1
00:00:00,690 --> 00:00:03,480
On point to point links a proposal agreement.

2
00:00:03,510 --> 00:00:08,910
Handshake sequence is used by rapid spending tree to achieve foster convergence.

3
00:00:09,420 --> 00:00:16,100
So rapid spanning tree uses this to quickly transition ports to the forwarding state, whereas 82 to

4
00:00:16,100 --> 00:00:22,950
1 D was just waiting for timers to expire before ports were set to the forwarding state.

5
00:00:23,100 --> 00:00:30,780
In this topology, if the link between the route switch on Port zero and Port one on the switch came

6
00:00:30,780 --> 00:00:34,110
up, they would send proposals to each other.

7
00:00:34,320 --> 00:00:42,420
Both ports are put into designated blocking and they both send a rapid spanning tree BPD with a proposal

8
00:00:42,420 --> 00:00:43,380
but set.

9
00:00:43,620 --> 00:00:51,690
Now this relies on a new BPU format which contains a proposal, but in the BPD view.

10
00:00:51,720 --> 00:00:56,550
So additional information in a rapid spanning tree BPU has been added.

11
00:00:56,850 --> 00:01:00,570
We have the proposal, but we have the port role.

12
00:01:00,600 --> 00:01:06,540
We have learning, forwarding and agreement bits as well as a topology change bit and topology change

13
00:01:06,540 --> 00:01:11,400
acknowledgement, but that are part of the rapid spanning tree BPU.

14
00:01:11,880 --> 00:01:17,880
So essentially what happens is when the port comes up, the switches send proposals to each other saying

15
00:01:17,880 --> 00:01:22,590
I want to have the designated port on the segment.

16
00:01:22,860 --> 00:01:30,780
However, because bridge, this switch here receives a BPU indicating a superior path cost.

17
00:01:30,900 --> 00:01:36,570
In other words, a better path is found via the route switch then itself.

18
00:01:37,670 --> 00:01:41,240
It immediately knows that port one is going to be its new route port.

19
00:01:41,240 --> 00:01:46,850
So it know straight away that this port should be its root port because this port has the best path

20
00:01:46,850 --> 00:01:48,360
back to the route switch.

21
00:01:48,380 --> 00:01:52,910
In other words, a superior or better or lower path cost.

22
00:01:53,930 --> 00:01:54,410
Bridge.

23
00:01:54,410 --> 00:02:00,050
A In other words, the switch starts a sync to ensure that all of its ports are in sync with this new

24
00:02:00,050 --> 00:02:00,860
information.

25
00:02:01,130 --> 00:02:04,310
A port is in sync if it meets the following criteria.

26
00:02:04,520 --> 00:02:05,930
It is in the blocking state.

27
00:02:05,930 --> 00:02:08,900
In other words, it's discarding or it's an edge port.

28
00:02:09,520 --> 00:02:13,180
So in this example, Port two is an alternate port.

29
00:02:13,210 --> 00:02:17,130
Port three is a designated port and port four is a edge.

30
00:02:17,140 --> 00:02:24,520
Port support two and port four already meet one of the criteria listed above.

31
00:02:24,640 --> 00:02:32,560
It's in the blocking state or it's an edge port so to be in sync bridge a must block port three and

32
00:02:32,560 --> 00:02:34,630
assigns it to the discarding state.

33
00:02:34,900 --> 00:02:41,590
Now that all ports are in sync bridge a can unblock its newly selected route port and reply to the route

34
00:02:41,590 --> 00:02:43,030
with an agreement message.

35
00:02:43,830 --> 00:02:49,060
So basically a proposal is sent from the root to switch a switch.

36
00:02:49,080 --> 00:02:52,920
A agrees that this is the best path back to the root.

37
00:02:53,130 --> 00:02:59,130
It blocks all ports or make sure that they are in sync because they edge ports and then sends back an

38
00:02:59,130 --> 00:02:59,920
agreement.

39
00:02:59,940 --> 00:03:02,910
As soon as that happens, the ports can be unblocked.

40
00:03:03,360 --> 00:03:07,680
So Bridge can unblock its port and replies with an agreement message.

41
00:03:08,190 --> 00:03:13,740
Once the root switch receives that agreement message, it can transition immediately to the forwarding

42
00:03:13,740 --> 00:03:14,250
state.

