chapter 2 Flashcards Preview

CIT_209 > chapter 2 > Flashcards

Flashcards in chapter 2 Deck (27):


spanning tree protocol: used when using redundant links between switches. this keeps a broadcast storm from occuring. without stp's the redundant links all get MAC address updates from the switches and then the same MAC address is on several connections from 1 switch to another and this causes broadcast storms. *STP helps figure out which connection is most reliable.


what is it and what is used today: STP and RSTP?

Spanning Tree Protocol: older technology (cisco propriatary) used to stop loops and broadcast storms.
**RSTP: Rapid spanning tree protocol: used instead of STP.


what command is used to display the cost of each interface port?

#show spanning-tree


how to turn STP debugging on so that all events will be shown in real time?

debug spanning-tree events


Broadcast storms?

Without some loop-avoidance process, each switch may flood broadcasts endlessly. This situation is commonly called a broadcast storm.


Multiple frame transmission?

Multiple copies of unicast frames may be delivered to destination stations. Many protocols expect to receive only a single copy of each transmission. Multiple copies of the same frame can cause unrecoverable errors.


MAC database instability?

Instability in the content of the MAC address table results from copies of the same frame being received on different ports of the switch. Data forwarding can be impaired when the switch consumes the resources that are coping with instability in the MAC address table


do ethernet frames have a time to live attribute?



MAC database instability.

this is when a mac address is broadcast from one switch to another one and then that switch sends the broadcast to redundant paths and this causes a loop that also provides incorrect info of where the mac address originated from and the mac address table becomes corrupted with incorrect info. This loop will continue until the connection is physically broken or the device is restarted. cusses high cpu load on switches. the host caught in the loop is not contactable and will cause a broadcast storm as the packets try to keep up with the changing mac address tables of the switches and loop throughout the network.


what happens to the network if a broadcast storm does develop?

it fails.



bridge protocol data unit (BPDU) frames:
frames that contain information about the Spanning tree protocol (STP).


what is a blocked port and what does it do?

a port that is configured so that it can not accept or forward packets. this is used on redundant links so that broadcast storms or loops do not occur. this prevents a layer 2 loop.


what happens when a blocked port is needed because the destination is unreachable due to a port / connection failing?

the port is automatically set to un-bloacked. the switch then sends a broadcast from the newly un-blocked port and all other un-blocked ports. the message continues onto the destination device.



STP: spanning tree protocol is the older version. now
Rapid Spanning Tree Protocol/ RSTP is the version used today. it is also a type of spanning tree protocol as there is also: Multiple spanning tree protocol MSTP



Multiple spanning tree protocol


what are the spanning tree port roles?

designated port, root port, alternate port.



spanning tree algorithm: used to decided which ports are put into blocked mode or active mode. this prevents loops.



Bridge ID:
all switches that use STP in a network have an id. these id's are used to determine the Root bridge. the one with the lowest id number is chosen.


BPDU? used for?

bridge protocol data unit (BPDU)
**a frame from each STP switch that is sent. this frame contains the BID (bridge ID) and root id. (the root is elected based on the switch with lowest BID)


alternate/ backup ports vs disabled ports?

a disabled port is a switch port that is shut down, alternative ports are ports that have been chosen by the STA (spanning tree algorithm) as back-up ports that are in a blocked state to stop loops from occurring but are available if the designated and root port is shutdown or disconnected.


how is the BID chosen?

The BID (bridge ID) contains a priority value,
the MAC address of the sending switch,
and an optional extended system ID. The lowest BID value is determined by the combination of these three fields


purpose of the root bridge?

a switch on the network that the STA (spanning tree algorithm) has decided is the reference point for all calculations. this switch is decided via the lowest BID. (Bridge ID)


root ports?

Switch ports closest to the root bridge


designated ports?

All non-root ports that are still permitted to forward traffic on the network
Designated ports are selected on a per-trunk basis. If one end of a trunk is a root port, then the other end is a designated port. All ports on the root bridge are designated ports


Alternate and backup ports

Alternate ports and backup ports are configured to be in a blocking state to prevent loops
Alternate ports are selected only on trunk links where neither end is a root port.


what command is entered to change the port cost? what does this allow/do?

#spanning-tree cost 'number u select'
this allows the admin to decide/ control the spanning tree paths to the root bridge.


command to change the spanning tree cost back to default is?

no spanning-tree cost