One Second to Know Rapid spanning tree protocol
- Posted on: 2019-04-30
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The design of the 802.1D Spanning Tree Protocol (STP) standard is that every connection is restored after a minute or so of interruption.
With the advent of Layer 3 switching technology in LAN environments, bridging solutions can now compete with routing solutions, such as Open Shortest Path First (OSPF) and Enhanced Interior Gateway Routing Protocol (EIGRP) in bridging solutions. Provide an alternate path in a shorter time.
Cisco enhanced the original 802.1D specification with features such as Uplink Fast, Backbone Fast, and Port Fast to speed up the convergence time of the bridged network. The disadvantage is that these mechanisms are proprietary and require additional configuration.
The Rapid Spanning Tree Protocol (RSTP, IEEE 802.1W) can be seen as an evolution of the 802.1D standard, not just a revolution.
The terminology in 802.1D remains essentially unchanged. And most of the parameters have not changed, so users familiar with 802.1D can easily configure the new protocol quickly and easily.
In most cases, RSTP performs better than Cisco proprietary extensions and does not require additional configuration.
802.1W can also be restored to 802.1D to interoperate with traditional bridges based on each port, but this just abandons its benefits.
The new 802.1D standard (IEEE 802.1D-2004) includes IEEE 802.1T-2001 and IEEE 802.1W standards.
Catalyst switch support for RSTP
The following table shows the support for RSTP in Catalyst switches and the minimum software version required to support this feature.
New port status and port role
The definition of 802.1D in these five different port states:
The state of a port is a mixed state (whether it is blocking or forwarding traffic), and it also plays a role in the active topology (root port, designated port, etc.).
For example, from an operational perspective, there is no difference between a blocked state port and a listening state port.
Both states discard the frame and cannot learn the MAC address. The actual difference is the role that the spanning tree assigns to the port.
It is safe to assume that the listening port is the designated port or root port and is entering the forwarding state.
However, after being in the forwarding state, it is not possible to infer from the port state whether the port is the root port or the designated port. RSTP solves this problem by separating port roles from states.
Only three port states are reserved in RSTP, corresponding to three possible operating states. The disable, block, and listen status words in 802.1D are merged into a unique drop state in 802.1W.
The role is now a variable assigned to the specified port. The roles of the root port and the designated port are still retained, and the role of the blocked port is split into the backup port and backup port roles.
The Spanning Tree Algorithm (STA) determines the role of the port based on Bridge Protocol Data Units (BPDUs).
For the sake of simplicity, one thing to keep in mind about BPDUs is that there is always a way to compare any two of them and then decide if one is more useful than the other.
This is based on the values stored in the BPDU, and sometimes based on the port that received them. Therefore, the information in this section explains practical ways to determine port roles.
Root port role
The port that receives the best BPDU on the bridge is the root port. In terms of path cost, the root port is the port closest to the root bridge.
The STA selects a root bridge in the entire bridged network (per VLAN). The BPDU sent by the root bridge is more useful than any BPDU sent by other bridges.
The root bridge is the only bridge in the network that does not have a root port. All other bridges receive BPDUs on at least one port.
Specify port role
If the port can send the best BPDU on the network to which it is connected, it is the designated port.
The 802.1D bridge links different network segments together to create a bridged domain. On a given network segment, there can only be one path leading to the root bridge.
If there are two paths, there will be a bridge loop in the network. All bridges connected to a given network segment will listen to the BPDUs of each bridge and agree to use the bridge that sends the best BPDUs as the designated bridge for the network segment.
The corresponding port on the bridge is the designated port of the network segment.
Backup port and backup port role
These two port roles correspond to the blocking state of 802.1D. A blocked port is neither a designated port nor a root port.
Blocked ports receive more BPDUs than BPDUs sent on their network segments. Moreover, the port must receive BPDUs to remain blocked. To this end, RSTP introduces these two roles.
One port receives a more useful BPDU from another bridge, which is blocked. Figure:
The backup port receives a more useful BPDU from the bridge it is on, and it is a blocked port, as shown in Figure:
This has been distinguished internally by 802.1D. This is essentially how Cisco UplinkFast works.
The basic principle is that the backup port provides a backup path to the root bridge, so it can be replaced when the root port fails.
Of course, the backup port can provide redundant connections to the same network segment, but cannot guarantee an alternate connection to the root bridge. Therefore, the uplink group excludes it.
RSTP calculates the final topology for spanning trees that use the same standard as 802.1D.
There is no change in the priority of using different bridges and ports. The name "blocking" is used to describe the drop status in the Cisco implementation.
Learning and listening status is still displayed in Catos version 7.1 and higher. This provides more information about the port, far exceeding the requirements of the IEEE standard.
However, the new features are now different, and the protocol determines the port role and its current state.
For example, a port can now be both a designated port and a blocked port. However, the duration of this situation is generally very short, which simply means that the port is in transition to the specified forwarding state.
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