What is STP &
Redundant Links?
Spanning Tree Protocol (STP) is a protocol which prevents
layer 2 loops. STP enables switches to become aware of each other so that they
can negotiate a Loop-Free path through network. In practical Scenario,
Redundant links are created to avoid complete network failure in an event of
failure of one link.
How STP works?
STP chooses a Reference point (Root Bridge) in the network
and calculates all the redundant paths to that reference point. Than it picks
one path by which to forward frames and blocks other redundant paths.
What are the
different port states?
1. Disabled - A port in the disabled state does not
participate in the STP.
2. Blocking - A blocked port does not forward frames. It
only listens to BPDUs. The purpose of the blocking state is to prevent the use
of looped paths.
3. Listening - A port in listening state prepares to forward
data frames without populating the MAC address table. The port also sends and
listens to BPDUs to make sure no loops occur on the network.
4. Learning - A port in learning state populates the MAC
address table but doesn’t forward data frames. The port still sends and
receives BPDUs as before.
5. Forwarding - The port now can send and receive data
frames, collect MAC addresses in its address table, send and receive BPDUs. The
port is now a fully functioning switch port within the spanning-tree topology.
What is the default time a port takes to transition from the
blocking state to the forwarding state?
The default time a port takes to transition from the
blocking state to the forwarding state is 50 seconds: 20 seconds for Max Age,
15 seconds for listening, and 15 seconds for learning.
What are STP Timers
and Explain different types of STP Timers?
STP uses three timers to make sure that a network converges
properly before a bridging loop can form.
1. Hello - The time interval between Configuration BPDUs
sent by the root bridge. It is 2 seconds by default.
2. Forward Delay - The time interval that a switch port
spends in both the Listening and Learning states. The default value is 15
seconds.
3. Max Age - Maximum length of time, a BPDU can be stored without
receiving an update. It can also be define as a time interval that a switch
stores a BPDU before discarding it. It is 20 seconds by default.
Explain types of STP
Port Roles?
1. Root port - The root port is always the link directly
connected to the root bridge, or the shortest path to the root bridge. It is
always on Non-Root Bridge.
2. Designated port - A designated port is one that has been
determined as having the best (lowest) cost. A designated port will be marked
as a forwarding port. It can be on both Root Bridge & Non Root Bridge. All
ports of Root Bridge are Designated Port.
3. Forwarding port - A forwarding port forwards frames.
4. Blocked port - A blocked port is the port that is used to
prevent loops. It only listens to frames. Any port other than Root port &
Designated port is Block Port.
What is the STP
blocking state?
When a switch starts, all ports are in the blocking state to
prevent any loop in the network. If there is a better path to the root bridge,
the port remains in the blocked state. Ports in the blocked state cannot send
or receive traffic, but they can receive BPDUs.
What is BPDU?
All the switches exchange information to select Root Bridge
as well as for configuration of the network. This is done through Bridge
Protocol Data Unit (BPDU). Each switch compares the parameters in the BPDU that
it sends to one neighbor with the one that it receives from another neighbor.
How often do Bridges
send BPDUs on active ports?
The default time that bridges send BPDUs is 2 seconds.
What does STP do when
it detects a topology change in the network due to a bridge or link failure?
If spanning tree detects a change in the network due
to a bridge or link failure, it send out TCN[topology change message] BPDU and at least one bridge interface changes from the
blocking state to the forwarding state, or vice versa.
What is the Max Age
timer?
The Max Age timer is how long a bridge stores a BPDU before discarding it. The default time is 20 seconds (ten missed hello intervals).
The Max Age timer is how long a bridge stores a BPDU before discarding it. The default time is 20 seconds (ten missed hello intervals).
What is STP forward
delay?
The forward delay is the time it takes for a port to move from the listening state to the learning state or from the learning state to the forwarding state. The default time is 30 seconds.
The forward delay is the time it takes for a port to move from the listening state to the learning state or from the learning state to the forwarding state. The default time is 30 seconds.
What is the
destination MAC address used by Bridge Protocol Data Units (BPDUs)?
Bridge Protocol Data Units (BPDUs) frames are sent out as at
multicast destination MAC address 01:80:c2:00:00:00.
What are Types of
BPDU?
Two types of BPDU exist -
1. Configuration BPDU - Used for Spanning-Tree Computation.
2. Topology Change Notification (TCN) BPDU - Used to
announce changes in the Network Topology.
How Root bridge is
elected?
The Bridge ID is used to elect the root bridge in the STP
domain. This ID is 8 bytes long and includes both the priority and the MAC
address of the device. Switch with the lowest Bridge ID is elected as the Root
bridge which means Switch with the lowest priority will become Root Bridge if
two or more switches have same priority than switch with lowest mac address
will become Root Bridge.
