Switching Loops
By
default, a switch will forward a broadcast or multicast out all ports, excluding
the port the broadcast/multicast was sent from.
When a loop
is introduced into the network, a highly destructive broadcast storm
can develop within seconds. Broadcast storms
occur when road casts are
endlessly switched through the loop, choking off all other traffic.Consider the following looped environment:
In the
above structure If the computer connected to Switch 4 sends out a broadcast,
the switch will forward the broadcast out all ports, including the ports
connecting to Switch 2 and
Switch 5. Those switches, likewise, will forward that broadcast out all ports,
including to their neighboring switches.
The
broadcast will loop around the switches infinitely. In fact, there will be two
separate broadcast storms cycling in opposite
directions through the switching
loop. Only powering off the switch or physically removing the loop will stop the storm.
So in this
situation we will use Spanning Tree Protocol (STP)
Switches
(and bridges) needed a mechanism to prevent loops from forming,and thus Spanning
Tree Protocol (STP, or IEEE 802.1D) was developed.
STP is enabled
by default on all VLANs on Catalyst switches.
STP-enabled
switches communicate to form a topology of the entire switching network, and
then shutting down (or blocking) a port if a loop exists. The blocked
port can be reactivated if another link on the switching network goes down,
thus preserving fault-tolerance. Once all switches agree on the topology
database, the switches are considered converged.
STP
switches send Bridge Protocol Data Units
to each other to form their topology databases. BPDU’s are sent out
all ports every two seconds, are
forwarded to a specific MAC multicast address: 0180.c202.0100
STP Types
Various
flavors of 802.1D STP exist, including:
• Common
Spanning Tree (CST) –A single STP process is used for all VLANs.
• Per-VLAN
Spanning Tree (PVST) – Cisco proprietary version of STP, which employs a
separate STP process for each VLAN.
• Per-VLAN
Spanning Tree Plus (PVST+) – Enhanced version of PVST that allows
CST-enabled switches and PVST-enabled switches to interoperate. This is default
on newer Catalyst switches.
The STP Process
To
maintain a loop-free environment, STP performs the following functions:
Ø A
Root Bridge is elected
Ø Root
Ports are identified
Ø Designated
Ports are identified
Ø If
a loop exists, a port is placed in Blocking state. If the loop is removed
the blocked port is activated again.
If
multiple loops exist in the switching environment, multiple ports will be placed in a blocking state.
Electing an STP Root Bridge
The first
step in the STP process is electing a Root Bridge, which serves as the
centralized point of the STP topology. Good design practice dictates that the
Root Bridge be placed closest to the center of the STP topology.
The Root
Bridge is determined by a switch’s priority. The default priority is 32,768,
and the lowest priority wins. In case of a tie in priority, the switch with
the lowest MAC address will be elected root bridge. The combination of a
switch’s priority and MAC address make up that switch’s Bridge ID.
Consider the following example:
Remember that the lowest priority
determines the Root Bridge. Switches 2,3, and 5 have the default priority
set. Switches 1 and 4 each have a priority of 100 configured. However, Switch 1
will become the root bridge, as it has the lowest MAC address.
Switches exchange BPDU’s to perform
the election process. By default, all switches “believe” they are the Root
Bridge, until a switch with a lower Bridge ID is discovered.
Root Bridge elections are a continuous
process. If a new switch with a lower Bridge ID is added to the topology, it
will be elected as the new Root Bridge.
Switch#
show spanning-tree interface Ge0/10
Interface
Fa0/10 in Spanning tree 5 is Forwarding
Port path
cost 150, Port priority 140
<snip>
Some useful and imp Basic STP Configuration
To disable
STP for a specific VLAN:
Switch(config)#
no spanning-tree vlan 10
To adjust
the Bridge Priority of a switch from its default of 32,768, to
increase
its chances of being elected Root Bridge of a VLAN:
Switch(config)#
spanning-tree vlan 10 priority 150
To change
an interface’s Path Cost from its defaults:
Switch(config)#
int fa0/24
Switch(config-if)#
spanning-tree cost 42
To force a
switch to become the Root Bridge:
Switch(config)#
spanning-tree vlan 10 root primary
The root
primary parameter in the above command automatically lowers the
switch’s
priority to 24,576. If another switch on the network has a lower
priority
than 24,576, the above command will lower the priority by 4096 less
than the
priority of the other switch.
It is
possible to assign a Secondary Root Bridge for redundancy. To force a
switch to
become a Secondary Root Bridge:
Switch(config)#
spanning-tree vlan 10 root secondary
The root
secondary parameter in the above command automatically lowers
the
switch’s priority to 28,672.
To specify
the diameter of the switching topology:
Switch(config)#
spanning-tree vlan 10 root primary diameter 7
The diameter
parameter in the preceding command indicates the length of
the STP
topology (number of switches). The maximum (and default) value
for the
diameter is 7. Note that the switching topology can contain more than
seven
switches; however, each branch of the switching tree can only
extend
seven
switches deep, from the Root Bridge.
The diameter
command will also adjust the Hello, Forward Delay, and Max
Age
timers. This is the recommended way to adjust timers, as the hello
timers are tuned specifically to the diameter
of the switching network.
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