Chapter-5 STP

Introduction

Examine a redundant design In a hierarchical design, redundancy is achieved at the distribution and core layers through additional hardware and alternate paths through the additional hardware.

Examine a redundant design

Layer 2 Loops Redundancy is an important part of the hierarchical design. When multiple paths exist between two devices on the network and STP has been disabled on those switches, a Layer 2 loop can occur. If STP is enabled on these switches, which is the default, a Layer 2 loop would not occur. Ethernet frames do not have a time to live (TTL) like IP packets traversing routers. As a result, if they are not terminated properly on a switched network, they continue to bounce from switch to switch endlessly or until a link is disrupted and breaks the loop. Broadcast frames are forwarded out all switch ports, except the originating port. This ensures that all devices in the broadcast domain are able to receive the frame. If there is more than one path for the frame to be forwarded out, it can result in an endless loop.

Broadcast Storms A broadcast storm occurs when there are so many broadcast frames caught in a Layer 2 loop that all available bandwidth is consumed. Consequently, no bandwidth is available bandwidth for legitimate traffic, and the network becomes unavailable for data communication. A broadcast storm is inevitable on a looped network. As more devices send broadcasts out on the network, more and more traffic gets caught in the loop, eventually creating a broadcast storm that causes the network to fail.

Duplicate Unicast Frames Broadcast frames are not the only type of frames that are affected by loops. Unicast frames sent onto a looped network can result in duplicate frames arriving at the destination device.

Loops in the Wiring Closet Redundancy is an important component of a highly available hierarchical network topology, but loops can arise as a result of the multiple paths configured on the network. You can prevent loops using the Spanning Tree Protocol (STP). If STP has not been implemented in preparation for a redundant topology, loops can occur unexpectedly. Network wiring for small to medium-sized businesses can get very confusing. Network cables between access layer switches, located in the wiring closets, disappear into the walls, floors, and ceilings where they are run back to the distribution layer switches on the network.

Loops in the Wiring Closet

Loops in the Cubicles

STP Topology Redundancy increases the availability of the network topology by protecting the network from a single point of failure, such as a failed network cable or switch. When redundancy is introduced into a Layer 2 design, loops and duplicate frames can occur. Loops and duplicate frames can have severe consequences on a network. The Spanning Tree Protocol (STP) was developed to address these issues. STP ensures that there is only one logical path between all destinations on the network by intentionally blocking redundant paths that could cause a loop. A port is considered blocked when network traffic is prevented from entering or leaving that port. This does not include bridge protocol data unit (BPDU) frames that are used by STP to prevent loops.

STP Algorithm STP uses the Spanning Tree Algorithm (STA) to determine which switch ports on a network need to be configured for blocking to prevent loops from occurring. The STA designates a single switch as the root bridge and uses it as the reference point for all path calculations.

Root Bridge Every spanning-tree instance (switched LAN or broadcast domain) has a switch designated as the root bridge. The root bridge serves as a reference point for all spanning-tree calculations to determine which redundant paths to block. Go to

BID Fields

Best Paths to the Root Bridge When the root bridge has been designated for the spanning-tree instance, the STA starts the process of determining the best paths to the root bridge from all destinations in the broadcast domain. The path information is determined by summing up the individual port costs along the path from the destination to the root bridge. The default port costs are defined by the speed at which the port operates. In the table, you can see that 10-Gb/s Ethernet ports have a port cost of 2, 1-Gb/s Ethernet ports have a port cost of 4, 100-Mb/s Fast Ethernet ports have a port cost of 19, and 10-Mb/s Ethernet ports have a port cost of 100.

Port Cost

BPDU Fields

BPDU Example

BID Fields The bridge ID (BID) is used to determine the root bridge on a network.

