Upgrade to remove ads
Chapter 4 (Spanning Tree Protocol)
Terms in this set (131)
identifies and prevent looping in a layer 2 network
What is Loop
is when a packet endlessly travel between devices causing your network to go down.
PVST+ (Per-VLAN Spanning Tree Protocol Plus)
By default, Cisco switches use PVST+
Feature of STP that SPEED of performance are
Feature of STP that that increase stability
1. Root Guard
2. Loop Guard
3. BPDU Guard
4. BPDU Filter
Some feature that are not directly connected to STP but is used to Complement STP OPERATION
What is an benefit of a redundancy topology
it can eliminate the possibility of a single point of failure
what are some draw back of a redundancy topology?
1. redundancy network can cause loop.
2. Broadcast Storm
3.Multiple Frame Transmission
4. Mac database Instability
Broadcast storm are flooded to all ports except the port on which the frame was receive. Broadcast frame travel in loop
Multiple Frame Transmission
Multiple copies of the same unicast frames may be delivered to destination station
Mac- Database instability
this is a direct result of the multiple copies unicast frame being receive on multiple port on the switch
How do you solve all the problem that redundancy topology cause such as 1. Broadcast Storm 2. Multiple Frame Transmission 3. Mac database instability.
How does STP prevent a loop free topology?
STP, block certain ports, so there is only one active path to each network segment.
Port in a Standby State ( STP put ports that are not in active state into a standby state, that's how it prevent loop)
they do not listen, Forward, or flood data frames
What are BPDUs
BPDUs are messages STP sent every 2 second to determine any topology change in the network.
the STP Standard are :
1. STP original IEEE 802.1D
2. CST (common Spanning Tree) 802.1D
3. PVST+ () Cisco Proprietary
4. RSTP () 802.1w
5. RSTP+ 802.1w
6. MST (Multiple Spanning Tree) 802.1s
The STP Operations
1. Elects one Root Bridge
2. Selects the Root Bridge on the non root switch
3. Selects one designated port on each segment
what does STP use to calculate it path cost
( you can also Set STP Path Cost manually)
How many Root Bridge can be in a network topology
One ( all data must flow through the Root Bridge)
All Port on the Root Bridge Forward
What Port on the Root Bridge Switch that can forward
How is a Root Port selected on a non root bridge sw
the lowest path cast from the non root switch to the Root Bridge
are nor Root Port or Designated Port.
Non-Designated port are normally in the block state to break loop
what is the different between root Port and Designated port
Root port do not receive or send dated
Designated port receive and send data traffic
STP Port Rule and Description
1. Root Port: lowest path cost to the Root Bridge.
2. Designated Port: receive and forward traffic
3. non-designated: Block Ports
4. Disable port: Shut down Ports
What are BPSU's used for ?
To exchange STP information
there are two type of BPDU
1. Configuration BPDUs- used for calclating STP
2. TCN - (topology change notification) : used to inform change in the network topology.
3. Message Type
4. Root path cost
6. Sender ID
7. Root Bridge ID
9. Port Id
10. Message Age
11. Max age
12. Hello time
13. Forward delay
How does a switch know if where is a redundancy in the network
it sent out a BPDU, if the BPDU return the switch automatically know there is a redundancy in the net work.
What are BID consists of
1. Priority--------------2 bytes
2. Mac-Address------6 bytes
3. Extended system ID -------- carry vlan ID
How are root bridge Elected
the Root Bridge is Elected based on the lowest BID
Links value and Cost
Link Value Cost
10 Gbps 1
1 Gbps 4
100mbps ---------- 19
10 mbps 100
what is the default port priority
what is the five STP Port States
1. Blocking- receive BPDUs.all ports start here
2. Listening- receive and send BPDus
3. Learning- it can learn and add new mac-address
4. Forwarding- send and receive data
What are the STP Port Roles
1. Root Port- shortest path to the Root switch
2. non-Designated port- Block port
3. Designated port- send out traffic
4. Disable- shutdown
PVST+ (the default STP for cisco switches)
PVST+ have separate root bridge per vlan. and separate forwarding link per vlan
what happen when the Root ridge receive a TCN
it was first send back an acknowledgement to the switch it receive the topology change notification from. 2nd the Root Bridge switch send out the change to other switches in he network topology
is when a switch change state to forwarding or Blocking
what port an topology change is send on ?
