WEBVTT 1 00:00.000 --> 00:04.870 So now we need to address what we've talked about before with port priorities 2 00:04.870 --> 00:07.000 and how this ties into everything. 3 00:07.000 --> 00:12.200 So we've said that cost is an inbound concept. If I have an advertised cost, 4 00:12.200 --> 00:18.000 whatever that is, let's say it's 100, and this port here is 150 cost. 5 00:18.000 --> 00:23.390 Well then when I receive it, I consider my inbound cost here, and therefore my 6 00:23.390 --> 00:28.000 cost to get to the root this way is going to be 250. 7 00:28.000 --> 00:33.000 I add the 100 to the 150 because again it's an inbound concept. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 7 00:28.000 --> 00:33.000 I add the 100 to the 150 because again it's an inbound concept. 8 00:33.000 --> 00:38.020 But now we're going to talk about port priorities. And we're talking about port 9 00:38.020 --> 00:39.000 priority. 10 00:39.000 --> 00:45.000 What we'll find is that it's actually an outbound concept. 11 00:45.000 --> 00:51.000 I'm not going to configure port priorities where I'm receiving the BPDU. 12 00:51.000 --> 00:54.850 So if I'm receiving the BPDU this way, we might expect that if I want to 13 00:54.850 --> 01:00.130 configure cost, I would put it here on the inbound interface, and therefore if WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 13 00:54.850 --> 01:00.130 configure cost, I would put it here on the inbound interface, and therefore if 14 01:00.130 --> 01:03.480 I want to configure port priority, then I would also put it on the same 15 01:03.480 --> 01:05.000 interfaces. 16 01:05.000 --> 01:06.000 And that would be wrong. 17 01:06.000 --> 01:12.670 Port priority is an outbound concept, meaning that it's going to have to go 18 01:12.670 --> 01:19.370 here at the interfaces that are sending the BPDU because it's an outbound, or 19 01:19.370 --> 01:23.000 in other words, an egress counting method. 20 01:23.000 --> 01:36.000 Now we recall that the port priority is actually just part of the equation. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 20 01:23.000 --> 01:36.000 Now we recall that the port priority is actually just part of the equation. 21 01:36.000 --> 01:36.000 Ultimately we're going to form a port ID, which consists of the priority value 22 01:36.000 --> 01:36.000 plus a port number. 23 01:36.000 --> 01:40.740 And as we explained in the past, this number is an internal value. Usually we 24 01:40.740 --> 01:45.000 can say that for example gig 01 is less than gig 02. 25 01:45.000 --> 01:49.480 At that point we can just use basic math, but it gets a little trickier when we 26 01:49.480 --> 01:54.000 say okay that's great and all but how does gig 01 fit into all of this. 27 01:54.000 --> 01:57.720 I mean it'd be one thing if it was one slash zero, but maybe the switch has a 28 01:57.720 --> 02:01.940 gig one or a gig zero, and then we have to go and look and find out what that WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 28 01:57.720 --> 02:01.940 gig one or a gig zero, and then we have to go and look and find out what that 29 02:01.940 --> 02:03.000 port number is. 30 02:03.000 --> 02:05.000 So it's an internal concept. 31 02:05.000 --> 02:09.850 But in a lot of cases we can just look at the values here and compare them and 32 02:09.850 --> 02:13.000 say gig 01 would be less than gig 012. 33 02:13.000 --> 02:18.700 Now the priority again, this is actually eight bit, and so the ranges from zero 34 02:18.700 --> 02:20.000 to 255. 35 02:20.000 --> 02:25.000 And Cisco's default value, if we remember, is 128. 36 02:25.000 --> 02:32.000 And furthermore, we must configure the priorities in increments of 16. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 36 02:25.000 --> 02:32.000 And furthermore, we must configure the priorities in increments of 16. 37 02:32.000 --> 02:38.080 So it's similar to how we can, or we have to configure our bridge priorities in 38 02:38.080 --> 02:42.000 increments of 4096, even though we have a reason for that with the VLANs. 39 02:42.000 --> 02:47.000 The port priority is also going to be in increments of 16 here. 40 02:47.000 --> 02:51.410 So we usually don't need more than just really two values, maybe one that's 41 02:51.410 --> 02:54.000 higher than 128, one that's lower than 28. 42 02:54.000 --> 02:57.740 And so certainly we have plenty we don't need to worry about running out of 43 02:57.740 --> 03:00.000 port priorities or anything like that. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 43 02:57.740 --> 03:00.000 port priorities or anything like that. 44 03:00.000 --> 03:05.200 So again, let's go ahead and zoom up here and take a look at a situation where 45 03:05.200 --> 03:08.000 we've got, let's just say two interfaces here. 46 03:08.000 --> 03:14.560 And so when we look at the port number, let's just say that we're sending the P 47 03:14.560 --> 03:17.000 2Us by the way in this direction. 48 03:17.000 --> 03:21.000 And so our port numbers are going to be zero and one. 49 03:21.000 --> 03:24.670 Maybe that's a gig zero slash zero and zero slash one, but regardless, we just 50 03:24.670 --> 03:26.000 have numbers on the ports. 51 03:26.000 --> 03:27.000 And let's flip it around. 52 03:27.000 --> 03:30.000 Let's say we've got port five down here. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 52 03:27.000 --> 03:30.000 Let's say we've got port five down here. 53 03:30.000 --> 03:34.790 So this is the higher number and the lower number port three here on the right 54 03:34.790 --> 03:36.000 on this switch. 55 03:36.000 --> 03:38.000 So here's the question. 