Save Digg Del. Network redundancy is a key to maintaining network reliability. In this sample chapter from Scaling Networks v6 Companion Guide , explore potential redundancy problems, symptoms, and protocols to manage them. Scaling Networks v6 Companion Guide.

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Arista Networks, Inc. If you click accept, you indicate that you consent to receive cookies from our website. Details about Arista's use of cookies can be found here. Section An Ethernet network functions properly when only one active path exists between any two stations.

A spanning tree is a loop-free subset of a network topology. STP allows a network to include spare links as automatic backup paths that are available when an active link fails without creating loops or requiring manual intervention. Several variations to the original STP improve performance and add capacity. Arista switches support these STP versions:. The following sections describe the supported STP versions, compatibility issues in networks containing switches running different STP versions, and supported alternatives to spanning tree.

RSTP is specified in RSTP provides rapid convergence after network topology changes. Standard The maximum number of PVST instances that can be created on a switch depends on the hardware platform. In most of the cases, it is However, PVST does not address slow network convergence after a network topology change. This architecture supports load balancing by providing numerous forwarding paths to data traffic.

Network fault tolerance is improved because failures in one instance do not affect other instances. Each region can support a maximum of 65 spanning-tree instances. You must configure identical parameters on all switches in the regions. A VLAN can be simultaneously assigned to only one spanning-tree instance. MST does not specify the maximum number of regions that a network can contain.

It gives a root switch to the region that contains all VLANs configured across all switches in the region but not assigned to a MST instance. The following restrictions are enforced based on port role. A network can contain switches running different spanning tree versions. In multi-instance topologies, the following instances correspond to the CST:.

The clear spanning-tree detected-protocols command forces MST ports to renegotiate with their neighbors. Switchport interface pairs associate two interfaces in a primary-backup configuration. When the primary interface is functioning, the backup interface remains dormant in standby mode.

When the primary interface stops functioning, the backup interface handles the traffic. An alternative implementation balances traffic between the primary and backup interfaces.

If either interface shuts down, the other handles traffic addressed to the pair. The following guidelines apply to switchport interface pairs. When spanning tree is disabled and switchport interface pairs are not configured, all interfaces forward packets as specified by their configuration.

Disabling all Spanning Tree Protocols on the switch is strongly discouraged. A layer 2 network consists of bridges and network segments. A loop exists when multiple active paths connect two components. Spanning tree protocols allow only one active path between any two network components. Loops are removed by blocking selected ports that connect bridges to network segments. Ports are assigned cost values that reflect their transmission speed and any other criteria selected by the administrator.

Ports with faster transmission speeds and other desirable characteristics are assigned lower costs. High cost ports are blocked in deference to lower cost ports. A network topology defines multiple possible spanning trees.

Network bridges collectively compute and implement one spanning tree to maintain connectivity between all network components while blocking ports that could result in loops. Administrators improve network performance by adjusting parameter settings to select the most efficient spanning tree. Spanning tree bridges continuously transmit topology information to notify all other bridges on the network when topology changes are required, such as when a link fails.

The following sections describe spanning tree configuration parameters. The root bridg e is the center of the STP topology. A spanning tree instance has one root bridge. All other network bridges calculate paths to the root bridge when selecting spanning tree links. STP calculates the distance to the root bridge to build a loop-free topology that features the shortest distance between devices among all possible paths.

Each switch is assigned a unique bridge ID number for each instance. All network switches collectively elect the root bridge by comparing bridge IDs. The root bridge is the switch with the lowest bridge ID. The bridge ID contains the following eight bytes, in order of decreasing significance:. A designated bridge is selected for each segment after a root bridge is selected; a switch can be a designated bridge for multiple segments. The following network calculations in Figure assume that each path has the same cost:.

Figure Spanning Tree Network Example. Messages from connected devices to the root bridge traverse a least-cost path, which has the smallest cost among all possible paths to the root bridge. The cost of a path is the sum of the costs of all path segments, as defined through port cost settings.

Active ports in a least cost-path fulfill one of two possible roles: root port and designated port. STP blocks all other network ports. STP also defines alternate and backup ports to handle traffic when an active port is inaccessible. Each bridge selects its root port after calculating the cost of each possible path to the root bridge. The following ports in Figure are root ports:. Each segment defines one DP. Switches can provide DPs for multiple segments. All ports on the root bridge are DPs.

The following ports in Figure are designated ports:. An alternate port is blocked until a network change transforms it into a root port. A backup port is blocked until a network change transforms it into a designated port. Root ports and designated ports are either in, or transitioning to, this state. Blocked ports receive BPDU packets. All ports except RPs and DPs are blocked, including alternate and backup ports. Port type is a configurable parameter that reflects the type of network segment that is connected to the port.

Proper port type configuration results in rapid convergence after network topology changes. RSTP port types include normal, network, and edge ports. Normal is the default port type. RSTP immediately transitions network ports to the discarding state. Edge ports transition directly to forwarding state because they do not create loops.

An edge port becomes a normal port when it receives a BPDU. Link type is a configurable parameter that determines candidates for RSTP fast state transition. Fast state transitions are allowed on point-to-point links that connect bridges. Fast state transitions are not allowed on shared ports regardless of the duplex setting.

Spanning tree rules specify a root bridge, select designated bridges, and assign roles to ports. STP rule implementation requires that network topology information is available to each switch. Switches exchange topology information through bridge protocol data units BPDUs.

Bridges enter the following addresses in outbound BPDU frames:. The hello timer specifies the period between consecutive BPDU messages; the default is two seconds.

Bridge timers specify parameter values that the switch includes in BPDU packets that it sends as a root bridge. Bridge timers include:. The switch recomputes the spanning tree topology if it does not receive another BPDU before the max-age timer expires. When edge ports and point-to-point links are properly configured, RSTP network convergence does not require forward-delay and max-age timers.

Recipients use this digest and other administratively configured values to identify bridges in the same MST region. Chapter PDF. X Arista Networks, Inc.


Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst Switches

The main purpose of STP is to ensure that you do not create loops when you have redundant paths in your network. Loops are deadly to a network. A console cable that is suitable for the Supervisor Engine in the switch. The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared default configuration. If your network is live, make sure that you understand the potential impact of any command.


Calculating an 802.1d Spanning-Tree Topology[1]

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Chapter Summary

You must be logged in to access this content. This is phenomenal. Calculating the topology is something which is exceedingly important. Understanding why and how a switch becomes the root and why certain ports are designated and others not is extremely important. If you encounter a technical issue on the site, please open a support case.

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