The three type of Topology Change
1. Direct Topology Change.
2. Indirect Topology Change.
3. insignificant Topology Change.
The three type of Topology Change
1. Direct Topology Change- when a link failure occur on an interface . can be detected on a interface.
2. Indirect Topology Change- no link failure is detected. no TCN is send. (EX: Firewall failure)
3. insignificant Topology Change- Pcs connected to an interface is turned off
Cisco enhanced The Original 802.1D Feature to speed up Convergence Time. these Feature are ?
Rapid Spanning Tree Protocol (IEEE 802.1w)
It significantly speed up the recalculation of spanning tree when topology change occur
RSTP Port Roles
1. Alternative: is an alternative path to the root switch. Alternative Port start up in a discarding state
2. Backup- is an additional switch port o the designated switch. start up in a discarding state
RSTP Port State
How many Root Bridge sw does RSTP have
One Root Bridge Switch
RSTP provide Rapid connectivity
when a switch failure occur in a network topology
When a Switch failure occur new root port and the designated port of the connecting bridge transition to forwarding through
an explicit handshake protocol between them
what STP Port State will not be find in RSTP port state. and what is it replace with
RSTP replace Listening state with Discarding State
Discarding port state
Prevent forwarding of Data Frames
what is different in topology change in RSTP
well, a lost of connectivity in STP is consider as topology change, in RSTP lost of connectivity is no longer consider as topology change .
RSTP Topology change
only when a non-edge port change port state to forwarding
when a TC occur in a RSTP
The switch that detected the link failure, will send BPDU to all of his port. the next sw will do the same except it will not send BPDU to the port it receive it BPDU from.
Why does RSTP not consider link failure a topology change?
Loss of connectivity does not provide new paths in topology. If a switch loses the link to a downstream switch, the downstream switch either has an alternate path to the root bridge or it does not.
RSTP Link Type
1. Point-to-Point Link---- Port connected to one single switch, and it operated into a full duplex.
2. Shared Link- port is connected to multiple switch with a shared media. operated in half duplex.
An Edge port is a switch that is never intended to be connected to another switch devices.
what happen when an edge port receive a BPDU
It will immediately loses the edge port status
Because BPDUs only goes between switches.
edge ports are connected to PC, server..
Command to verify Port role
#Show Spanning Tree
How do you manually set the Root Bridge
By changing the STP priority
what Switches do you want to configure as the Root Bridge in a network
1. Distribution switches
2. Core switches
what STP version used Extended system ID for one of the BID
If a sw priority is set to Zero
That sw will become the Root Bride .
what Port ID consist of
1. Port Priority
2. Port Number
What are the three STP Timer
1. Hello Timer---- -----2 second default
2. Forward Delay -----15 second default
3. Max Age -----------20 second default
What are the three STP Timer
1. Hello Timer- time between each BPDUs that is send on a port.
2. Forward dalay- the time spend in listening and learning state
3. Max (maximum) Age- the time that passes before a port save his configuration BPDU information
what switch should make changes to the STP timer.
Root Switch. ( don't ever change STP timer tho) instead use RSTP
what is the convergence time for STP
that is the convergence time RSTP
lest then 1 second
what are the Cisco Toolkit that improve performance
1. Backbonefast: fast convergence in distribution and core layer when change occur
2. portfast: configure access port to transition to forwarding state
3. uplikefast: fast fail-over on access switch
What are the key feature of STP Toolkit that ensure Stability
1. BPDU guard: Disable Portfast when a BPDU is receive.
2. BPDU filter: suppresses BPDU on port
3. Root Guard: prevent external switches from becoming root
4. Loop Guard:prevent alternative port form becoming designated port if no BPDU is receive.
UplinkFast ( Cisco Proprietary solution)
- enable fast fail-over (IT reduce convergence Time)
- uplinkfast can only be enable on access switches with redundancy
sw2(config)# spanning-tree uplink
- enable fast convergence on distribution and core layer switches when failer occur
what does a switch do when a receive an inferior BPDU
it first check to see if it have an active path to the root bridge.
when will a switch send and inferior BPDU?
when it does not have root port connection to the root bridge switch
what a PCs is first turned on it goes through all the STP states
all this take about 30 seconds. 15 second for listening to learning another 15 from Learning to Forwarding
Portfast (portfast is disable by default)
- portfast are enable on a per port or globally
- portfast are enable only on access port
- enabling portfast pcs goes from blocking directly to forwarding
- Port enable with port should not receive BPDUs
what happen when a Portfast port receive a BPDU?
