In this tutorial I will
explain Spanning Tree Protocol (STP) with examples. For backup purpose we
usually create redundant links. Redundant links are extremely useful in preventing
entire network down situation that is triggered from single link failure. With
all of its advantages, redundant link is a double edged sword. It creates
network loops that flood down frames. STP removes loops as well as other evils
of redundant link.
Layer 2 loop example
Above figure illustrates a
simple layer 2 loop. In this network, LAN segment has two links to server.
Ø Primary
/ Main Link (S1, S2, S3, S6)
Ø Redundant
/ Backup Link (S1, S4, S5, S6)
Multiple layer 2
connections between LAN segment and server create following problems
ü Broadcast
Storms
ü Duplicate
Frame copies
ü Unstable
MAC Table
Before we dig these
problems in more detail remember that a switch always floods three kinds of
frames: unknown unicast, broadcast and multicast.
Broadcast Storms
Assume that PC0 perform an
ARP request to find the MAC address of Server. ARP (Address Resolution
Protocol) uses broadcast method to locate the MAC address of device.
In this circumstance PC0
will generate single broadcast frame. Switch S1 will receive it from PC0. Switch
S1 will flood this broadcast frame from all remaining ports except the incoming
port.
Without any loop removing mechanism, switches will flood broadcasts
endlessly throughout the network. This is known as broadcast storm. Next figure
illustrates how a broadcast frame is continually being flooded throughout the
network.
Endless Cycle One
PC0 => S1 => S2
=> S3 => S6 => (Server and) S5 => S4 => S1 => (PC0 and) S2
=> S3 …………..
Endless Cycle Two
PC0 => S1 => S4
=> S5 => S6 => (Server and) S3 => S2 => S1 => (PC0 and) S4
=> S5 …………..
Duplicate frame copies
In
looped network, a device could receive duplicate copies of same frame from
different switches. Assume that PC0 sends a unicast frame to Server. Switch
floods unknown unicast from all of its ports, except the incoming port. Above
figure demonstrates how the Server will receive duplicate copies of unicast
frame simultaneously. Duplicate frame copies create additional overhead on the
network.
Unstable MAC Table
When
switch receives a frame, it checks source MAC address in frame and associate
that interface with finding MAC address. Next time when switch receives a frame
for this MAC address, it will forward that frame from this interface. These
entries are stored in MAC Address Table. Switch uses MAC Address Table to
forward the frame. Looped network can make
MAC Address Table unstable. For example, assume that PC0 sends a unicast frame
to Server. Switch S6 receives this frame from two interfaces (interface
connected to S3 and interface connected to S5). When it receives frame from S3,
it associate PC0 MAC address with the interface that is connected with S3.
Again when it receives same frame from S5, it think that location of PC0 has
been changed and it would update the entry in MAC address table.
Same
thing will happen again when it receives frame from switch S3 again. MAC
address table would be totally confused about the PC0 location because the
switch S6 is receiving the PC0’s frame from more than one link. Situation
becomes bad to worse when switch stuck in constantly updating the MAC Address
Table with source locations and fail to forward the frame. This is known as
thrashing the MAC Table.
STP
is the answer of all issues explained above. STP was developed to solve each
problem that is trigged due to layer two loops. Before we explain how the STP works,
you need to be familiar with some basic terms of STP and their function within
STP.
BPDU
BPDU
(Bridge Protocol Data Unit) is a multicast frame that is used to share
information about switch and its interface connections. Switches use BPDU to
learn the network topology; other switches connections and any existing loops.
BPDU frames are sent out as multicast in every two seconds.
Root Bridge
All
decisions in STP are made from the perspective of Root Bridge. Switch with the lowest switch ID is selected as Root
Bridge. BPDU contains Switch ID. Switch ID is made from
priority of the switch and MAC address of switch itself. Default priority is
set to 32768. Switch with the lowest MAC address will be selected as the root
switch, if you don’t change the default priority value. You can override root selection process by
changing the priority value. If you want one switch to be Root Bridge, change
its priority value to less than 32768.
Selection
process of Root Bridge runs each time a network change occurs like as adding
new switch in topology, removing existing switch or Root Bridge failure. If
other switches in network do not receive BPDUs from Root Bridge within 20
seconds, they assume that Root Bridge has failed and will begin a new election
process to choose a new Root Bridge.
Non-Root Bridge
All
other switches in network except Root Bridge are the non-root bridges. Non-root
Bridge receives update from Root Bridge and updates its STP database.
Port Costs
STP assigns each port a cost, called port cost. Port cost is used
to choose the best path when multiple links are available between two switches.
