1 00:00:08,140 --> 00:00:14,800 In the previous video I mentioned that in auto RP you have to have at 2 00:00:14,800 --> 00:00:18,640 least two routers playing two special roles, one called an RP candidate 3 00:00:18,640 --> 00:00:22,060 and one called an mapping agent. 4 00:00:22,060 --> 00:00:26,740 And I said that one router could play both roles, theoretically, but it's 5 00:00:26,740 --> 00:00:29,300 better to split them apart on a different router so you have a little 6 00:00:29,300 --> 00:00:33,520 bit more resiliency, a little bit more redundancy in case the router fails. 7 00:00:33,520 --> 00:00:39,080 So let's start by looking at more details of the RP candidates. 8 00:00:39,080 --> 00:00:46,240 So I mentioned how RP candidates send out messages called RP announcements. 9 00:00:46,240 --> 00:00:49,400 And it's within these RP announcements that they say, hey, I would like 10 00:00:49,400 --> 00:00:53,700 to be an RP for this particular group. 11 00:00:53,700 --> 00:01:00,120 So those RP announcements are actually encapsulated in UDP. 12 00:01:00,120 --> 00:01:03,760 And they use UDP port number 496. 13 00:01:03,760 --> 00:01:06,960 I know you might come across some certification someday that asks you 14 00:01:06,960 --> 00:01:09,840 a nitty gritty detail like that. 15 00:01:09,840 --> 00:01:15,820 And even after it's initially sent, it's reset every 60 seconds, right, 16 00:01:15,820 --> 00:01:20,580 because maybe this RP was not the one that was elected, but he still wants 17 00:01:20,580 --> 00:01:24,580 to make himself known in case the elected RP goes away. 18 00:01:24,580 --> 00:01:26,820 So it's reset every 60 seconds. 19 00:01:26,820 --> 00:01:35,640 And it's sent to a special address of 224.0.1.39. 20 00:01:35,640 --> 00:01:39,160 We saw this in the previous section, this is a reserved address. 21 00:01:39,160 --> 00:01:44,420 Now, if I now only mapping agents pay attention to this, what do I mean 22 00:01:44,420 --> 00:01:46,380 by pay attention to it? 23 00:01:46,380 --> 00:01:56,520 Well, if a router, for example, let's say that this router, let's get 24 00:01:56,520 --> 00:02:03,260 rid of that. Let's say that this router here was the RP candidate. 25 00:02:03,260 --> 00:02:10,120 And so we're assuming here that all these interfaces are in the same way. 26 00:02:10,120 --> 00:02:19,220 So he sends out via dense mode, he floods his RP announce messages. 27 00:02:19,220 --> 00:02:20,980 And that's going to 239. 28 00:02:20,980 --> 00:02:26,260 No, not 239.224.0.1.39. 29 00:02:26,260 --> 00:02:36,180 Now, let's say that our mapping agent was over here. 30 00:02:36,180 --> 00:02:41,140 Router two. So what's router eight going to do with this message? 31 00:02:41,140 --> 00:02:44,440 Because after all, he can't really look at it to see if this guy's an 32 00:02:44,440 --> 00:02:49,280 RP. And you might say, well, he'll just silently pass it on, right? 33 00:02:49,280 --> 00:02:51,620 He'll just transparently pass it. 34 00:02:51,620 --> 00:02:56,900 Well, but the problem with that is the rules of PIM state that you cannot 35 00:02:56,900 --> 00:03:00,960 forward a multicast message without creating multicast state. 36 00:03:00,960 --> 00:03:03,540 You can't just forward it. 37 00:03:03,540 --> 00:03:07,560 Even in dense mode, you still have to create some sort of state in the 38 00:03:07,560 --> 00:03:10,040 multicast routing table. 39 00:03:10,040 --> 00:03:14,280 So when router eight receives this message, he's actually going to do 40 00:03:14,280 --> 00:03:18,500 two things. He's going to create an s, entry because he does know the 41 00:03:18,500 --> 00:03:21,100 source. The source is coming from router three. 42 00:03:21,100 --> 00:03:24,220 He knows the group 2201.39. 43 00:03:24,220 --> 00:03:26,580 So he will create an s, entry. 44 00:03:26,580 --> 00:03:31,540 He'll also create a star, entry because that's required when you've got 45 00:03:31,540 --> 00:03:38,540 an s, entry. And the outgoing interface list, remember when you've got 46 00:03:38,540 --> 00:03:43,620 both a star, and an s, entry, the s, entry is typically the one that's 47 00:03:43,620 --> 00:03:44,920 actually used for forwarding. 48 00:03:44,920 --> 00:03:49,420 So whatever is in the outgoing interface list of the s, that's what we 49 00:03:49,420 --> 00:03:50,940 use to forward packets. 