↓ Skip to main content

Spanning Tree Protocol

· loading · loading · 
2.0 Network Access
2.4 Configure and verify (Layer 2/Layer 3) EtherChannel (LACP)2.5 Describe the need for and basic operations of Rapid PVST+ Spanning Tree Protocol and identify
basic operations
2.5.a Root port, root bridge (primary/secondary), and other port names2.5.b Port states (forwarding/blocking)
2.5.c PortFast benefits
    • RSTP is most common nowCisco defaults to RSTP
      MAC table instability
  • The switches arrive on different ports.MAC address tables keep changing because frames with the same source MAC
    Broadcast storms
  • forwarding of a frame repeatedly on the same links
    Multiple frame transmission
    • side effect of looping framesMultiple copies are delivered to a host, confusing the host.
      What Spanning Tree Does
  • interfaces does not process any frames
  • except STP/RSTP messages and some other overhead messages
  • blocking state
STP Convergence
  • switches collectively realize that something has changed in the LAN topology
  • determine whether they need to change which ports block and which port forward
    How Spanning Tree works
    three criteria to choose whether to put and interface in forwarding state:
  • STP puts all working interfaces on the root switch in forwarding state
elect a root switch.
  • select the port with the least administrative cost (root port)– cost between itself and the root switch (root cost) (root cost path)root port (RP) gets put in a forwarding state
    • That switch is the designated switch, and that switch’s interface, attached to that
  • The switch with the lowest root cost, as compared with the other switches attached to the same link, is placed in forwarding state.
nonroot switches

Spanning Tree Protocol Concepts

  • That switch is the designated switch, and that switch’s interface, attached to that segment, is called the designated port (DP)

STP States

  • forwarding
  • root switch is always the designated switch
All the root switches ports
  • forwarding
  • port with the least cost to the root switch (lowest root cost)
All nonroot switch's root ports
  • forwardingswitch forwarding the Hello on to the segment with the lowest root cost is the designated
    switch for the segment
Each LAN's designated port
    • blockingNot used for forwarding frames
  • frames received on these interfaces to not forward
All other working ports

The STP Bridge ID and Hello BPDU

    • 82 - -byte value unique to each switch. byte priority field
  • 6 - byte system ID- based on a universal (burned-in) MAC address
  • bridge ID (BID)
  • configuration BPDUs, which switches used to exchange information with each other (switches) The most common BPDU, called
  • a
  • switches can tell which switch sent which Hello BPDU
  • sending switch’s BID
  • BID the sender currently believes to be the root switch
  • Root Bridge ID
  • Sender’s root cost- STP cost between this switch and current root
  • Timer values on the root switch- Hello timer, MaxAge timer, and forward delay timer
  • hello BPDU,
  • bridge protocol data units (BPDU)

Electing the Root Switch

    • based on the BIDs in the BPDUs.root switch is the switch with the lowest numeric value for the BID.

  • Because the two- essentially the switch with the lowest priority becomes the root.-part BID starts with the priority value,

  • If a tie occurs based on the priority portion of the BID, - the switch with the lowest MAC address portion of the BID is the root.

  • Mac addresses are the second part of the BID

  • Mac addresses are the second part of the BID

  • all switches claim to be the root by sending Hello BPDUs listing their own BID as the root BID.
    that switch stops advertising itself as root and starts forwarding the superior Hello. - The Hello sent by the better switch lists the better switch’s BID as the root.

  • If a switch hears a Hello that lists a better (lower) BID-

  • superior hello (better hello)- the listed root’s BID is better (numerically lower),

  • Inferior hello (worse Hello), meaning that - the listed root’s BID is not as good (numerically higher)

  • each nonroot switch chooses its one and only root port.which it has the least STP/RSTP cost to reach the root switch (least root cost).A switch’s RP is its interface through

Choosing Each Switch’s Root Port

  • its interface through which it has the least STP/RSTP cost to reach the root switch (least root cost).

  • each nonroot switch chooses its one and only root port.

  • The STP/RSTP port cost is simply an integer value assigned to each interface, per VLAN

  • The switches also look at their neighbor’s root cost, as announced in Hello BPDUs received from each neighbor.

  • SW3 calculates its cost to reach the root over the two possible paths by adding the advertised cost (in Hello messages) to the interface costs listed in the figure.

  • The root switch sends Hellos, with a listed root cost of 0. The idea is that the root’s cost to reach itself is 0.

  • Each switch places its root port into a forwarding state.

  • tiebreaker to use in case the best root cost ties for two or more paths.

    • Choose based on the lowest neighbor bridge ID.
      • Choose based on the lowest neighbor port priority.Choose based on the lowest neighbor internal port number.

Choosing the Designated Port on each LAN Segment

  • final step to choose the STP/RSTP topology is to choose the designated port on each LAN segment.

  • The designated port (DP) on each LAN segment is the switch port that advertises the lowestHello onto a LAN segment. -cost
    When a nonroot switch forwards a Hello, the nonroot switch sets the root cost field in the Hello
    to that switch’s cost to reach the root. In effect, the switch with the lower cost to reach the root, among all switches connected to a segment, becomes the DP on that segment.