IT go into BLOCKING mode ( and lost it portfast status, u will have to renable portfast)
when Portfast is enable on a access port
the PC do not go through all the STP State. it goes directly from Blocking to Forwarding
protect the the integrity of the port that have portfast enable
When a portfast interface that is enabled with BPDU Guard receive a BPDU what happen to the port
that port will go into a "err-disabled state" That means the port is shut down and must be manually reenabled or automatically recovered through the error-disabled timeout function.
in order for a port to be enable for BPDU Guard
the port must be enable for portfast.
-suppresses BPDU on Port
- Prevent BPDU from bring sent out
- BPDU Filter prevent sharing of STP information
- BPDU are configured on Access Port
- server Provider are the one who use BPDU filter to prevent the sharing of STP INFORMATION
- ignores all BPDU receive
- it sends NO BPDUs
When a port that is configured for BPDU FILTER receive a BPDU
Ignore the BPDU
- Prevent other surrounding switches from becoming ROOT SWITCH by forcing all his interface to becoming Desalinated Port.
-Root Guard is enable on all access port
- The Root Guard err-disable state is
" Root-inconsistent state"
- If a better BPDUs is receive on a port, ROOT GUARD is DISABLED
- ROOT GUARD should be enable on all access port, so that root bride is not established through these port
Root Guard : After a Root Guard feature is enable on a port, the Switch does allow that port to become an STP ROOT PORT. )
The port remain as a designated port ( is will happen on a Root Bridge switch)
When a ROOT Guard configure interface receive a SUPERIOR BPDU (BETTER) that port will go into a
ROOT-INCONSISTENT err-disabled state
- recovery from this state is Automic
ROOT GUARD can be enable on
- access switches : any access port
- distribution switches: any access port
- core switches: any access
- if no BPDUs are receive on a non-destination port that is enable with Loop Guard the port will transition into loop-inconsistency
- recover from Loop inconsistency is atomic ( just need to receive BPDUs again)
- enable on a per port basis
- Loop Guard is disabled by default
Prevent STP looping by not letting alternative port or non-destination port to just transition into forwarding state .
Loop inconsistent ( same as Blocking )
If loop guard is enable on a ether-channel interface
the entire interface is Block if BPDUs are not receive. because ether-channel are logical
Loop Guard with Ether-channel
Loop Guard will put the entire channel into a loop-inconsistent state if any physical link in the bundle fails.
ROOT Guard and Loop Guard
should not be enable on the same port.
- Root Guard-- is enable on designated port
- Loop Guard -- is enable on non-designated port
When Traffic flow in both direction (this is the normal state of traffic)
When traffic is only flowing one direction. this can cause loop in a topology
Unidirectional Link Detection (UDLD)
Enable device to detect what a unidirectional link exist in the topology if so shut it down.
- Is useful for FIBER PORT.
- Cisco Proprietary protocol
- prevent Layer 2 Loop
- UDLD frames are send every 15 second by default
- err-disables state "SHUTDOWN"
- In ether-channel UDLD only shut down link that are effected.
How does a port detect a bidirectional vs Unidirectional
Port configured with UDLD sends UDLD frames and if it receives echos response - bidirectional
- If no response ---- Unidirectional
what is the propose of UDLD.
is to prevent one way communication between adjacent devices
After UDLD detects a unidirectional link, it can take two courses of action depending on the mode
UDLD have two MODE
1. Normal mode: port continue its normal operation: syslog message is generated
2. Aggressive mode- try to reestablish the link
- when unidirectional is detected, the port continue its normal operation
- syslog message is generated
- Try to reestablish the link
- send one message per second for 8 second if none of the messages are send back the port is place in an err- disabled state
UDLD Best Practices
- UDLD is deployed on any FIBER-OPTIC interconnection
- used Aggressive mode fro best practices
- turn on UDLD in global configuration
FlEXLINK (CISCO Proprietary)
Provide an alternative solution to STP and allow user to turn off STP and still provide basic link redundancy.
