To reduce network downtime, we often employ redundant topologies. STP allows a network with a redundant structure to automatically adjust the data forwarding path of the network in the event of a failure. STP reconvergence takes a long time, usually 30-50 seconds. In order to reduce this time, some supplementary technologies are introduced, such as uplinkfast, backbonefast, etc. RSTP makes a fundamental improvement to STP in the protocol to form a new protocol, thereby reducing the convergence time. STP also has many improvements, such as PVST, MST protocols, and security measures.

Basic STP protocol:

In order to increase the redundancy of the local area network, we often introduce redundant links into the network, but this will cause switching loops. Switching loops cause three problems: broadcast storms, multiple copies of the same frame, and unstable switch CAM tables. STP (STP, Spanning Tree Protocol) can solve these problems. The basic idea of ​​STP is to block some switch interfaces and build a forwarding tree without loops. STP uses BPDU (Bridge Protocol Data Unit) to communicate with other switches to determine which switch should block which interface. There are several key fields in the BPDU, such as:

Root bridge ID, path cost, port ID, etc.

In order to form a topology without loops in the network, the switches in the network must perform the following three steps:

(1) Election of the root bridge

(2) Select the root port

(3) Select the specified port.

Which switch wins in these steps will depend on the following factors (in order):

(1) lowest root bridge ID;

( 2) The lowest root path cost;

(3) The lowest sender bridge ID;

(4) The lowest sender port ID.

Each switch has a unique bridge ID, which consists of two parts: bridge priority + MAC address. The bridge priority is a 2-byte number, the default priority of the switch is 32768; the MAC address is the MAC address of the switch. The switch with the lowest bridge ID is the root bridge. The interfaces on the root bridge are all designated ports and will forward data packets. After the root bridge is elected, other switches become non-root bridges. Each non-root bridge elects a root path to the root bridge. STP uses the path cost to determine the best path to the root bridge (the cost is cumulative, and the cost of the link with a large bandwidth is lower). The path with the lowest cost value is the root path, and this interface is the root port; if the cost is the same, the Election order Election root. The root port forwards packets.

Other interfaces of the switch also need to decide whether to designate the port or block the port, and the switches will further compete according to the above four factors. The designated port is for forwarding data frames. The remaining other interfaces will be blocked and no packets will be forwarded. In this way, the network constructs a forwarding tree without loops. When the topology of the network changes, the network transitions from one state to another, reopening or blocking certain interfaces. The port of the switch goes through several states: Disable, Blocking, Listening, Learning, and finally Forwarding.

PVST, portfast, uplinkfast, backbonefast

When there are multiple VLANs on the network, PVST (Per Vlan STP) will construct a an STP tree. The benefit of this is that you can independently control which interfaces forward data for each VLAN, enabling load balancing. The disadvantage is that if the number of VLANs is large, it will bring a heavy burden to the switch. The default mode of Cisco switches is PVST.

The convergence time of STP usually takes 30-50 seconds. To reduce the convergence time, there are some improvements. The Portfast feature enables the Ethernet interface to enter the forwarding state as soon as a device is connected. This is very suitable if only computers or other devices that do not run STP are connected to the interface.

Uplinkfast is often used on access layer switches. When it is connected to the main link on the backbone switch, it can immediately switch to the backup link without 30 seconds or 50 seconds. second. Uplinkfast only needs to be configured on the access layer switch. Backbonefast is mainly used between backbone switches. When the link between backbone switches fails, it can switch to the backup link in 20 seconds less than the original 50 seconds. Backbonefast needs to be configured on all switches.


RSTP actually combines some measures to reduce the convergence time of STP in the STP protocol to form a new protocol. In RSTP, the roles of interfaces are: root interface, designated interface, backup interface (Backup Interface), and alternate interface (AlternateInterface). The states of the interface are: Discarding, Learning, and Forwarding. The interface is also divided into edge interface (Edge Port), point-to-point interface (Point-to-Point Port), shared interface (Share Port).

In PVST, the switch builds an STP tree for each VLAN, which will not only bring a lot of CPU load, but also take up a lot of bandwidth. MST maps multiple VLANs to one STP instance, thereby reducing the number of STP instances. MST can be used with STP and PVST. To switches running STP, PVST, an MST domain looks like a switch.


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