  • All DPs are placed into a forwarding state

    • these would be unlikely today.
      • In that case, the one switch hears its own BPDUs.
          • the lowest interface STP/RSTP priority and, if that ties, the lowest internal interface number.
      • So, if a switch ties with itself, two additional tiebreakers are used:
    • A single switch can connect two or more interfaces to the same collision domain by connecting to a hub.
Two additional tiebreakers are needed in some cases
  • switch ports connected to endpoint devices should become DPs and settle into a forwarding state.

Configuring to influence the STP Topology

  • configure the bridge ID and change STP/RSTP port costs.
    • set the priority used by the switch, while
      giving a switch the lowest priority value among all switches will cause that
      switch to win the root election.
    • continues to use the universal MAC address as the final 48 bits of the BID. -
  • change the BID, the engineer can
to favor one link, give the ports on that link a lower cost, or to avoid a link, give the ports a
higher cost.

Link Costs
10 Mbps– 2,000,000100 (old)

    • 200,00019 (old)
100Mbps
    • 20,0004 (old)
1gbps
  • 2000
  • 2 (old)
10Gbps
  • 200
  • N/A (old)
100Gbps
    • (^20) N/A (old)

  • the cost defaults based on the operating speed of the link, not the maximum speed

  • (config) # spanning - Cisco Catalyst switches can be configured to use the long values as defaults - tree pathcost method long

1Tbps

Details specific to STP
STP Activity When the Network Remains Stable

  • An Each nonroot switch forwards the Hello on all DPs, but only after changing items listed in STP root switch sends a new Hello BPDU every 2 seconds by default.
    the Hello.

  • (As a result, the Hello flows once over every working link in the LAN.)
    When forwarding the Hello BPDU, each switch sets the root cost to that local switch’s
    calculated root cost. The switch also sets the “sender’s bridge ID” field to its own bridge ID. (The root’s bridge ID field is not changed.)

Step 1. interfaces (those in a forwarding state).The root creates and sends a Hello BPDU, with a root cost of 0, out all its working
Step 2. The nonroot switches receive the Hello on their root ports. After changing the
Hello to list their own BID as the sender’s BID and listing that switch’s root cost, the switch forwards the Hello out all designated ports.
Step 3. Steps 1 and 2 repeat until something changes.
When a switch ceases to receive the Hellos, or receives a Hello that lists different details, something has failed, so the switch reacts and starts the process of changing the spanning-
tree topology.
  • STP convergence process requires the use of three timers
    All switches use the timers as dictated by the root switch, which the root lists in its
    periodic Hello BPDU messages.
STP Timers That Manage STP Convergence

periodic Hello BPDU messages.
  • 2 seconds by default
  • Period between hellos created by the root
hello
  • 10 times the hello (20 seconds by default hello)How long the switch will go without receiving any hellos before it attempts to
    change the stp topology
MaxAge

Forward Delay-- 15 secondshow long the port stays in listening and learning state

After MaxAge expires, the switch essentially makes all its STP choices again, based on any
Hellos it receives from other switches.

Changing Interface States with STP
Roles, like root port and designated port, relate to how STP analyzes the LAN topology.
States, like forwarding and blocking, tell a switch whether to send or receive frames.
When STP converges, a switch chooses new port roles, and the port roles determine the state (forwarding or blocking).
Switches using STP can simply move immediately from forwarding to blocking state, but they must take extra time to transition from blocking state to forwarding state.
when a port that formerly blocked needs to transition to forwarding, the switch first puts
the port through two intermediate interface states.

  • interface does not forward frames. switch removes old stale (unused) MAC table entries for which no frames are
    received from each MAC address during this period.

  • These stale MAC table entries could be the cause of the temporary loops.

  • Transitory

Listening:
    • do not forward framesswitch begins to learn the MAC addresses of frames received on the interface.
  • Transitory
Learning:
  • does not forward frames
    • does not learn mac addressesstable
Blocking
  • learns mac addresses
    • forwards framesstable
Forwarding
    • does not forward or learn mac addressesstable
Disabled

blocking > listening, > learning > forwarding.
STP leaves the interface in each interim state for a time equal to the forward delay timer, which defaults to 15 seconds.
a convergence event that causes an interface to change from blocking to forwarding
requires 30 seconds to transition from blocking to forwarding.
a switch might have to wait MaxAge seconds (default 20 seconds) before even choosing to
move an interface from blocking to forwarding state.
Rapid STP Concepts

    • 802.1wSits in 802.1q standards document
Comparing STP and RSTP
  • elect the root switch using the same rules and tiebreakers.
    • switches select their root ports with the same rules.elect designated ports on each LAN segment with the same rules and tiebreakers. - (RSTP calls the blocking state the discarding state.)
  • place each port in either forwarding or blocking state
  • similarities
  • they can both be used in the same network.
  • RSTP improves network convergence when topology changes occur, usually converging within a few seconds (or in slow conditions, in about 10 seconds).
RSTP defines more cases in which the switch can avoid waiting for a timer to expire, such as the following:
a switch can replace its root port, without any waiting to reach a forwarding state (in some
conditions).
  • replace a designated port, without any waiting to reach a forwarding state (in some conditions).
    • lowers waiting times for cases in which RSTP must wait for a timer.MaxAge is only 3 times the hello
  • uses the term alternate port to refer to a switch’s other ports that could be used as the root port if the root port ever fails.
    The backup port concept provides a backup port on the local switch for a designated port.- backup ports apply only to designs that use hubs, so they are unlikely to be useful today.)
RSTP Port roles