- Flexlink have active and backup interface
what if an admin turn off STP what are other alternative beside STP can he use still give he link redundancy
How do you know that STP is turn OFF
STP is turn OFF- when one link is active and the other like is in the standby mode
IF Preemption is enable on FLEXLINK
- When active port is shutdown and re enable that port will take on the active port role again
IF Preemption is turned OFF FLEXLINK
- When an active port is shutdown, the standby port will take the role of the active port when the active is back on line, it will take the role of the standby.
on what switches do you want to configured FLEXLINK
ACCESS LAYER SWITCHES
what is TTL (TIME-TO-LIVE)
If a frame do not have a TTL filed, the fame will travel around the network devices endlessly, resulting in network flooding.
what is the benefit of using multilayer switches in the distribution and core layer
Because it allow multipathing ( up to 16 paths) using equal-cost multipathing without dependency of STP
What is ECMP (Equal-Cost Multipathing)
when a router have multiple equal cost path to a prefix.
what are the negative of using layer 2 switches in your distribution and core layer
- layer 2 switches do no support MULTIPATHING
- It have NETWORKWIDE FAILURE DOMAIN
How can A loop be introduced on the user-facing access layer ports
1. Wiring mistakes
2. misconfigured end user stations
3. malicious users can create a loop
STP Best Practice
1. Portfast- Apply to all access port
2. ROOT GUARD- Apply to port the root is never expected
3. LOOP GUARD- Apply to all non-designated port
4. UDLD- detect the exists of unidirectional
MST (Multiple Spanning-Tree)
- reduce the total number of Spanning Tree instance to match the physical topology.
- reduce CPU cycles on the switch.
- MST is a concept of mapping one or more vlans into a single instance.
- MST region is an interconnected Bridge with the same MST configuration.
what is the main purpose of spanning-tree instance?
to reduce the total number of spanning tress instance to match the physical topology
The MST Region attribute
.1. numerical configuration name
2. configuration revision number
3. 4096 vlans configuration
For Switches to be apart of the same MST region they must share the follow
1. same configuration numerical name
2. same revision number
3. same vlan
To ensure a consistent VLAN-to-instance mapping, it is necessary for the protocol to be able to identify the boundaries of the regions exactly.
For that purpose, the characteristics of the region are included in BPDUs. such as
1. revision number ---- give you the method of tracking change on the region
2. configuration name
you will know if the port is in different region
when the designated port is in a different region
MST do not carry BPDU for every active STP instance
But a special Instance (Instance 0) is created to carry STP related Information.
what is Instance 0 (The IST is the one that make up MSTIs)
instance 0 is the internal spanning tree (IST), maintain loop free toplogy
- IST presents the whole MST region as a single virtual bridge to the inside stp
-MSTIs do not send BPDUs outside a region, only the IST does.
- never interact with the outside of the region
- IST presents the whole MST region as a single virtual bridge to the inside stp
- run the RSTP automatically by default, without any extra configuration
- Fast converce
How are BPDU exchange between MST instance and CST instance
over the native vlan
in a MST configuration all vlan original belong to what instance by default
Instance 0 (internal instance)
By default all vlan are mapped to IST Instance
IST Instance 0
- You must explicitly map vlan to other instances from the IST Instance
what is the M-RECORD FIELD
Is the information IST send per instance
what does a mismatch digest list mean for MST configuration
A mismatch in digest indicate that the VLAN lists do not match between switches
digest like should away match if vlan is configure correctly
THIS SET IS OFTEN IN FOLDERS WITH...
Chapter 3 Multiplayer
YOU MIGHT ALSO LIKE...
LAN Switching and Wireless CH 5
CCNA 3 Chapter 5: STP
CCNA C Chapter 2: LAN Redundancy
OTHER SETS BY THIS CREATOR
ch 8 unix
chapter 7 (test2)