50 00:03:50,940 --> 00:03:57,440 So in this particular case, in router eight, when he creates his s, he's 51 00:03:57,440 --> 00:04:00,560 going to say, all right, well, all of my interfaces are in sparse dense 52 00:04:00,560 --> 00:04:05,240 mode, except for the one where I receive this packet is what I'm going 53 00:04:05,240 --> 00:04:07,760 to put into my outgoing interface list. 54 00:04:07,760 --> 00:04:25,380 So in his s, entry, you'll see that zero one and zero zero dot you know, 55 00:04:25,380 --> 00:04:28,420 flood this RP announced message. 56 00:04:28,420 --> 00:04:32,100 But if you at this point in time, if you actually look in him, you say, 57 00:04:32,100 --> 00:04:33,360 okay, who's your RP? 58 00:04:33,360 --> 00:04:35,720 Who's your RP? He'll say, I don't know. 59 00:04:35,720 --> 00:04:38,600 He doesn't know because he can't actually look at this message to decide 60 00:04:38,600 --> 00:04:41,820 who his RP should be. 61 00:04:41,820 --> 00:04:44,180 That message has to get to the mapping agent. 62 00:04:44,180 --> 00:04:47,980 And then the mapping agent will send an RP discover message. 63 00:04:47,980 --> 00:04:52,000 And that's what router eight can use to figure out what RP he's going 64 00:04:52,000 --> 00:05:01,080 to select. So within this RP announcement, and I'm actually going to do 65 00:05:01,080 --> 00:05:04,980 a sniffer trace of this in just a second so you can see it, it contains 66 00:05:04,980 --> 00:05:10,160 the address of the RP, you know, his unicast IP address, what groups he's 67 00:05:10,160 --> 00:05:14,120 saying that he's capable of being the RP for, which by default is everything. 68 00:05:14,120 --> 00:05:17,480 Everything beginning with one one one zero and binary. 69 00:05:17,480 --> 00:05:19,480 And his whole time. 70 00:05:19,480 --> 00:05:27,060 So how do I make a router an RP candidate with this command? 71 00:05:27,060 --> 00:05:30,420 IP PIMS send dash RP dash announced. 72 00:05:30,420 --> 00:05:32,600 And then interface. 73 00:05:32,600 --> 00:05:36,480 And the interface you select here like fast ethernet zero zero, serial 74 00:05:36,480 --> 00:05:38,380 one one, whatever. 75 00:05:38,380 --> 00:05:42,200 This interface command here is going to actually control the source address 76 00:05:42,200 --> 00:05:49,600 that he selects for those RP announced messages, the source address. 77 00:05:49,600 --> 00:05:56,120 Why do we care? Well, the reason why we care is because this is what the 78 00:05:56,120 --> 00:05:59,900 mapping agent uses to figure out who the best router is. 79 00:05:59,900 --> 00:06:04,660 So if a mapping agent receives two or more RP announced messages from 80 00:06:04,660 --> 00:06:08,340 two routers and they're advertising their capability to be the RP for 81 00:06:08,340 --> 00:06:13,160 the exact same groups, the mapping agent will simply say, okay, whichever 82 00:06:13,160 --> 00:06:16,360 one of you has the highest IP address, you're the winner. 83 00:06:16,360 --> 00:06:18,620 You're the one I'm going to tell everybody about. 84 00:06:18,620 --> 00:06:23,100 So this is how you can influence that by on the actual RP itself specifying 85 00:06:23,100 --> 00:06:27,480 the interface, they'll be higher or lower than other routers that you're 86 00:06:27,480 --> 00:06:29,160 configuring this for. 87 00:06:29,160 --> 00:06:34,160 Scope, this basically controls the TTL. 88 00:06:34,160 --> 00:06:37,320 So how far this RP announced message can go. 89 00:06:37,320 --> 00:06:39,000 And it's not optional. 90 00:06:39,000 --> 00:06:42,040 You have to set it, non optional keyword. 91 00:06:42,040 --> 00:06:44,640 What is optional is the group list. 92 00:06:44,640 --> 00:06:49,120 So this gives you the ability to say, okay, instead of being the RP for 93 00:06:49,120 --> 00:06:54,240 everything, I only want you to be the RP for certain groups. 94 00:06:54,240 --> 00:06:56,960 Now there's something actually kind of interesting about this. 95 00:06:56,960 --> 00:06:59,080 And I'm going to show it to you rather than tell you I'm just going to 96 00:06:59,080 --> 00:07:03,920 do it. So in order to replicate this and just do a quick snipper trace 97 00:07:03,920 --> 00:07:08,520 of it, step number one is I have to go to all my interfaces right here 98 00:07:08,520 --> 00:07:13,100 and configure them for sparse dense mode. 99 00:07:13,100 --> 00:07:25,420 So let's just do that and then we'll go to the other side. 100 00:07:25,420 --> 00:07:31,600 Okay, so that's all we need on him. 101 00:07:31,600 --> 00:07:45,040 Go to router eight. 102 00:07:45,040 --> 00:07:46,960 Okay, he's done. 103 00:07:46,960 --> 00:07:49,920 Let's go on router three. 