Configuring RSTP

Most newer Cisco switches use RSTP by default. RSTP prevents frame looping out of the box and no additional configuration is necessary. To check whether a switch runs RSTP, the show spanning-tree command is used:SW1#show spanning-tree VLAN0001 Spanning tree enabled protocol rstp Root ID Priority 32769 Address 0004.9A47.1039 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority Read more [...]

How RSTP works

Just like STP, RSTP creates a topology database of the network. To prevent loops, some interfaces on switches are placed in forwarding state and other interfaces in discarding state. How does RSTP decides in which state the port will be placed? A couple of criteria exist:1. all switches in a network elect a root switch. All working interfaces on the root switch are placed in forwarding state. 2. all other switches, called nonroot switches, determine the best path to get to the root switch. The port Read more [...]

What is RSTP?

RSTP (Rapid Spanning Tree Protocol) is an evolution of STP. It was originally introduced as IEEE 802.1w standard and in 2004 IEEE decided to replace STP with RSTP in 802.1D standard. Finally, in 2011, in the IEEE decided to move all the RSTP details into 802.1Q standard.RSTP is backwards-compatible with STP and there are many similarities between the two protocols, such as:the root switch is elected using the same set of rules in both protocolsroot ports are selected with the same rules, as well Read more [...]

Selecting STP designated port (DP)

We’ve already learned that, on the shared Ethernet segments, the switch with the best path to reach the root switch is placed in forwarding state. That switch is called the designated switch and its port is known as the designated port. In order to avoid loops, the non-designated port on the other end of the link is placed in blocking state.The designated switch is determined based on the following criteria:the switch with the lowest cost to reach the root becomes the designated switch on that Read more [...]

Selecting STP root port

As we’ve mentioned before, all working interfaces on the root switch are placed in forwarding state. All other switches (called nonroot switches) determine the best path to get to the root switch and the port used to reach the root switch is placed in forwarding state. The best path is the one with the lowest cost to reach the root switch. The cost is calculated by adding the individual port costs along the path from the switch to the root.Take a look the following example:SW1 has won the election Read more [...]

Electing the Root Switch in STP

The STP process works by default on Cisco switches and begins with the root switch election. The election is based on the bridge IDs (BIDs) sent in the BPDUs. Each switch that participates in STP will have a 8-byte switch ID that comprises of the following fields:2-byte priority field – by default, all switches have the priority of 32768. This value can be changed using configuration commands.6-byte system ID – a value based on the MAC address of each switch.A switch with the lowest BID Read more [...]

How STP works

STP uses the Spanning-Tree Algorithm (SPA) to create a topology database of the network. To prevent loops, SPA places some interfaces in forwarding state and other interfaces in blocking state. How does STP decides in which state the port will be placed? A couple of criteria exist:1. all switches in a network elect a root switch. All working interfaces on the root switch are placed in forwarding state. 2. all other switches, called nonroot switches, determine the best path to get to the root switch. Read more [...]

What is STP?

Spanning Tree Protocol (STP) is a network protocol designed to prevent layer 2 loops. It is standardized as IEEE 802.D protocol. STP blocks some ports on switches with redundant links to prevent broadcast storms and ensure loop-free topology. With STP in place, you can have redundant links between switches in order to provide redundancy.To better understand the importance of STP and how STP prevents broadcast storms on a network with redundant links, consider the following example:SW1 sends a broadcast Read more [...]