  • Nonroot switch’s port that has the best path to the root
    Root Port

  • Nonroot switch’s port that has the best path to the root
    Alternate port- Replaces the root port when the root port fails
    Designated port- port designated to forward onto a collision domain

  • Replaces designated port when designated port fails

Backup Port

Disabled ports- administratively disabled
  • each switch independently generates its own Hellos.
    • (rather than waiting on timers to expire to learn new information)
  • allows for queries between neighbors

RSTP and the Alternate (Root) Port Role

  • both the RP and the alternate port must receive Hellos that identify the same root switch.
alternate port
    • the role from root port to a disabled port, and the state from forwarding to discarding - its role changes to be the root port, with a forwarding state.
without waiting on any timers, the switch changes roles and state for the alternate
port:

the switch changes the former root port’s role and state:

the new root port also does not need to spend time in other states, such as learning state,
instead moving immediately to forwarding state.
  • Step 1. The link between SW1 and SW3 fails, so SW3’s current root port (Gi0/1) fails.
    Step 2. SW3 and SW2 exchange RSTP messages to confirm that SW3 will now transition its former alternate port (Gi0/2) to be the root port. This action causes SW2 to flush the
    required MAC table entries.Step 3. SW3 transitions Gi0/1 to the disabled role and Gi0/2 to the root port role.
    Step 4. SW3 transitions Gi0/2 to a forwarding state immediately, without using learning state, because this is one case in which RSTP knows the transition will not create a loop.

RSTP States and Processes

  • RSTP keeps both the learning and forwarding states as compared with STP, for the same purposes

  • RSTP does not even define a listening state,

  • RSTP renames the blocking state to the discarding state and redefines its use slightly.

  • RSTP uses the discarding state for what STP defines as two states: disabled state and blocking state.

  • RSTP switches tell each other (using messages) that the topology has changed.

  • Those messages also direct neighboring switches to flush the contents of their MAC tables in a way that removes all the potentially loop-causing entries, without a wait.
    As a result, RSTP creates more scenarios in which a formerly discarding port can immediately transition to a forwarding state, without waiting, and without using the
    learning state, as shown in the example in Figure 9-9.

RSTP backup port role creates a way for RSTP to quickly replace a switch’s designated port
on some LAN.

RSTP Port Types
several links between two switches. RSTP considers these links to be point-to-point links
and the ports connected to them to be point-to-point ports

  • Ports that instead connect to a single endpoint device at the edge of the network, like a PC or server, are called point-to-point edge ports, or simply edge ports.

  • “shared” to describe ports connected to a hub.
    hubs also force the attached switch port to use halfall half-duplex ports may be connected to hubs, treating ports that use half duplex -duplex logic. RSTP assumes that
    as shared ports.

  • RSTP converges more slowly on shared ports as compared to all point-to-point ports.
    Optional STP Features
    Etherchannel

  • The switches treat the EtherChannel as a single interface with regard to STP.
    Layer 2 EtherChannels combine links that switches use as switch ports, with the
    switches using Layer 2 switching logic to forward and receive Ethernet frames over the EtherChannels. Layer 3 EtherChannels also combine links, but the switches use
    Layer 3 routing logic to forward packets over the EtherChannels.
    PortFast

  • allows a switch to immediately transition from blocking to forwarding, bypassing listening and learning states.

PortFast

only ports on which you can safely enable PortFast are ports on which you
know that no bridges, switches, or other STP-speaking devices are connected.

Cisco switches enable RSTP point-to-point edge ports by enabling PortFast on the
port.

BPDU Guard
  • An attacker could connect a switch to one of these ports, one with a low STP/RSTP
An attacker could connect a switch to one of these ports, one with a low STP/RSTP priority value, and become the root switch. The new STP/RSTP topology could have
worse performance than the desired topology.

Tactually forward much of the traffic in the LAN. Without the networking staff he attacker could plug into multiple ports, into multiple switches, become root, and
realizing it, the attacker could use a LAN analyzer to copy large numbers of data
frames sent through the LAN.

Users could innocently harm the LAN when they buy and connect an inexpensive consumer LAN switch (one that does not use STP/RSTP). Such a switch, without any
STP/RSTP function, would not choose to block any ports and could cause a loop.

Cisco BPDU Guard feature helps defeat these kinds of problems by disabling a port if
any BPDUs are received on the port.that should be used only as an access port and never connected to another switch.So, this feature is particularly useful on ports

In addition, the BPDU Guard feature helps prevent problems with PortFast. PortFast
should be enabled only on access ports that connect to user devices, not to other LAN switches. Using BPDU Guard on these same ports makes sense because if
another switch connects to such a port, the local switch can disable the port before
a loop is created.