quiz2
Bridge Priority:
The default priority value for all Cisco switches is the decimal value 32768
Extended System ID
The extended system ID value is a decimal value added to the bridge priority value in the BID to identify the VLAN for this BPDU.
What additional information is contained in the 12-bit extended system ID of a BPDU?
Vlan ID
after the election fo the root bridge has been completed, how will switches find the best paths to the root bridge?
each switch will analyze the sum of all port costs to reach the root and use the path with the lowest cost.
Currently each EtherChannel can consist of up to
eight compatibly-configured Ethernet ports
All switches in the broadcast domain participate in the
election process.
how often are bpdu's are sent?
every 2 seconds
*if you have a layer 2 loops, a switch is unable to
forward frames
Which port state will switch ports immediately transition to when configured for PortFast?=
forwarding
The path information, known as the internal root path cost
is determined by the sum of all the individual port costs along the path from the switch to the root bridge.
*LACP
link aggregation control protocol
ether channel automatically configures
load balancing
root port determined by
lowest cost to root bridge
*MSTP
maps mutiple vlan's into the same spanning tree instance
*EtherChannel does what?
multiple physical Ethernet links together into one single logical link, It provides fault-tolerance, *load sharing*, *increased bandwidth*, and redundancy between switches, routers, and servers.
can you ether port a gig and a ethernet connection togeather?
no, they must be the same
The show etherchannel summary command displays
one line of information per port channel.
SU on show ethersummary
port is down
SD on show ethersummary
port not established
The Bridge ID contains a
priority value, the MAC address of the switch, and an extended system ID (vlan ID).The lowest BID value is determined by the combination of these three fields.
*PVST+ standard
provides a separate 802.1D spanning tree instance for each VLAN
STP compensates for a failure in the network by
recalculating and opening up previously blocked ports.
STP stops
redundent paths/loops
The STA designates a single switch as the root bridge and uses it as the
reference point for all path calculations.
BPDUs are used to elect the
root bridge, root ports, designated ports, and alternate ports.
*RSTP port rolls
root port Designated port Backup port, alternate port
Switches exchange BPDUs to
select the root bridge.
*STP
spanning tree protocol, layer 2 loop prevention protocol
inconsistant mac talbe if
stp is not turned on to prevent flood of frames
An EtherChannel link can only be created between
switches
Which two statements describe a switch port that is configured with PortFast?
switchport immediately transitions from blocking to forwarding state, or listening to forwarding state
When two switches are configured with the same priority and have the same extended system ID, how do the switches determine bridge priority
the MAC address with the lowest value, expressed in hexadecimal, will have the lower BID.
bridge id values determine which switch will be
the root bridge
if they all match the switch with the lowest mac address is
the root bridge
*In EtherChannel, it is mandatory that all ports have
the same speed, duplex setting, and VLAN information. Any port modification after the creation of the channel also changes all other channel ports.
how do routers stop loops
time to live
Switches do not have TTL
time to live, unlike routers they cannot discard data
portfast eliminates the time wasted on
trying to descover the DHCP server
what is the port roll of back up ports on RSTP port rolls?
used for hubs
*MST
wich provides up to 16 instances of RSTP
PAgP modes
On, PAgP desirable, PAgP auto
*only 2 protocols usable on ether channel
PAGP, and LACP
default protocol spanning tree is
PVST+
cisco ios runs what protocols for STP
PVST+ and RSTP
*PAGP
Port Aggregation Protocol, cisco only version
BID of the root bridge, known as the
Root ID.
*STP port rols
Root port, designated port, blocked port
If a switch port transitions directly from the blocking state to the forwarding state without information about the full topology during the transition, the port can temporarily create a data loop. For this reason,
STP has five ports states, four of which are operational port states blocking, listening, learning, forwarding. The disabled state is considered non-operational
difference between listening state and learning state
both recieve and send data, learning updates mac table, listening does not, the both do not forward frame data
When the root bridge has been elected for a given spanning tree instance, the STA starts determining the best paths to the root bridge from all destinations in the
broadcast domain
*layr 2 loop
broadcast storm
Without STP enabled, Layer 2 loops can form, causing?
broadcast, multicast and unknown unicast frames to loop endlessly. This can bring down a network quickly.
STP is enabled when?
by default to prevent broadcast storms
The individual EtherChannel group member port configuration must be
consistent on both devices.
If one end of a segment is a root port, the other end is a
designated port.
All ports on the root bridge are
designated ports.
*STP port states
disable, blocking, listening, learning, forwarding
The switches running STP are able to compensate for failures by ?
dynamically unblocking the previously blocked ports and permitting traffic to traverse the alternate paths.
STP prevents loops from occurring by configuring a loop-free path through the network using strategically placed what?
"blocking-state" ports.
range of bridge priority
0 - 61440
PAgP packets are sent every
30 seconds
*how long from blocking state to forwarding state on a port?
30 seconds, from listening forwarding delay (15sec) to learning forward delay (15sec)
Lowest Sender Port Priority
This topology has two switches which are connected with two equal-cost paths between them. S1 is the root bridge, so both of its ports are designated ports
a bridge priority can be changed in incriments of
4096
EtherChannel
A link aggregation technology is needed that allows redundant links between devices that will not be blocked by STP
on access ports, you have 2 options for port status
BPDU and port-fast
During STA and STP functions, switches use (BPDUs)
Bridge Protocol Data Units to share information about themselves and their connections.
*RSTP port stats
Discarding, learning, forwarding
why is a port configured alternate and block
blocked logically, if other path fails the blocked port opens up
Using the STA, STP builds a loop-free topology in a four-step process
Elect the root bridge., Elect the root ports., Elect designated ports., Elect alternate (blocked) ports.
*STP standard is
IEE 802.1D
*RSTP standard
IEE802.1W
by default, a port can only be in 2 states
blocking and forwarding
A Layer 2 loop can result in
MAC address table instability, link saturation, and high CPU utilization on switches and end-devices, resulting in the network becoming unusable.
When a switch port is configured with PortFast, that port transitions from
blocking to forwarding state immediately
PAgP auto
This PAgP mode places an interface in a passive negotiating state in which the interface responds to the PAgP packets that it receives but does not initiate PAgP negotiation.
PAgP desirable
This PAgP mode places an interface in an active negotiating state in which the interface initiates negotiations with other interfaces by sending PAgP packets.
Selecting a Root Bridge
This bridge (switch) is the reference point for the entire network to build a spanning tree around.
PAgP on
This mode forces the interface to channel without PAgP. Interfaces configured in the on mode do not exchange PAgP packets.
*port channel
When an EtherChannel is configured, a port channel is created from the bundled physical interfaces.
port fast shoud only be enabled on what type of port?
access ports
