Original subtitles

1

So let’s look at Rapid Spanning Tree protocol or 802.1W

2

this is an evolution of the IEEE 802.1D standard

3

however, Rapid Spanning Tree provides rapid failover and convergence times.

4

The big difference that you need to remember here

5

is that Rapid Spanning Tree is not based on timers like 802.1D

6

so it offers an improvement over the 30 seconds interval or longer

7

that 802.1D takes to move a port to the forwarding state

8

What Rapid Spanning Tree does is it uses a bridge to bridge handset mechanism

9

which allows ports to move directly to forwarding

10

rather than waiting for the port to move from listening to learning to forwarding.

11

It is backward compatible with 802.1D

12

is transparent to end users and is standard based

13

but it does introduce some enhancements

14

including new port role assignments and ports states

15

a new BPDU format and BPDU processing, a bridge to bridge handshake mechanism

16

and different topology change notifications and processing procedures.

17

So what are port states and port roles in 802.1D and Rapid Spanning Tree?

18

There are only 3 ports states in Rapid Spanning Tree

19

learning, forwarding, and discarding

20

in 802.1D we have disabled, blocking and listening

21

and this has been merged into the discarding state.

22

So when you administratively disable a port

23

that’s called disabled in 802.1D

24

but it's called discarding in 802.1W or Rapid Spanning Tree.

25

A blocking port that does not forward user data frames

26

and ignores incoming data frames is called discarding in 802.1W

27

A listening port is not used in 802.1W

28

a learning port is known as a learning port

29

and a forwarding port is known as forwarding port in 802.1W

30

So we have learning, forwarding and discarding

31

disabled, blocking and listening

32

have been merged into the discarding state in 802.1W

33

Cisco still uses the term blocking for discarding

34

so just see those terms as interchangeable terms

35

blocking is discarding and discarding is blocking.

36

So what about Port Roles?

37

The role is now a variable assigned to a given port.

38

Previously we had root ports and designated ports and those remain

39

but blocking ports are now split

40

into what are called back up and alternate port roles.

41

Spanning tree will determine the role of the port

42

by looking at the BPDUs received

43

and deciding which one is more useful than another.

44

A more useful BPDU is a BPDU that has a lower path cost

45

or a better path to get to the root bridge.

46

So let’s start with a root port with the Spanning Tree Protocol

47

the Spanning Tree algorithm elects a single root bridge

48

for the entire bridged network

49

now with PVST that stands on a per-VLAN basis

50

but in 802.1D or Rapid Spanning Tree

51

there’s only 1 root bridge or root switch for the entire layer 2 topology.

52

The root bridge send BPDUs that are more useful

53

than the one sent by any other bridge.

54

The port receiving the best BPDU on a bridge or switch is known as the root port.

55

In other words, this is the port

56

that is closest to the root bridge in terms of path cost.

57

So in this topology, this switch is the root switch.

58

This port would be the root port of switch A

59

and this would be the root port of switch B.

60

the root bridge doesn’t have a root port.

61

All other bridges have at least one root port.

62

What is a designated port?

63

This is the best port on a segment to use to get to the root bridge.

64

So all bridges connected to a given segment

65

listen to each other’s BPDUs and agree on the bridge

66

sending the best BPDU as the designated bridge for the segment.

67

So in this topology, this switches the root

68

so for this segment, this is the designated port

69

or best port to use to get to the root bridge.

70

On this segment, this is the best port to use to get to the root bridge

71

so this is the designated port.

72

Once again, just imagine that you’ve got a PC

73

connected to the middle of this cable

74

which is the best way to get to the root bridge?

75

This way or this way?

76

and as we can see, this is the best path or best way to get to the root bridge

77

so this is the root port it's much quicker to go this way

78

than it is to go this way. so this is the designated port.

79

on this segment, let’s assume that we’ve got a hub connected here

80

this port has been chosen as the designated port

81

and that maybe because the switch here has a lower bridge ID in switch A

82

and this is port 1 which is lower than port 2

83

so this is the designated port on that segment.

84

Now what about alternate and backup port roles?

85

This corresponds to the blocking state in 802.1D

86

A blocked port is defined as any port that is not a designated or root port.

87

A port remains blocked as long as it receives more useful

88

in other words better BPDUs than the one it would send out on the segment.

89

Therefore port must receive BPDUs in order to stay blocked

90

if it doesn’t receive BPDUs it will transition to the forwarding state.

91

So in Rapid Spanning Tree there are 2 types of blocked ports

92

an alternate port is a port that is blocked

93

because it's receiving more useful BPDUs from another bridge on the segment.

94

So in this example, this port is the designated port

95

on let say switch B, on switch A

96

this port is an alternate port because more useful or better.

97

BPDUs are being received on this segment from switch B then from switch A

98

and that maybe because the priority of switch B is lower than the priority of switch A.

99

a backup port is a port that is blocked

100

because it's receiving more useful BPDUs from the same bridge that it's on.

101

so in this example were assuming that this port, this port

102

and this port are connected to a hub

103

this port a becomes the backup port its connected to the same switch

104

as this designated ports but it may be a higher port number

105

therefore it becomes a backup port.