Explain Root path
cost?
Root path cost is the Cumulative Cost of all links to the
Root Bridge.
How Root Ports are
elected?
Non Root Bridges use Root path cost to determine which port
will be the Root port. The port with the lowest root path cost is elected as
the root port and is placed in the forwarding state.
What is the
difference between Path cost and Root Path cost?
Path cost is the value assigned to each port. It is added to
BPDUs received on that port to calculate the root path cost. Root path cost is
defined as the cumulative cost to reach the root bridge. This value is
calculated by adding the receiving port's path cost to the value contained in
the BPDU. In a BPDU, Root path cost is transmitted not the path cost.
What is Path Cost or
Spanning Tree Path Cost value?
The Spanning Tree Cost Value is inversely proportional to
the associated bandwidth of the link and therefore a path with a low cost value
is more preferable than a path with high cost value.
Link Bandwidth
Cost Value
10 Gbps
2
1 Gbps 4
100 Mbps 19
10 Mbps 100
What is Root Port?
Once the Root Switch is elected, every other Switch in the
network must select a single port on itself to reach the Root Switch. The
single selected port on a Switch with least Path Cost to reach the Root Bridge
is called the Root Port. Root Bridge will never have a Root Port.
What is Extended
System ID?
The Extended System ID is utilized by spanning-tree to
include the VLAN ID information inside 16-bit STP Bridge Priority value.
Extended System ID is the least significant 12-bits in 16-bit STP Bridge
Priority value.
Explain Root Guard ?
Root guard is used to protect root bridge. Root Guard stops
a new switch introduced in the network with a lower bridge ID to become the
root bridge. If a port with Root Guard feature enabled receives a superior
BPDU, it moves the port into a root-inconsistent state (equal to a listening
state) thus maintaining the current Root Bridge status. It is enabled on
interface level.
switch(config-if)# spanning-tree guard root
What is BPDU Guard ?
When we enable portfast on a port , we do not expect BPDU's
on that port. Suppose a switch is connected by mistake on the port where
portfast is enabled, loop can form. An even greater consequence is that
connected switch has potential to become root bridge. The BPDU Guard feature
was developed to protect the integrity of switch ports that have PortFast
enabled. If any BPDU (superior to the current root or not) is received on a
port where BPDU Guard is enabled, that port immediately is put into the
error-disable state. The port is shut down in an error condition and must be
either manually re-enabled or automatically recovered through the error-disable
timeout function.
BPDU Guard can be enabled on both interface & global
level. It is basically enabled on access layer switches. In this all vlans are
effected.
Switch(config)# spanning-tree portfast bpduguard default
Switch(config-if)# spanning-tree bpduguard enable
Explain Sudden Loss
of BPDUs?
Suppose switch does not receive BPDUs, it will think that
the topology must have changed, so blocked ports can be unblocked again. What
if the absence of BPDUs is actually a mistake and not a topology change,
bridging loops easily can form.
There are two features that help detect or prevent the
unexpected loss of BPDUs:-
1. Loop Guard
2. Unidirectional Link Detection (UDLD)
What is Loop Guard?
Loop Guard keeps track of the BPDU activity on
non-designated ports. It does not allow non-designated ports to become
designated ports in case of sudden loss of BPDUs. While BPDUs are received, the
port is allowed to behave normally. When BPDUs go missing, Loop Guard moves the
port into the loop-inconsistent state (port is effectively blocking at this
point to prevent a loop from forming and to keep it in the non-designated
role). When BPDUs are received on the port again, Loop Guard allows the port to
move through the normal STP states and become active.
It can be enabled on both interface & global level. It
affects per vlan basis.
Switch(config)# spanning-tree loopguard default
Switch(config-if)# spanning-tree guard loop
What is BPDU Filter?
STP runs on a switch to prevent loops. However, in special cases
when we need to prevent BPDUs from being sent or processed on one or more
switch ports, we can use BPDU filtering to effectively disable STP on those
ports. It prevents port from sending and receiving BPDUs. It can be enabled on
both interface & global level.
Switch(config)# spanning-tree portfast bpdufilter default
Switch(config-if)# spanning-tree bpdufilter { enable |
disable }
What is the
difference between BPDU Guard and BPDU Filter?
BPDU Guard works aggresively and puts the port in
error-disable state while BPDU filter does not shut the port, it only filters
BPDU. BPDU Guard only prevents receiving BPDUs while BPDU filter prevents both
sending and receiving BPDU.
If Both BPDU Guard and BPDU Filter is enabled on a port than
only BPDU filter will work.
Comments
Post a Comment