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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% So let’s look at Rapid Spanning Tree protocol or 802.1W 2 00:00:06,000 --> 00:00:11,000 align:middle line:84% this is an evolution of the IEEE 802.1D standard 3 00:00:11,000 --> 00:00:17,000 align:middle line:84% however, Rapid Spanning Tree provides rapid failover and convergence times. 4 00:00:17,000 --> 00:00:21,000 align:middle line:84% The big difference that you need to remember here 5 00:00:21,000 --> 00:00:26,000 align:middle line:84% is that Rapid Spanning Tree is not based on timers like 802.1D 6 00:00:26,000 --> 00:00:33,000 align:middle line:84% so it offers an improvement over the 30 seconds interval or longer 7 00:00:33,000 --> 00:00:38,000 align:middle line:84% that 802.1D takes to move a port to the forwarding state 8 00:00:38,000 --> 00:00:43,000 align:middle line:84% What Rapid Spanning Tree does is it uses a bridge to bridge handset mechanism 9 00:00:43,000 --> 00:00:46,000 align:middle line:84% which allows ports to move directly to forwarding 10 00:00:46,000 --> 00:00:51,000 align:middle line:84% rather than waiting for the port to move from listening to learning to forwarding. 11 00:00:51,000 --> 00:00:56,000 align:middle line:84% It is backward compatible with 802.1D 12 00:00:56,000 --> 00:01:00,000 align:middle line:84% is transparent to end users and is standard based 13 00:01:00,000 --> 00:01:03,000 align:middle line:84% but it does introduce some enhancements 14 00:01:03,000 --> 00:01:06,000 align:middle line:84% including new port role assignments and ports states 15 00:01:06,000 --> 00:01:13,000 align:middle line:84% a new BPDU format and BPDU processing, a bridge to bridge handshake mechanism 16 00:01:13,000 --> 00:01:19,000 align:middle line:84% and different topology change notifications and processing procedures. 17 00:01:19,000 --> 00:01:26,000 align:middle line:84% So what are port states and port roles in 802.1D and Rapid Spanning Tree? 18 00:01:26,000 --> 00:01:30,000 align:middle line:84% There are only 3 ports states in Rapid Spanning Tree 19 00:01:30,000 --> 00:01:34,000 align:middle line:84% learning, forwarding, and discarding 20 00:01:34,000 --> 00:01:38,000 align:middle line:84% in 802.1D we have disabled, blocking and listening 21 00:01:38,000 --> 00:01:41,000 align:middle line:84% and this has been merged into the discarding state. 22 00:01:41,000 --> 00:01:45,000 align:middle line:84% So when you administratively disable a port 23 00:01:45,000 --> 00:01:48,000 align:middle line:84% that’s called disabled in 802.1D 24 00:01:48,000 --> 00:01:52,000 align:middle line:84% but it's called discarding in 802.1W or Rapid Spanning Tree. 25 00:01:52,000 --> 00:01:57,000 align:middle line:84% A blocking port that does not forward user data frames 26 00:01:57,000 --> 00:02:02,000 align:middle line:84% and ignores incoming data frames is called discarding in 802.1W 27 00:02:02,000 --> 00:02:06,000 align:middle line:84% A listening port is not used in 802.1W 28 00:02:06,000 --> 00:02:09,000 align:middle line:84% a learning port is known as a learning port 29 00:02:09,000 --> 00:02:13,000 align:middle line:84% and a forwarding port is known as forwarding port in 802.1W 30 00:02:13,000 --> 00:02:17,000 align:middle line:84% So we have learning, forwarding and discarding 31 00:02:17,000 --> 00:02:20,000 align:middle line:84% disabled, blocking and listening 32 00:02:20,000 --> 00:02:25,000 align:middle line:84% have been merged into the discarding state in 802.1W 33 00:02:25,000 --> 00:02:29,000 align:middle line:84% Cisco still uses the term blocking for discarding 34 00:02:29,000 --> 00:02:32,000 align:middle line:84% so just see those terms as interchangeable terms 35 00:02:32,000 --> 00:02:36,000 align:middle line:84% blocking is discarding and discarding is blocking. 