Spanning-tree Protocol:
- STP stop the loops
which occurs when you have multiple links between switches
- STP stops avoiding
Broadcast Storms, Multiple Frame copies & Database instability
- STP is a open
standard (IEEE 802.1D)
- STP enabled by
default on all Cisco Catalyst Switches
How Spanning-tree
Protocol works:
- Selecting the Root
Bridge
- Selecting the Root
Port
- Selecting
Designated port and Non Designated port
01 Selecting the
Root Bridge
- The bridge with
the Best(Lowest) Bridge ID
- Bridge ID =
Priority + MAC address of the switch(least is best)
- Out of all the
switches in the network, one is elected as a root bridge that becomes
the focal point in the network
02 Selecting the
Root Port
- Shortest path to
the Root Bridge
- Every Non-root
bridge looks the best way to go Root Bridge
- For every non-root
bridge there is only one root port
- Least cost(Speed)
- Bridge-ID of
forwarding switch
- Least port of
forwarding switch
- Rot port selection
based on Cost(least cost(Speed))
10mbps(BW) –
100(Cost)
100mbps(BW) –
19(Cost)
1-Gigabit
Ethernet(BW) – 4(Cost)
10-Gigabit
Ethernet(BW) – 2(Cost)
03 Selecting
Designated port & Non Designated port
- Least cost(Speed)
- The least local
switch-ID
- Lowest local port
number
BPDU:
- All switches
exchange information through what is call Bridge Protocol Data
Units(BPDUs)
- BPDUs are sent
every 2 sec and dead in 20 sec
- A BPDU contains
information regarding ports, switches, port priority and addresses
Lab: