning-tree portfast

STP convergence

Step 1. Elect a root bridge

Step 2. Elect root ports

Step 3. Elect designated and non-designated ports

Variant of STP

RTSP Rapid Spanning Tree Protocol is using the term Edge ports instead of Port Fast

RTSP is having less Port States -> Faster Convergence

Rapid Spanning Tree Protocol is using 3 Link Type:

• Shared
• Edge
• Point to Point

RTSP Port Roles:

• Altenate
• Designated
• BackUp

STP waits for the network to converge before placing ports into forwarding state. RSTP places designated ports into forwarding state immediately.