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These are the user uploaded subtitles that are being translated: 1 00:00:00,000 --> 00:00:06,000 align:middle line:84% On point-to-point links, a proposal agreement handshake sequence is used 2 00:00:06,000 --> 00:00:09,000 align:middle line:84% by Rapid Spanning Tree to achieve fast convergence. 3 00:00:09,000 --> 00:00:15,000 align:middle line:84% So Rapid Spanning Tree uses this to quickly transition ports to the forwarding state 4 00:00:15,000 --> 00:00:20,000 align:middle line:84% where as 802.1D was just waiting for timers to expire 5 00:00:20,000 --> 00:00:24,000 align:middle line:84% before ports where set to the forwarding state. 6 00:00:24,000 --> 00:00:27,000 align:middle line:84% In this topology, if the link between the root switch on port 0 7 00:00:27,000 --> 00:00:35,000 align:middle line:84% and port 1, on this switch, came up they would send proposals to each other. 8 00:00:35,000 --> 00:00:37,000 align:middle line:84% Both ports are put into designated blocking 9 00:00:37,000 --> 00:00:44,000 align:middle line:84% and they both send a Rapid Spanning Tree BPDU with the proposal bit set 10 00:00:44,000 --> 00:00:52,000 align:middle line:84% Now, these rely on a new BPDU format which contains a proposal bit in the BPDU. 11 00:00:52,000 --> 00:00:57,000 align:middle line:84% So additional information in a Rapid Spanning Tree BPDU has been added 12 00:00:57,000 --> 00:01:01,000 align:middle line:84% we have the proposal bit, we have the port role 13 00:01:01,000 --> 00:01:04,000 align:middle line:84% we have we have learning forwarding and agreement bits 14 00:01:04,000 --> 00:01:08,000 align:middle line:84% as well as a topology change bit and topology change acknowledgment bit 15 00:01:08,000 --> 00:01:12,000 align:middle line:84% that are part of the Rapid Spanning Tree BPDU. 16 00:01:12,000 --> 00:01:15,000 align:middle line:84% So essentially what happens is when the port comes up 17 00:01:15,000 --> 00:01:18,000 align:middle line:84% the switches sent proposal to each other 18 00:01:18,000 --> 00:01:23,000 align:middle line:84% saying I want to have the designated port on this segment. 19 00:01:23,000 --> 00:01:27,000 align:middle line:84% However, because bridge A, this switch here 20 00:01:27,000 --> 00:01:31,000 align:middle line:84% receives the BPDU indicating a superior path cost 21 00:01:31,000 --> 00:01:37,000 align:middle line:84% in other words, a better path is found via the root switch then itself 22 00:01:37,000 --> 00:01:42,000 align:middle line:84% it immediately knows that port 1 is gonna be its new root port. 23 00:01:42,000 --> 00:01:45,000 align:middle line:84% So it knows right away that this port should be its root port 24 00:01:45,000 --> 00:01:49,000 align:middle line:84% because this port has the best path back to the root switch. 25 00:01:49,000 --> 00:01:53,000 align:middle line:84% In other words a superior or better or lower path cost. 26 00:01:53,000 --> 00:01:58,000 align:middle line:84% Bridge A, in other words, this switch start a sync 27 00:01:58,000 --> 00:02:01,000 align:middle line:84% to ensure that all of its ports are in sync with this new information. 28 00:02:01,000 --> 00:02:05,000 align:middle line:84% A port is in sync if it meets the following criteria 29 00:02:05,000 --> 00:02:09,000 align:middle line:84% it is in the blocking state, in other words, its discarding ports or it's an edge port. 30 00:02:09,000 --> 00:02:13,000 align:middle line:84% So in this example, port 2 is an alternate port 31 00:02:13,000 --> 00:02:18,000 align:middle line:84% port 3 is a designated port and port 4 is an edge port. 32 00:02:18,000 --> 00:02:25,000 align:middle line:84% So port 2 and port 4 already meet one of the criteria listed above. 33 00:02:25,000 --> 00:02:28,000 align:middle line:84% It’s in the blocking state or it's an edge port. 34 00:02:28,000 --> 00:02:35,000 align:middle line:90% So to be in the sync, bridge A must block port 3 and assigns it to the discarding state. 35 00:02:35,000 --> 00:02:37,000 align:middle line:84% Now that all ports are in sync 36 00:02:37,000 --> 00:02:41,000 align:middle line:84% bridge A can unblock its newly selected root port 37 00:02:41,000 --> 00:02:44,000 align:middle line:84% and reply to the root with an agreement message. 38 00:02:44,000 --> 00:02:49,000 align:middle line:84% So basically a proposal is sent from the root to switch A. 39 00:02:49,000 --> 00:02:53,000 align:middle line:84% Switch A agrees that this is the best path back to the root 40 00:02:53,000 --> 00:02:57,000 align:middle line:84% it blocks all ports or make sure that they are in sync 41 00:02:57,000 --> 00:03:00,000 align:middle line:84% because they're edge ports and then sends back an agreement. 42 00:03:00,000 --> 00:03:03,000 align:middle line:84% As soon as that happens the port can be unblocked. 