Original subtitles

1

On point-to-point links, a proposal agreement handshake sequence is used

2

by Rapid Spanning Tree to achieve fast convergence.

3

So Rapid Spanning Tree uses this to quickly transition ports to the forwarding state

4

where as 802.1D was just waiting for timers to expire

5

before ports where set to the forwarding state.

6

In this topology, if the link between the root switch on port 0

7

and port 1, on this switch, came up they would send proposals to each other.

8

Both ports are put into designated blocking

9

and they both send a Rapid Spanning Tree BPDU with the proposal bit set

10

Now, these rely on a new BPDU format which contains a proposal bit in the BPDU.

11

So additional information in a Rapid Spanning Tree BPDU has been added

12

we have the proposal bit, we have the port role

13

we have we have learning forwarding and agreement bits

14

as well as a topology change bit and topology change acknowledgment bit

15

that are part of the Rapid Spanning Tree BPDU.

16

So essentially what happens is when the port comes up

17

the switches sent proposal to each other

18

saying I want to have the designated port on this segment.

19

However, because bridge A, this switch here

20

receives the BPDU indicating a superior path cost

21

in other words, a better path is found via the root switch then itself

22

it immediately knows that port 1 is gonna be its new root port.

23

So it knows right away that this port should be its root port

24

because this port has the best path back to the root switch.

25

In other words a superior or better or lower path cost.

26

Bridge A, in other words, this switch start a sync

27

to ensure that all of its ports are in sync with this new information.

28

A port is in sync if it meets the following criteria

29

it is in the blocking state, in other words, its discarding ports or it's an edge port.

30

So in this example, port 2 is an alternate port

31

port 3 is a designated port and port 4 is an edge port.

32

So port 2 and port 4 already meet one of the criteria listed above.

33

It’s in the blocking state or it's an edge port.

34

So to be in the sync, bridge A must block port 3 and assigns it to the discarding state.

35

Now that all ports are in sync

36

bridge A can unblock its newly selected root port

37

and reply to the root with an agreement message.

38

So basically a proposal is sent from the root to switch A.

39

Switch A agrees that this is the best path back to the root

40

it blocks all ports or make sure that they are in sync

41

because they're edge ports and then sends back an agreement.

42

As soon as that happens the port can be unblocked.

43

So bridge A can unblock its port and replies with an agreement message

44

once the root switch receives that agreement message

45

it can transition immediately to the forwarding state.

46

So proposal, agreement, unblock port

47

in other words, set it to the forwarding state.

48

Now that happens very, very quickly.

49

Now by the same token, switch A can send the proposal to this switch.

50

So this process continues, proposal to this switch.

51

The switch sends back an agreement

52

this port can go to the forwarding state

53

this happens very quickly because it doesn’t rely on timers.

54

This way of handshakes propagates quickly towards the edge of the network

55

and quickly restores connectivity after a change in the topology.

56

If a designated discarding port does not receive an agreement

57

to its proposal, it slowly transitions to the forwarding state

58

using the traditional 802.1D listening, learning sequence.

59

This could happen if the remote bridge doesn’t understand

60

Rapid Spanning Tree BPDUs or if the remote bridge port is blocking.

61

So in this topology at the moment

62

sh spanning-tree g0/1

63

is a designated port in the forwarding state but it's shared port.

64

In this topology, g0/0 has been shutdown

65

so the root port on switch 3 is g0/1

66

and the alternate port is g0/2

67

but if i no shut g0/0

68

so I’ve enabled this port again.

69

sh spanning-tree notice g0/0 is the root port of switch 3.

70

This port is now the designated port but is in a blocking state.

71

0/2 is in the blocking state backup port.

72

On switch 2 the status has changed to blocking

73

because in this segment this is the best port to use to get back to root bridge.

74

However, it takes longer for this process to complete

75

than it would be if it was point-to-point link

76

shared ports take longer to converge.

77

so let’s do that again, I'll shut the port.

78

So gigabit 0/0 is now gonna be shutdown.

79

Previously gigabit 0/1 on this switch was the alternate port.

80

Now it’s the designated port

81

but it's blocking, still blocking, sh spanning-tree on switch 3.

82

Notice alternate port is blocking

83

root port is forwarding, now the port is in the learning state.

84

So traffic is still being dropped on this port

85

even though this is the port to use to get to the root bridge.

86

Now it’s forwarding, can take 30 seconds on a shared ports

87

because we’re using traditional 802.1D, listening, learning sequences on that port.

88

To use proposals and agreements the ports need to be configured

89

as point-to-point ports or need to negotiate to use full duplex.