43
00:03:14,550 --> 00:03:18,240
So proposal, agreement, unblock port.

44
00:03:18,240 --> 00:03:20,820
In other words, set it to the forwarding state.

45
00:03:21,150 --> 00:03:23,280
Now that happens very, very quickly.

46
00:03:23,430 --> 00:03:27,030
Now, by the same token, switch a can send a proposal to the switch.

47
00:03:28,850 --> 00:03:30,380
So this process continues.

48
00:03:30,470 --> 00:03:31,980
Proposal to the switch.

49
00:03:32,000 --> 00:03:33,780
The switch sends back an agreement.

50
00:03:33,800 --> 00:03:35,810
The support can go to the forwarding state.

51
00:03:36,050 --> 00:03:39,590
This happens very quickly because it doesn't rely on timers.

52
00:03:40,040 --> 00:03:46,430
This wave of handshakes propagates quickly towards the edge of the network and quickly restores connectivity

53
00:03:46,430 --> 00:03:48,170
after a change in the topology.

54
00:03:48,650 --> 00:03:55,700
If a designated discarding port does not receive an agreement to its proposal, it slowly transitions

55
00:03:55,700 --> 00:04:01,610
to the forwarding state using the traditional 8 to 1 D listening learning sequence.

56
00:04:02,120 --> 00:04:08,660
This could happen if the remote bridge doesn't understand rapid spending tree beeps or if the remote

57
00:04:08,660 --> 00:04:10,310
bridges port is blocking.

58
00:04:10,850 --> 00:04:14,630
So in this topology at the moment shows spanning tree.

59
00:04:16,290 --> 00:04:22,140
Gigabit zero one is a designated port in the forwarding state, but it's a shared port.

60
00:04:23,470 --> 00:04:26,620
In this topology, gigabit zero zero has been shut down.

61
00:04:26,710 --> 00:04:33,460
So the route put on switch three is gigabit zero one and the alternate port is gigabit zero two.

62
00:04:33,880 --> 00:04:37,420
But if I no shut gigabit zero zero.

63
00:04:38,740 --> 00:04:40,840
So I've enabled this port again.

64
00:04:42,840 --> 00:04:44,730
Shows spanning tree notice.

65
00:04:44,730 --> 00:04:49,260
Gigabit zero zero is the root port of switch three.

66
00:04:49,470 --> 00:04:53,040
This port is now the designated port but is in a blocking state.

67
00:04:53,310 --> 00:04:55,400
Zero two is in the blocking state.

68
00:04:55,410 --> 00:05:06,390
Back up port on switch to the status has changed to blocking because on this segment this is the best

69
00:05:06,390 --> 00:05:08,700
port to use to get back to the root bridge.

70
00:05:09,410 --> 00:05:15,740
However, it takes longer for this process to complete than it would be if it was a point to point link.

71
00:05:15,740 --> 00:05:17,990
Shade ports take longer to converge.

72
00:05:18,020 --> 00:05:19,580
So let's do that again.

73
00:05:19,610 --> 00:05:21,410
I'll shut the port.

74
00:05:22,070 --> 00:05:24,740
So gigabit to zero zero is now going to be shut down.

75
00:05:27,060 --> 00:05:30,450
Previously, Gigabit zero one in the switch was the alternate port.

76
00:05:30,810 --> 00:05:33,390
Now it's the designated port, but it's blocking.

77
00:05:34,550 --> 00:05:35,720
Still blocking.

78
00:05:38,490 --> 00:05:41,730
Show spending tree on switch three notice.

79
00:05:41,730 --> 00:05:44,700
Alternate port is blocking route port is forwarding.

80
00:05:47,340 --> 00:05:52,080
Now the port is in the learning state, so traffic is still being dropped on this port, even though

81
00:05:52,080 --> 00:05:54,510
this is the port to use to get to the route bridge.

82
00:05:56,070 --> 00:06:03,480
Now it's forwarding can take 30 seconds on a shared port because we're using traditional ETA 2 to 1

83
00:06:03,480 --> 00:06:06,930
DX listening learning sequences on that port.

84
00:06:08,800 --> 00:06:11,080
To use proposals and agreements.

85
00:06:11,080 --> 00:06:17,470
The ports need to be configured as point to point ports or need to negotiate to use full duplex.