36 00:02:36,000 --> 00:02:38,000 align:middle line:84% So what about Port Roles? 37 00:02:38,000 --> 00:02:42,000 align:middle line:84% The role is now a variable assigned to a given port. 38 00:02:42,000 --> 00:02:47,000 align:middle line:84% Previously we had root ports and designated ports and those remain 39 00:02:47,000 --> 00:02:49,000 align:middle line:84% but blocking ports are now split 40 00:02:49,000 --> 00:02:52,000 align:middle line:84% into what are called back up and alternate port roles. 41 00:02:52,000 --> 00:02:55,000 align:middle line:84% Spanning tree will determine the role of the port 42 00:02:55,000 --> 00:02:58,000 align:middle line:84% by looking at the BPDUs received 43 00:02:58,000 --> 00:03:01,000 align:middle line:84% and deciding which one is more useful than another. 44 00:03:01,000 --> 00:03:06,000 align:middle line:84% A more useful BPDU is a BPDU that has a lower path cost 45 00:03:06,000 --> 00:03:09,000 align:middle line:84% or a better path to get to the root bridge. 46 00:03:09,000 --> 00:03:17,000 align:middle line:84% So let’s start with a root port with the Spanning Tree Protocol 47 00:03:17,000 --> 00:03:20,000 align:middle line:84% the Spanning Tree algorithm elects a single root bridge 48 00:03:20,000 --> 00:03:23,000 align:middle line:84% for the entire bridged network 49 00:03:23,000 --> 00:03:27,000 align:middle line:84% now with PVST that stands on a per-VLAN basis 50 00:03:27,000 --> 00:03:31,000 align:middle line:84% but in 802.1D or Rapid Spanning Tree 51 00:03:31,000 --> 00:03:36,000 align:middle line:84% there’s only 1 root bridge or root switch for the entire layer 2 topology. 52 00:03:36,000 --> 00:03:41,000 align:middle line:84% The root bridge send BPDUs that are more useful 53 00:03:41,000 --> 00:03:44,000 align:middle line:84% than the one sent by any other bridge. 54 00:03:44,000 --> 00:03:49,000 align:middle line:84% The port receiving the best BPDU on a bridge or switch is known as the root port. 55 00:03:49,000 --> 00:03:52,000 align:middle line:84% In other words, this is the port 56 00:03:52,000 --> 00:03:55,000 align:middle line:84% that is closest to the root bridge in terms of path cost. 57 00:03:55,000 --> 00:03:59,000 align:middle line:84% So in this topology, this switch is the root switch. 58 00:03:59,000 --> 00:04:02,000 align:middle line:84% This port would be the root port of switch A 59 00:04:02,000 --> 00:04:05,000 align:middle line:84% and this would be the root port of switch B. 60 00:04:05,000 --> 00:04:08,000 align:middle line:84% the root bridge doesn’t have a root port. 61 00:04:08,000 --> 00:04:11,000 align:middle line:84% All other bridges have at least one root port. 62 00:04:11,000 --> 00:04:15,000 align:middle line:84% What is a designated port? 63 00:04:15,000 --> 00:04:20,000 align:middle line:84% This is the best port on a segment to use to get to the root bridge. 64 00:04:20,000 --> 00:04:23,000 align:middle line:84% So all bridges connected to a given segment 65 00:04:23,000 --> 00:04:27,000 align:middle line:84% listen to each other’s BPDUs and agree on the bridge 66 00:04:27,000 --> 00:04:33,000 align:middle line:84% sending the best BPDU as the designated bridge for the segment. 67 00:04:33,000 --> 00:04:38,000 align:middle line:84% So in this topology, this switches the root 68 00:04:38,000 --> 00:04:41,000 align:middle line:84% so for this segment, this is the designated port 69 00:04:41,000 --> 00:04:44,000 align:middle line:84% or best port to use to get to the root bridge. 