56 03:38.000 --> 03:43.390 If this switch here has to block one of these ports, which port does it end up 57 03:43.390 --> 03:46.000 blocking and which one becomes the root port? 58 03:46.000 --> 03:53.650 So the answer here is to again compare the port IDs and the port ID, we prefer 59 03:53.650 --> 03:57.000 the lower number, just like with everything else in spanning tree. 60 03:57.000 --> 04:00.000 And so when I look at this and say, okay, well, I prefer the lower number, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 60 03:57.000 --> 04:00.000 And so when I look at this and say, okay, well, I prefer the lower number, 61 04:00.000 --> 04:04.140 which is three, and therefore I block this, except again, it's an outbound 62 04:04.140 --> 04:05.000 concept. 63 04:05.000 --> 04:08.000 We're not looking at the inbound interfaces. 64 04:08.000 --> 04:10.000 We don't care what these are. 65 04:10.000 --> 04:13.000 We only care what the outbound is. 66 04:13.000 --> 04:17.930 And the outbound says that this port is the lower one and should be favored and 67 04:17.930 --> 04:23.000 therefore we will block this even though it's actually the lower ID locally. 68 04:23.000 --> 04:28.950 So this is an interesting concept because the control actually rests on the 69 04:28.950 --> 04:33.000 switch that isn't having to make the decisions. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 69 04:28.950 --> 04:33.000 switch that isn't having to make the decisions. 70 04:33.000 --> 04:37.250 So if I wanted to, I could then get on here and say, well, let's change the 71 04:37.250 --> 04:38.000 priority. 72 04:38.000 --> 04:42.000 Maybe we want to move the blocked port over here or something like that. 73 04:42.000 --> 04:47.000 So by default, it's 128 and 128. 74 04:47.000 --> 04:49.000 And so we have to go with the lower port number. 75 04:49.000 --> 04:53.850 But now I could change this to say, you know, instead of this being 128, let's 76 04:53.850 --> 04:57.000 make it 80, another multiple of 16. 77 04:57.000 --> 05:02.250 Well, now 80 compared to 128, just like again with bridge IDs and such, we're WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 77 04:57.000 --> 05:02.250 Well, now 80 compared to 128, just like again with bridge IDs and such, we're 78 05:02.250 --> 05:05.760 not even going to look at the MAC address when the bridge priority tells us 79 05:05.760 --> 05:07.000 which one is better. 80 05:07.000 --> 05:08.000 Same thing here. 81 05:08.000 --> 05:11.000 The port priority tells us which one's better. 82 05:11.000 --> 05:14.000 This is now the superior BPDU. 83 05:14.000 --> 05:20.230 Therefore, I'm going to bring this port online and end up blocking the left 84 05:20.230 --> 05:21.000 port. 85 05:21.000 --> 05:25.430 At this point, it has nothing to do with the port numbers as everything to do 86 05:25.430 --> 05:27.000 with the port priority. 87 05:27.000 --> 05:33.100 Now, in the real world here, as we often like to bring this back to reality, in WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 87 05:27.000 --> 05:33.100 Now, in the real world here, as we often like to bring this back to reality, in 88 05:33.100 --> 05:36.000 the real world, we almost never worry about port priorities. 89 05:36.000 --> 05:41.150 There's just not a lot of reason to worry about which link is getting blocked 90 05:41.150 --> 05:44.000 because we're usually going to use ether channels. 91 05:44.000 --> 05:48.680 It would take a pretty special situation to say, first of all, we can't bundle 92 05:48.680 --> 05:52.000 these connections together and make them both online. 93 05:52.000 --> 05:55.000 And then second of all, that we care which one is being chosen. 94 05:55.000 --> 05:59.400 Probably the closest possibility might be a situation where we have two 95 05:59.400 --> 06:04.350 switches with an ether channel here and an ether channel here, at which point WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 95 05:59.400 --> 06:04.350 switches with an ether channel here and an ether channel here, at which point 96 06:04.350 --> 06:08.200 both of these links can't be online at the same time, one of them would have to 97 06:08.200 --> 06:09.000 get blocked. 98 06:09.000 --> 06:13.470 So we are leveraging ether channels, but we still need to worry about blocking 99 06:13.470 --> 06:14.000 one. 100 06:14.000 --> 06:21.220 And then, if we maybe one is 10 gig and one is one gig ether channel, well, at 101 06:21.220 --> 06:25.000 that point, spanning tree cost would just favor the higher bandwidth. 102 06:25.000 --> 06:30.000 And so we don't even necessarily need to worry about bandwidth at this point, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 102 06:25.000 --> 06:30.000 And so we don't even necessarily need to worry about bandwidth at this point, 103 06:30.000 --> 06:34.010 but we might have to come up with some other reason why we'd really care which 104 06:34.010 --> 06:37.000 one of these ends up getting blocked, I guess, is what we're getting at. 105 06:37.000 --> 06:39.780 So chances are in the real world, we're never going to have to worry about this 106 06:39.780 --> 06:40.000 . 107 06:40.000 --> 06:43.610 However, when we're preparing for an exam like this, when we're trying to 108 06:43.610 --> 06:47.550 understand the basic mechanisms of spanning tree, we do need to understand this 109 06:47.550 --> 06:48.000 . 110 06:48.000 --> 06:51.000 We do need to understand how to manipulate this. 111 06:51.000 --> 06:55.000 And in the end, understand that if I want to change which port gets blocked, 112 06:55.000 --> 06:59.000 the control lies on the upstream switch, not down here. 113 06:59.000 --> 07:04.530 If I want to change the cost, then I can do it down here. If I want to change WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 113 06:59.000 --> 07:04.530 If I want to change the cost, then I can do it down here. If I want to change 114 07:04.530 --> 07:06.000 the port priority, it's going to be up here. 115 07:06.000 --> 07:10.340 Either way, I can control which port gets blocked, but I'm going to have to do 116 07:10.340 --> 07:14.000 it differently depending on which switch I'm configuring.