43 00:03:03,000 --> 00:03:08,000 align:middle line:84% So bridge A can unblock its port and replies with an agreement message 44 00:03:08,000 --> 00:03:12,000 align:middle line:84% once the root switch receives that agreement message 45 00:03:12,000 --> 00:03:15,000 align:middle line:84% it can transition immediately to the forwarding state. 46 00:03:15,000 --> 00:03:19,000 align:middle line:84% So proposal, agreement, unblock port 47 00:03:19,000 --> 00:03:21,000 align:middle line:84% in other words, set it to the forwarding state. 48 00:03:21,000 --> 00:03:24,000 align:middle line:84% Now that happens very, very quickly. 49 00:03:24,000 --> 00:03:27,000 align:middle line:84% Now by the same token, switch A can send the proposal to this switch. 50 00:03:27,000 --> 00:03:32,000 align:middle line:84% So this process continues, proposal to this switch. 51 00:03:32,000 --> 00:03:34,000 align:middle line:84% The switch sends back an agreement 52 00:03:34,000 --> 00:03:36,000 align:middle line:84% this port can go to the forwarding state 53 00:03:36,000 --> 00:03:40,000 align:middle line:84% this happens very quickly because it doesn’t rely on timers. 54 00:03:40,000 --> 00:03:45,000 align:middle line:84% This way of handshakes propagates quickly towards the edge of the network 55 00:03:45,000 --> 00:03:49,000 align:middle line:84% and quickly restores connectivity after a change in the topology. 56 00:03:49,000 --> 00:03:53,000 align:middle line:84% If a designated discarding port does not receive an agreement 57 00:03:53,000 --> 00:03:57,000 align:middle line:84% to its proposal, it slowly transitions to the forwarding state 58 00:03:57,000 --> 00:04:02,000 align:middle line:84% using the traditional 802.1D listening, learning sequence. 59 00:04:02,000 --> 00:04:05,000 align:middle line:84% This could happen if the remote bridge doesn’t understand 60 00:04:05,000 --> 00:04:11,000 align:middle line:84% Rapid Spanning Tree BPDUs or if the remote bridge port is blocking. 61 00:04:11,000 --> 00:04:13,000 align:middle line:84% So in this topology at the moment 62 00:04:13,000 --> 00:04:19,000 align:middle line:84% sh spanning-tree g0/1 63 00:04:19,000 --> 00:04:23,000 align:middle line:84% is a designated port in the forwarding state but it's shared port. 64 00:04:23,000 --> 00:04:27,000 align:middle line:84% In this topology, g0/0 has been shutdown 65 00:04:27,000 --> 00:04:31,000 align:middle line:84% so the root port on switch 3 is g0/1 66 00:04:31,000 --> 00:04:34,000 align:middle line:84% and the alternate port is g0/2 67 00:04:34,000 --> 00:04:38,000 align:middle line:84% but if i no shut g0/0 68 00:04:38,000 --> 00:04:41,000 align:middle line:84% so I’ve enabled this port again. 69 00:04:41,000 --> 00:04:50,000 align:middle line:84% sh spanning-tree notice g0/0 is the root port of switch 3. 70 00:04:50,000 --> 00:04:53,000 align:middle line:84% This port is now the designated port but is in a blocking state. 71 00:04:53,000 --> 00:04:57,000 align:middle line:84% 0/2 is in the blocking state backup port. 72 00:04:57,000 --> 00:05:04,000 align:middle line:84% On switch 2 the status has changed to blocking 73 00:05:04,000 --> 00:05:09,000 align:middle line:84% because in this segment this is the best port to use to get back to root bridge. 74 00:05:09,000 --> 00:05:13,000 align:middle line:84% However, it takes longer for this process to complete 75 00:05:13,000 --> 00:05:16,000 align:middle line:84% than it would be if it was point-to-point link 76 00:05:16,000 --> 00:05:18,000 align:middle line:84% shared ports take longer to converge. 77 00:05:18,000 --> 00:05:22,000 align:middle line:84% so let’s do that again, I'll shut the port. 78 00:05:22,000 --> 00:05:25,000 align:middle line:84% So gigabit 0/0 is now gonna be shutdown. 79 00:05:25,000 --> 00:05:31,000 align:middle line:84% Previously gigabit 0/1 on this switch was the alternate port. 80 00:05:31,000 --> 00:05:33,000 align:middle line:84% Now it’s the designated port 81 00:05:33,000 --> 00:05:42,000 align:middle line:84% but it's blocking, still blocking, sh spanning-tree on switch 3. 82 00:05:42,000 --> 00:05:44,000 align:middle line:84% Notice alternate port is blocking 83 00:05:44,000 --> 00:05:50,000 align:middle line:84% root port is forwarding, now the port is in the learning state. 84 00:05:50,000 --> 00:05:52,000 align:middle line:84% So traffic is still being dropped on this port 85 00:05:52,000 --> 00:05:55,000 align:middle line:84% even though this is the port to use to get to the root bridge. 86 00:05:55,000 --> 00:06:00,000 align:middle line:84% Now it’s forwarding, can take 30 seconds on a shared ports 87 00:06:00,000 --> 00:06:07,000 align:middle line:84% because we’re using traditional 802.1D, listening, learning sequences on that port. 88 00:06:07,000 --> 00:06:14,000 align:middle line:84% To use proposals and agreements the ports need to be configured 89 00:06:14,000 --> 00:06:19,000 align:middle line:84% as point-to-point ports or need to negotiate to use full duplex. 10647