9. Spanning Tree Protocol Concepts Flashcards
What is the standard for STP
802.1D
What are two goals of STP
- All devices in a VLAN can send frames to all other devices by making sure STP does not block too many ports
- Frames have a short life and do not loop around the network indefinitely
How does STP/RSTP prevent loops
Add an additional check on each interface
If port is in STP/RSTP forwarding state in that VLAN, use as normal
If port is in STP/RSTP blocking state, block all user traffic and do not send or receive traffic on that interface in that VLAN
Does STP/RSTP affect the interface’s connected or operational state?
No
What is a broadcast storm
When any kind of ethernet frames loop around a LAN indefinitely which saturate all the links with copies of that single frame crowding out good frames
What is a broadcast storm
The forwarding of a frame repeatedly on the same links, consuming
significant parts of the links’ capacities
What is MAC table instability
The continual updating of a switch’s MAC address table with
incorrect entries, in reaction to looping frames, resulting in frames
being sent to the wrong locations
What are three problems by not using STP on redundant links
Broadcast storms
MAC table instability
Multiple frame transmission
What is multiple frame transmission
A side effect of looping frames in which multiple copies of one frame are delivered to the intended host, confusing the host
How does STP/RSTP prevent loops
STP/RSTP prevents loops by placing each switch port in either a forwarding state or a blocking
state.
Interfaces in the forwarding state act as normal, forwarding and receiving frames.
However, interfaces in a blocking state do not process any frames except STP/RSTP messages
(and some other overhead messages).
Interfaces that block do not forward user frames,
do not learn MAC addresses of received frames, and do not process received user frames.
What is STP convergence
process by which the switches collectively
realize that something has changed in the LAN topology and determine whether they need
to change which ports block and which ports forward.
Which ports does STP place into forwarding state
- All the root switch’s ports
- Each nonroot switch’s root port
- Each LAN’s designated port
What is a root port
for a nonroot switch, the port with the least administrative cost back to the root switch
What is a root port
for a nonroot switch, the port with the least administrative cost (root cost) back to the root switch
What is a designated port
On a single link between two switches, the port with the lowest root cost
What is STP/RSTP disabled state
failed or not connected interfaces not considered in the spanning tree algorithm
Which ports does STP place into forwarding state
- All the root switch’s ports
- Each nonroot switch’s root port
- Each LAN’s designated port
What is a root port
For a nonroot switch, the port with the least administrative cost (root cost) back to the root switch
There is only one root port on each switch
What are the fields inside a Hello BPDU
Root Bridge ID
Sender’s bridge ID
Sender’s root cost
Timer values on the root switch
What is the root bridge ID in a Hello BPDU
Bridge ID of the of the switch that the sender believes is the root switch
What are the timer values inside a Hello BPDU
Hello timer
MaxAge timer
forward delay timer
Which switch becomes the root switch
Switch with the lowest BID =
Switch with the lowest priority. If priority is tied then switch with the lowest MAC
How is the root switch chosen
All switches claim to be root switch and sends out
Hello BPDU as such
If it encounters a lower BID then it stops advertising itself as the root and forwards Hello BPDU with new root
This continues until the root is chosen
How does a switch determine the root cost out of a port
It adds its interface cost to the root cost listed in each Hello BPDU
If two paths on a switch has the same best root cost, how is a tiebreaker determined
- Choose based on lowest neighbor BID
- Choose based on lowest neighbor port priority
- Choose based on the lowest neighbor internal port number
How is designated port chosen if the advertise costs tie between 2 switches
Switch with the lower BID wins
What can network engineers change to influence STP choices
Configure priority
Change STP/RSTP port costs
What is the default port cost of a 10Mbps link
2,000,000
What is the default port cost of a 100Mbps link
200,000
What is the default port cost of a 1 Gbps link
20,000
What is the default port cost of a 10Gbps link
2000
What is the default port cost of a 100Gbps link
200
What is the default port cost of a 1Tbps link
20
What is the steady state of operations for Hellos in a stable STP topology
1, root creates and sends Hellop BPDU with root cost of 0 out of all its working interfaces
- Non root switches receive the hello on their root port. After they change the hello to list its own BID as the sender BID and adding its root cost, they send out the hello out of all designated ports
- Repeat steps 1 and 2 until something changes
What are the three times that manage STP convergence
Hello timer - 2 seconds
MaxAge - 10 x Hello timer
Forward delay - 15 seconds
What is the MaxAge timer in STP
10 x hello timer
How long switch should wait after not hearing Hellos before trying to change STP topology
What is the forward delay timer in STP
Delay that affects the process that occurs when an interface changes from blocking state to forwarding state.
A port stays in an interim listening state, and then an interim learning state, for the number of seconds defined by the forward delay timer.
What is the steady state of operations for Hellos in a stable STP topology
1, root creates and sends Hello BPDU with root cost of 0 out of all its working interfaces
- Non root switches receive the hello on their root port. After they change the hello to list its own BID as the sender BID and adding its root cost, they send out the hello out of all designated ports
- Repeat steps 1 and 2 until something changes
What are the three timers that manage STP convergence
Hello timer - 2 seconds
MaxAge - 10 x Hello timer (def 20 sec)
Forward delay - 15 seconds
What is the MaxAge timer in STP
10 x hello timer
How long switch should wait after not hearing Hellos on a port before trying to change STP topology
What happens after the MaxAge expires on a switch
It goes over the STP choices again based on the Hellos it receives from other switches
Why does STP need to put a port that is going from blocking to forwarding into intermediate states
To prevent temporary loops
What are the two intermediate states when a port is going from blocking to forwarding in STP
Listening
Learning
What is the listening state in STP
Interface does not forward frames.
Switch removes old stale MAC table entries for which no frames are received from each MAC address during this period
These MAC table entries could be cause of temporary loops
What is the learning state in STP
Interface do not forward frames
Switch begins to learn MAC address frames received on the interface
With default how long can it take for STP to move an interface from blocking to fowarding
20 sec MaxAge to determine there is an issue
15 sec in listening
15 sec in learning
—————————————–
50 seconds total for an interface to converge
What is the standard for RSTP
802.1w
In what ways is RSTP different from STP
- RSTP adds mechanism by which a switch can replace its root port without any waiting to reach a forwarding state
- RSTP adds mechanism to replace a designated port without waiting to reach a forwarding state
- RSTP lowers waiting times for cases in which RSTP must wait for a timer
- Each switch generates their own hellos instead of forwarding from the root switch
- Neighbors can query each other for information instead of waiting on timers to expire
What is the MaxAge on RSTP
3 x Hello
What are the different port roles in RSTP
Root port Alternate port Designated port Backup port Disabled port
What is the alternate port in RSTP
port that replaces the root port when the root port fails
What is the backup port in RSTP
port that replaces a designated port when a designated port fails
What is the requirement to be an alternate port
The alternate port must receive Hellos that identify the same root switch
What is the RSTP process when a root port fails on a switch
- the switches with the link that contains the alternate port exchange RSTP messages that the alt will now be the root and the switch on the other end flushes required MAC entries to avoid a loop
2 role of the root port is changed from root to disabled - state is changed from forwarding to discarding
- role of the alternate port is changed to be the root port and forwarding state immediately without waiting on timers
What are the different STP ports states compared to RSTP states
Disabled - Discarding Blocking - Discarding Listening - Not used Learning - Learning Forwarding - Forwarding