70 00:04:44,000 --> 00:04:48,000 align:middle line:84% On this segment, this is the best port to use to get to the root bridge 71 00:04:48,000 --> 00:04:50,000 align:middle line:84% so this is the designated port. 72 00:04:50,000 --> 00:04:53,000 align:middle line:84% Once again, just imagine that you’ve got a PC 73 00:04:53,000 --> 00:04:56,000 align:middle line:84% connected to the middle of this cable 74 00:04:56,000 --> 00:04:58,000 align:middle line:84% which is the best way to get to the root bridge? 75 00:04:58,000 --> 00:05:01,000 align:middle line:84% This way or this way? 76 00:05:01,000 --> 00:05:06,000 align:middle line:84% and as we can see, this is the best path or best way to get to the root bridge 77 00:05:06,000 --> 00:05:09,000 align:middle line:84% so this is the root port it's much quicker to go this way 78 00:05:09,000 --> 00:05:13,000 align:middle line:84% than it is to go this way. so this is the designated port. 79 00:05:13,000 --> 00:05:16,000 align:middle line:84% on this segment, let’s assume that we’ve got a hub connected here 80 00:05:16,000 --> 00:05:20,000 align:middle line:84% this port has been chosen as the designated port 81 00:05:20,000 --> 00:05:27,000 align:middle line:84% and that maybe because the switch here has a lower bridge ID in switch A 82 00:05:27,000 --> 00:05:30,000 align:middle line:84% and this is port 1 which is lower than port 2 83 00:05:30,000 --> 00:05:33,000 align:middle line:84% so this is the designated port on that segment. 84 00:05:33,000 --> 00:05:38,000 align:middle line:84% Now what about alternate and backup port roles? 85 00:05:38,000 --> 00:05:42,000 align:middle line:84% This corresponds to the blocking state in 802.1D 86 00:05:42,000 --> 00:05:47,000 align:middle line:84% A blocked port is defined as any port that is not a designated or root port. 87 00:05:47,000 --> 00:05:51,000 align:middle line:84% A port remains blocked as long as it receives more useful 88 00:05:51,000 --> 00:05:56,000 align:middle line:84% in other words better BPDUs than the one it would send out on the segment. 89 00:05:56,000 --> 00:06:01,000 align:middle line:84% Therefore port must receive BPDUs in order to stay blocked 90 00:06:01,000 --> 00:06:06,000 align:middle line:84% if it doesn’t receive BPDUs it will transition to the forwarding state. 91 00:06:06,000 --> 00:06:10,000 align:middle line:84% So in Rapid Spanning Tree there are 2 types of blocked ports 92 00:06:10,000 --> 00:06:13,000 align:middle line:84% an alternate port is a port that is blocked 93 00:06:13,000 --> 00:06:19,000 align:middle line:84% because it's receiving more useful BPDUs from another bridge on the segment. 94 00:06:19,000 --> 00:06:23,000 align:middle line:84% So in this example, this port is the designated port 95 00:06:23,000 --> 00:06:26,000 align:middle line:84% on let say switch B, on switch A 96 00:06:26,000 --> 00:06:30,000 align:middle line:84% this port is an alternate port because more useful or better. 97 00:06:30,000 --> 00:06:37,000 align:middle line:84% BPDUs are being received on this segment from switch B then from switch A 98 00:06:37,000 --> 00:06:42,000 align:middle line:84% and that maybe because the priority of switch B is lower than the priority of switch A. 99 00:06:42,000 --> 00:06:45,000 align:middle line:84% a backup port is a port that is blocked 100 00:06:45,000 --> 00:06:51,000 align:middle line:84% because it's receiving more useful BPDUs from the same bridge that it's on. 101 00:06:51,000 --> 00:06:55,000 align:middle line:84% so in this example were assuming that this port, this port 102 00:06:55,000 --> 00:06:57,000 align:middle line:84% and this port are connected to a hub 103 00:06:57,000 --> 00:07:02,000 align:middle line:84% this port a becomes the backup port its connected to the same switch 104 00:07:02,000 --> 00:07:07,000 align:middle line:84% as this designated ports but it may be a higher port number 105 00:07:07,000 --> 00:07:12,000 align:middle line:84% therefore it becomes a backup port. 12359