The Spanning Tree Protocol is an xloop-prevention network protocol. It creates a loop-free Layer 2 topology thus allowing redundancy. Using this topology information, we can determine the most efficient loop-free path through the switched network. A root switch election occurs when all switches behave as if they are the root switch at first. They continue to do so until they receive traffic from a superior switch switch priority determines.
This is known as a root switch election. On running the command the network administrator can see the priority of the switch and the value it has been set to. The switch which has the lowest value becomes the root switch. The network administrator can change the switch priority that we will see when we begin the configurations. Another thing to bear in mind is that depending on the STP mode used, the network may have multiple root switches. Image source. Following the election of the root switch, each port is assigned a role based on its location within the STP topology.
To avoid loops, all ports with the alternative or backup STP roles are blocked. This is done until the best route has been determined and every port is allocated a role. Each activated port on a switch participates in STP. And each of these ports must go through an interface state phase before being allowed to forward traffic. Figure 1 depicts the The time it takes for a port to convert and the process used to transition have both improved since the RSTP was introduced.
This allows a switched network to start routing traffic faster and without unnecessary delays. This was a complaint about the Ethernet networks are susceptible to broadcast storms if loops are introduced. However, an Ethernet network needs to include loops because they provide redundant paths in case of a link failure.
Spanning-tree protocols address both of these issues because they provide link redundancy while simultaneously preventing undesirable loops. RSTP is the default spanning-tree protocol for preventing loops on Ethernet networks.
Connects to devices that are not STP-capable, such as PCs, servers, routers, or hubs that are not connected to other switches, by using edge ports. Spanning-tree protocols intelligently avoid loops in a network by creating a tree topology spanning tree of the entire bridged network with only one available path between the tree root and a leaf. All other paths are forced into a standby state.
The tree root is a switch within the network elected by the STA spanning-tree algorithm to use when computing the best path between bridges throughout the network and the root bridge. Frames travel through the network to their destination—a leaf such as an end-user PC—along branches.
A tree branch is a network segment, or link, between bridges. Switches that forward frames through an STP spanning tree are called designated bridges. Each port has both a role and a state. The five port roles used in RSTP are:. Root port—The port closest to the root bridge has the lowest path cost from a bridge. This is the only port that receives frames from and forwards frames to the root bridge. Designated port—The port that forwards traffic away from the root bridge toward a leaf.
A designated bridge has one designated port for every link connection it serves. A root bridge forwards frames from all of its ports, which serve as designated ports. Alternate port—A port that provides an alternate path toward the root bridge if the root port fails and is placed in the discarding state. This port is not part of the active spanning tree, but if the root port fails, the alternate port immediately takes over.
Backup port—A port that provides a backup path toward the leaves of the spanning tree if a designated port fails and is placed in the discarding state.
A backup port can exist only where two or more bridge ports connect to the same LAN for which the bridge serves as the designated bridge. A backup port for a designated port immediately takes over if the port fails.
Each port has both a state and a role. RSTP places each port of a designated bridge in one of three states:. A port in this state discards all frames it receives and does not learn MAC addresses.
Learning—The port prepares to forward traffic by examining received frames for location information in order to build its MAC address table. Forwarding—The port filters and forwards frames. We have BPDU guard which disables a port in the event of a loopback, packet storm, etc. For the connections between the switches we do have STP so that we can build a mesh or ring between them and have no single point of failure.
I didn't set this network up, but the guy who did has been doing networking longer than I've been alive, so I trust he knows what he's doing. So to perhaps muddy the waters a bit, this is my example of using and not using STP. This is one of the reasons we don't have STP on the device ports. There was a huge problem of people plugging in their own switches or blue linksys routers and messing up the entire network before it was in multiple subnets. The only time you should even think of STP as an issue is if you have a big network and have daisy-chained more than 7 switches in succession.
After 7 hops the STP algorithm begins to breakdown so it is recommended to have routers in place before you get that deep into connection so many switches. Read this interesting article about the perils of misconfiguring STP-enabled devices and not building a hierarchical network. There could be a number of reasons why STP is disabled - do you have a design doc that explains your architecture?
Maybe the uplinks are Layer 3 not 2? The more important quesiton is how is someone causing a loop? Is this an IT person patching a switch to another? Create a Network Bridge Configuration. Bridge Mode. All rights reserved. All other tradenames are the property of their respective owners. Submit Search. Account Settings Logout. About Spanning Tree Protocol Administrators who must configure a highly-available network can set up redundant links between switches to avoid network downtime.
Bridge priority values: Minimum bridge priority — 0 Default bridge priority — 32, Range — 0—32, Path cost Path cost is a value associated with the link speed bandwidth of the links between bridges. This list shows the path costs specified by IEEE Configuration — Sent by a root bridge to the entire network at an interval specified by the hello value.
For the Firebox, the default interval is 2 seconds. You can change the hello value from the Fireware CLI. Root Bridge and Port Selections When you enable Spanning Tree Protocol, the topology convergence selection process automatically occurs. From the Fireware CLI, you can specify the Firebox as the root bridge: Change the bridgeprio value to a number than is lower than all other bridges.
To specify a particular switch as a backup root bridge, specify the next highest bridgeprio value to that switch. Step 2 — Select the Root Ports The root port is the port on a non-root bridge that is closest to the root bridge. Step 3 — Select the Designated Ports Designated ports exist on root and non-root bridges: All ports on a root bridge are designated ports.
On a non-root bridge, the designated port is the uplink to the next non-root bridge in the tree.
0コメント