Configure and Verify the BID

Port Roles The root bridge is elected for the spanning-tree instance. The location of the root bridge in the network topology determines how port roles are calculated. This topic describes how the switch ports are configured for specific roles to prevent the possibility of loops on the network. There are four distinct port roles that switch ports are automatically configured for during the spanning-tree process. Root Port The root port exists on non-root bridges and is the switch port with the best path to the root bridge. Root ports forward traffic toward the root bridge. The source MAC address of frames received on the root port are capable of populating the MAC table. Only one root port is allowed per bridge.

Port Roles

Designated Port The designated port exists on root and non-root bridges. For root bridges, all switch ports are designated ports. For non-root bridges, a designated port is the switch port that receives and forwards frames toward the root bridge. Non-designated Port The non-designated port is a switch port that is blocked, so it is not forwarding data frames and not populating the MAC address table with source addresses. A non-designated port is not a root port or a designated port. For some variants of STP, the non- designated port is called an alternate port. Disabled Port The disabled port is a switch port that is administratively shut down. A disabled port does not function in the spanning-tree process. There are no disabled ports in the example.

Configure Port Priority To configure the port priority value using the spanning- tree port-priority value interface configuration mode command.

Verifying Port Roles and Port Priority To verify the port roles and port priorities for the switch ports, use the show spanning-tree privileged EXEC mode command.

Port States

BPDU Timers

Roles and Timers

STP Convergence Steps

Step 1: Electing a Root Bridge The first step of the spanning-tree convergence process is to elect a root bridge. The root bridge is the basis for all spanning-tree path cost calculations and ultimately leads to the assignment of the different port roles used to prevent loops from occurring. A root bridge election is triggered after a switch has finished booting up, or when a path failure has been detected on a network.

STP Convergence Steps Step 2: Elect Root Ports The root bridge has been determined, the switches start configuring the port roles for each of their switch ports. The first port role that needs to be determined is the root port role. Every switch in a spanning-tree topology, except for the root bridge, has a single root port defined. The root port is the switch port with the lowest path cost to the root bridge. Normally path cost alone determines which switch port becomes the root port.

STP Convergence Steps Step 3: Electing Designated Ports and Non-Designated Ports A switch determines which of its ports is the root port, the remaining ports must be configured as either a designated port (DP) or a non-designated port (non-DP) to finish creating the logical loop-free spanning tree. Each segment in a switched network can have only one designated port. When two non-root port switch ports are connected on the same LAN segment, a competition for port roles occurs. The two switches exchange BPDU frames to sort out which switch port is designated and which one is non- designated.

Cisco and STP Variants

PVST+ Cisco developed PVST+ so that a network can run an STP instance for each VLAN in the network. With PVST+, more than one trunk can block for a VLAN and load sharing can be implemented.

PVST+ Bridge ID

Default Switch Configuration

RSTP RSTP (IEEE 802.1w) is an evolution of the 802.1D standard. The 802.1w STP terminology remains primarily the same as the IEEE 802.1D STP terminology. Most parameters have been left unchanged, so users familiar with STP can rapidly configure the new protocol.

RSTP Characteristics

RSTP BPDU RSTP (802.1w) uses type 2, version 2 BPDUs, so an RSTP bridge can communicate 802.1D on any shared link or with any switch running 802.1D.

Edge Ports RSTP edge port is a switch port that is never intended to be connected to another switch device. It immediately transitions to the forwarding state when enabled.

Edge Port

Non-Edge Port

Link Types The link type provides a categorization for each port participating in RSTP. The link type can predetermine the active role that the port plays as it stands by for immediate transition to forwarding state if certain conditions are met.

Use Layer 3 Switching Layer 3 switching means routing approximately at the speed of switching. A router performs two main functions:  It builds a forwarding table. The router generally exchanges information with peers by way of routing protocols.  It receives packets and forwards them to the correct interface based on the destination address.

Use Layer 3 Switching

Final Points

Troubleshoot a Failure To troubleshoot a bridging loop, you need to know at least these items:  Topology of the bridge network  Location of the root bridge  Location of the blocked ports and the redundant links

PortFast Configuration Error

Summary