Cost of port is determined by the bandwidth of connected media link. Switch
always use lower port cost to forward the frames. Two set of port costs exist.
|
Bandwidth
|
Old Cost
Value
|
New Cost
Value
|
|
10 Gbps
|
1
|
2
|
|
1 Gbps
|
1
|
4
|
|
100 Mbps
|
10
|
19
|
|
10
Mbps
|
100
|
100
|
Some old series switches like Catalyst 1900 uses old cost value. Cisco
already discontinued these old series switches. New series switches like 2960
uses new cost value. Lower cost value is always preferred over higher cost
value while selecting link.
Path
Costs
Path cost is an accumulated value of port costs from Root Bridge
to other switches in network. It is always calculated from Root Bridge. Default
path cost at Root Bridge is 0. BPDU contains path cost information. When Root
Bridge advertises BPDU out from its interface, it set path costs to 0. When
connected switch receives this BPDU, it increments path cost by adding the port
cost value of its incoming port. For example if switch receives this BPDU on
Gigabit interface then path cost would be 0 (Value it receive from Root Bridge)
+ 4 (port cost value, see above table) equal to 4. Now this switch will set
path cost value to 4 in BPDU frame and forward it. Assume that next switch is
connected with this switch and receives updated BPDU on fast Ethernet port.
Path cost for new switch would be 23. Path costs value received in BPDU + Port
cost (4+ 19 = 23).
Root
Port
Root port is a port that is directly connected with the Root
Bridge, or has the shortest path to the Root Bridge. Shortest path is path that
has lowest path cost value. Remember that switch can go through many other
switches to get the root. So it’s not always the shortest path but it is the
fastest path that will be used.
Designated
Port
Designated port is the port that is selected as having the lowest
port cost. Designated port would be marked as forwarding port.
Non-Designated
Port
Non-designated port is the port that is selected as having the
higher port cost than the designated port. Non-designated port would be marked
as blocking port.
Forwarding
Port
Forwarding port is used to forward the frames.
Blocking
Port
Blocking port remains disable to remove loops.
STP port
states
Ports on switch running STP go through the five different states. During
STP convergence, switches will move their root and designated ports
through the various states: blocking, listening, learning, and forwarding,
whereas any other ports will remain in a blocked state.
Blocking :- In blocking state,
switch only listen and process BPDUs on its ports. Any other frames except BPDUs
are dropped. In this state, switch try to find out which port would be root
port, which ports would be designated ports and which ports would remains in blocking state to remove loops. A
port will remain in this state for twenty seconds. By default all ports are in
blocking state, when we powered on the switch. Only root port and designated
ports will move into next state. All remaining ports will remain in this state.
Listening :- After twenty seconds,
root port and designated ports will move into listening state. In this state
ports still listen and process only BPDUs. All other frames except BPDUs are
dropped. In this state switch will double check the layer 2 topology to make
sure that no loops occur on the network before processing data frames. Ports
remain in this state for fifteen seconds.
Learning :- Root port and designated ports enter in learning state from
listening state. In this state ports still listen and process BPDUs. However,
in this state ports start processing user frames. Switch examines source
address in the frames and updates its MAC Address Table. Switch will not
forward user frames to destination ports in this state. Ports stay in this
state for fifteen seconds.
Forwarding :- In forwarding state, ports will listen and process BPDUs. In
this state ports will also process user frames, update MAC Address Table and
forward user traffic through the ports.
Disable :- Disable ports are manually shut down or removed from STP by an
administrator. All unplugged ports also remain in disable state. Disable ports
do not participate in STP.
Convergence
Convergence is a state where all ports on switch have transitioned
to either forwarding or blocking modes. During the STP converging, all user
data frames would be dropped. No user
data frame will be forwarded until convergence is complete. Usually convergence takes place in fifty
seconds (20 seconds of blocking state + 15 seconds of listing state + 15
seconds of learning state).
STP Operations
First thing a STP enable network do, is the election of Root
Bridge. Switches share BPDUs with each
other to select the Root Bridge. Switch that has lowest priority will become
root. Default priority is set to 32768.
If priority value is same then switch with lowest MAC address would be selected
as root. In our network switch S3 has lowest MAC address. Since we did not
change priority value, switch S3 would be chosen as Root Bridge.
ü Every switch selects single port (that has shortest path cost)
from all its ports and marked it as root port.
ü If two switches have multiple connections, only single connection
that has shortest path cost would be marked as designated port.
ü Any port that is not either a root port or designated port would
be blocked.
Next figure show, how STP changes a
physically looped topology in virtually looped free topology.
show spanning-tree command
show spanning-tree command provides useful information about STP
operation. This information could be divided in three subsets. First set
contains information about Root Bridge. Second set contains information about
switch itself. Third set lists active interfaces status those are participating
in STP operation.
On Root Bridge first and second subset information would be same.
On non-root bridge first subset shows information about Root
Bridge while second subset shows information about switch itself.
Priority value is a sum of default priority value and VLAN ID.
That’s all for this article. In next article I will explain
another CCNA topic in detail with examples.
