Stp (Spanning Tree Protocol)

Front 1

Switching Loops create 3 problems:
Broadcast Storms
MAC table instability
Duplicate Frames

Back 1

Spanning Tree Protocol(STP). STP is a protocol that runs on bridges and switches to find and block redundant looped paths during normal operation.

Front 2

Spanning Tree Protocol (STP) is IEEE 802.1d

Back 2

STP eliminates Layer 2 loops in switched networks with redundant paths.

Front 3

STP’s basic function is to create a loop-free path to a root bridge

Back 3

The Root is the switch with the lowest Bridge ID.
The ID is determined by a combination of an admin-
istrative Priority and the MAC address of the switch

Front 4

The default Administrative priority is 32768 (8000 hex).

Back 4

The Root is elected based on the Bridge ID and the Priority. The switch with the lowest Priority will always be the Root.

Front 5

Bridge Protocol Data Units (BPDUs) are exchanged between bridges every 2 seconds. Amongst the fields they contain, they contain 'Root Priority, Root ID, Root Path Cost, Bridge Priority and Bridge ID'.

Back 5

Root Port:
This port exists on each non-root bridge and is the switch port with the best path to the root bridge. Root ports forward traffic toward the root bridge with the source MAC address of frames.

Front 6

Designated:
This port type exists on root bridges 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 as needed. Only one designated port is allowed per segment. If multiple switches exist on the same segment, an election process determines the designated switch, and the corresponding switch port begins forwarding frames for the segment. Designated ports are capable of populating the MAC table.

Back 6

The Root switch has NO root ports (because it IS Root) and has ONLY Designated ports.

Front 7

The order a switch goes through when deciding its Root and designated ports:
1. The port with the lowest cumulative Root Path Cost will be the Root port/Designated Port
2. If tied between multiple ports, the port that connects to the neighboring switch with the lowest Bridge ID becomes the Root port/Designated port.
3. If there are multiple connections to that same switch, the port with the lowest assigned STP priority will be the Root port/Designated port.
4. If tied, the port with the lowest hardware number (Fa0/1 is lower then Fa0/2) will be the Root port/Designated port.

Back 7

Blocked Port:
A port that is part of a redundant path and is blocked by STP to prevent a loop.

Front 8

Convergence is the term to describe the process STP goes through to achieve a stable, loop-free network. Convergence has only occurred when all ports are either in a Blocked or Forwarding state.

Back 8

Port States: With STP each port goes through 4 states:
Blocking, Listening, Learning, Forwarding.

Front 9

Blocking - when a switch boots all ports start in a blocking state. Blocked ports send no data but do listen for BDPU's from other switches.
All ports go to blocking mode if a Topology Notification Change (TCN) is received.

Back 9

Max Age Timer - is by default 20 seconds (10 2 second updates).
If the Max Age Timer is reached all switches go to a blocked state and STP begins convergence again.

Front 10

Listening State:Listening state enables a Blocked port to begin sending its own BPDUs. By default the listening state is 15 seconds.

Back 10

Learning State:
The learning state is when a switch begins populating its own MAC table.
The Learning state also has a 15 second default.

Front 11

Forward Delay Timer:
30 seconds it takes the Listening state and the Learning state to complete.

Back 11

Forwarding State:
When ports start forwarding.

Front 12

In a converged STP system, all ports are either Blocking or Forwarding. Know the four STP port states and what exactly the port is doing in each one!

Back 12

Rapid Spanning Tree Protocol (RSTP) IEEE 802.1w)

Front 13

There are no timers in RSTP.

Back 13

RSTP port states:
Discarding
Discarding
Learning
Forwarding

Front 14

RSTP Switches wait for only 3 missing BPDU's before doing the recalculation process.

Back 14

RSTP also has an Alternate and Backup ports.

Front 15

RSTP Alternate Port:
The Alternate port is the port that will become the Root port if the primary Root port fails.

Back 15

RSTP Backup Port:
The Backup port is the port that will become the
Designated port if the primary Designated port fails.

Front 16

Edge ports:
Ports that are connected to non-STP-capable devices.

Back 16

PORTFAST:
Cisco's Portfast command feature. With PortFast configured a switch port will stop sending BPDUs and transition to the forwarding state almost immediately.

Front 17

If a port configured with PORTFAST does recieve a BPDU it will disable PORTFAST and start STP on that port to prevent loops.

Back 17

You can configure the switch port with BPDU Guard to shut down the port if it receives a BPDU.

Front 18

Switch(config-if)#spanning-tree portfast

Switch(config)#spanning-tree portfast default

To turn PortFast off, use the spanning-tree portfast disable interface configuration command.

Back 18

The most common problem that can be avoided by using STP is broadcast storms.

To turn PortFast off, use the spanning-tree portfast disable interface configuration command.

Front 19

PVST+
Allows multiple instances of STP in order to accomidate VLANS.

Back 19

VLANs break STP and RSTP. Without PVST+ STP and RSTP see VLANS as a single switch.

Front 20

PVST+ can be used to load balance.

Back 20

Rapid Per VLAN Spanning tree (also known as PVRST) does the same thing for RSTP as PVST+ does for STP.

Front 21

MIST 802.1s Multiple Spanning Trees (or Multiple instances of Spanning Trees) allows multiple instances of RSTP with each VLAN being associated with a particular instance.

Back 21

To build a swtichs BID for a particular per-VLAN STP instance the switch must use a base priority setting of a multiple of 4096 (2^12).

Front 22

Answering STP questions:
1. Determine the Root Switch.
2. For each nonroot switch, determine its one root port (RP) and cost to reach the root switch through that RP.
3. For each segment determine the designated port (DP) and the cost advertised by the DP onto that segment.

Back 22

Determine the root switch:
1. Pick the lowest BID.
2. If the question lists the priority and the MAC address separately (show command output) pick the switch with the lowest priority.
3. In case of a tie pick the lower MAC address value.

Front 23

Cisco switches default to PVST+.

Back 23

Technique to find BID's on local switches:
1. Pick a switch to begin. Find the switchs BID using Show spanning-tree vlan vlan-id
If the root BID and the local BID are equal, the local switch is the root switch.
3. If the root BID is not equal to the local switchs BID:
A) Find the RP interface on the local switch (also in the show spanning-tree command output)
B) Using Cisco Discovery Protocol (CDP) or other documentation determine which switch is on the other end of the RP interface found in 3A.
C) Log into the switch on the other end of the RP interface and repeat this process starting at step1.

Front 24

The Per-VLAN Spanning Tree Protocol (PVSTP+) allows a separate instance of STP
for each VLAN that exists on your switch.

Back 24

The Per-VLAN Spanning Tree Protocol (PVSTP+) allows a separate instance of STP
for each VLAN that exists on your switch.

Front 25

NonDesignated Port

A nondesignated port is a switch port that is not forwarding (blocking) data frames and is not populating its MAC address table with the source addresses of frames that are seen on the attached segment.

Back 25

Disabled port: A disabled port is a switch port that is shut down or disabled due to an operational issue with the switch and/or port. STP also places ports in different operating states. These operating states are transitioned by the system as STP converges:

Blocking: A port is blocking when STP has determined that a better path to the root exists. BPDUs are received but not sent.

Listening: The switch is listening on the port and processing BPDUs but is not updating the MAC address table. A switch stays in listening mode before transitioning to learning using the forwarding timer (default: 15 seconds).

Learning: The switch is listening on the port and processing BPDUs and is updating the MAC address table. A switch stays in learning mode before transitioning to learning using the forwarding timer (default: 15 seconds).