Preventing
Loops with Spanning Tree Protocol
Switching loops
form because parallel switches are unaware of each other. STP was developed to
overcome the possibility of switching loops so that redundant switches and
switch paths could be used for their benefits. Basically, the protocol enables
switches to become aware of each other so they can negotiate a loop-free path
through the network.
STP is
communicated among all connected switches on a network. Each switch executes the
spanning-tree
algorithm based on information
received from other neighboring switches. The algorithm chooses a reference
point in the network and calculates all the redundant paths to that reference
point. When redundant paths are found, the spanning-tree algorithm picks one path by which to
forward frames and disables, or blocks, forwarding on the other redundant
paths.
As its name
implies, STP computes a tree structure that spans all switches in a subnet or
network. Redundant paths are placed in a Blocking or Standby state to prevent
frame forwarding. The switched network is then in a loop-free condition.
However, if a forwarding port fails or
becomes disconnected, the spanning-tree algorithm recomputes the spanning-tree
topology so that the appropriate blocked links can be reactivated.
Spanning-Tree Communication: Bridge Protocol Data Units
STP operates as
switches communicate with one another. Data
messages are exchanged in the form of bridge protocol data units (BPDUs). A switch sends a BPDU frame out a port,
using the unique MAC address of the port itself as a source address. The switch
is unaware of the other switches around it, so BPDU frames are sent with a destination address of the well-known STP
multicast address 01-80-c2-00-00-00.
Two types of BPDU exist:
■ Configuration BPDU, used for spanning-tree
computation
■ Topology Change Notification (TCN) BPDU, used to
announce changes in the network topology
The Configuration BPDU message contains
the fields are shown in Table 2.3.
Table 2.2 Configuration BPDU
Message Content
Field Description Number of Bytes
Protocol ID (always
0) 2
Version
1
Message Type
(Configuration or TCN BPDU) 1
Flags 1
Root Bridge ID 8
Root Path Cost 4
Sender Bridge ID 8
Port ID 2
Message age (in
256ths of a second) 2
Maximum age (in
256ths of a second) 2
Hello time (in
256ths of a second) 2
Forward delay (in
256ths of a second) 2
The exchange of
BPDU messages work toward the goal of electing reference points as a foundation
for a stable spanning-tree topology. Also, loops can be identified and removed by
placing specific redundant ports in a Blocking or Standby state. Notice that
several key fields in the BPDU are related to switch identification, path costs, and timer values. These all work together so that
the network of switches can converge on a common spanning-tree topology and
select the same reference points within the network.
By default, BPDUs are sent out all switch ports every
2 seconds so that current topology information is exchanged, and loops are
identified quickly.
“Next, we will discuss about Root Bridge Selection
Process”
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