104 00:07:49,920 --> 00:08:20,880 Oh, and while I'm here, let me delete my static here. 105 00:08:20,880 --> 00:08:34,280 Okay, almost done here. 106 00:08:34,280 --> 00:08:35,780 Okay, so we're all set up. 107 00:08:35,780 --> 00:08:37,760 Everything's in sparse dense mode now. 108 00:08:37,760 --> 00:08:40,480 Okay, so let's see here. 109 00:08:40,480 --> 00:08:46,240 Let's go ahead and just continue with having router three be the RP. 110 00:08:46,240 --> 00:08:50,640 So now we're going to configure him to send those RP announced messages. 111 00:08:50,640 --> 00:09:04,130 Now before I do that, let's just go ahead and capture this in a little 112 00:09:04,130 --> 00:09:10,680 bit. And according to my physical topology, R3, 01 is port 0 slash six 113 00:09:10,680 --> 00:09:23,860 on the switch. All right, so ready to go. 114 00:09:23,860 --> 00:09:28,080 So let's go back to R3. 115 00:09:28,080 --> 00:09:33,080 And what I'm going to do is I'm actually going to configure R3 instead 116 00:09:33,080 --> 00:09:35,740 of announcing his capability for everything. 117 00:09:35,740 --> 00:09:39,040 I'm going to say, I only want you to announce yourself as a potential 118 00:09:39,040 --> 00:09:47,340 RP for anything in the 238 range and specifically say you're not the RP 119 00:09:47,340 --> 00:09:49,680 for the 239 range. 120 00:09:49,680 --> 00:09:50,620 And there's a reason I'm doing this. 121 00:09:50,620 --> 00:10:03,080 I'll show you. So let's do this. 122 00:10:03,080 --> 00:10:10,060 Okay, access dash list one deny 239. 123 00:10:10,060 --> 00:10:17,340 And you don't have to worry about the implicit deny at the end of the 124 00:10:17,340 --> 00:10:21,320 access list. That does not affect this particular feature. 125 00:10:21,320 --> 00:10:25,020 So this feature only looks at what you specifically configured as permits 126 00:10:25,020 --> 00:10:32,980 or denies. And now I say IPPIM send dash RP announce. 127 00:10:32,980 --> 00:10:36,160 Okay. And now I say, okay, what do I want your source interface to be? 128 00:10:36,160 --> 00:10:39,420 Well, we'll just have it be fast ethernet zero slash zero. 129 00:10:39,420 --> 00:10:45,640 Scope. I will just put it as like 100 hops. 130 00:10:45,640 --> 00:10:50,280 And now I'm going to say group list in reference to access list one. 131 00:10:50,280 --> 00:10:53,200 Now before I hit the enter key, let's do a snipper trace to see what his 132 00:10:53,200 --> 00:11:02,540 message looks like. 133 00:11:02,540 --> 00:11:04,980 Okay, so there's the RP announce. 134 00:11:04,980 --> 00:11:09,340 Okay, so there it is. 135 00:11:09,340 --> 00:11:12,840 Not sure why he sent two of them. 136 00:11:12,840 --> 00:11:18,040 Oh, wait a second. 137 00:11:18,040 --> 00:11:18,820 I think I know why. 138 00:11:18,820 --> 00:11:21,920 Let's see here. Let's just check the source MAC address. 139 00:11:21,920 --> 00:11:26,220 So this one has a source MAC address of five AC nine. 140 00:11:26,220 --> 00:11:33,700 Seven C C zero. Yeah, so this is actually seven C C zero. 141 00:11:33,700 --> 00:11:44,040 And I think if I go to router eight. 142 00:11:44,040 --> 00:11:51,080 Let's see here. zero zero dot 83. 143 00:11:51,080 --> 00:12:07,120 Seven C C zero. That what we saw in the router three. 144 00:12:07,120 --> 00:12:08,340 Yes, seven C C zero. 145 00:12:08,340 --> 00:12:15,160 So what we're actually seeing here, this is kind of interesting that in 146 00:12:15,160 --> 00:12:20,800 the whiteboard when router three sent his RP announce. 147 00:12:20,800 --> 00:12:32,540 Actually, what router eight did is he not selected it back. 148 00:12:32,540 --> 00:12:34,620 I'm not sure why they do that. 149 00:12:34,620 --> 00:12:36,860 But so we actually saw it coming back. 150 00:12:36,860 --> 00:12:39,880 That's why it looks like it's it's the exact same message. 151 00:12:39,880 --> 00:12:42,780 But one has a source MAC of router three. 152 00:12:42,780 --> 00:12:45,540 One has a source MAC of router eight. 153 00:12:45,540 --> 00:12:48,300 That's why it's showing up twice in the snipper trace here. 154 00:12:48,300 --> 00:12:51,060 But the main thing I wanted to show you was the body of the message. 155 00:12:51,060 --> 00:12:56,820 So notice it's encapsulated in UDP port 496. 156 00:12:56,820 --> 00:13:00,900 And then we have the Cisco auto RP behind that. 157 00:13:00,900 --> 00:13:03,160 And here's what I want to show you. 158 00:13:03,160 --> 00:13:09,720 He says here's my IP address three four three three. 159 00:13:09,720 --> 00:13:13,320 And I'm specifically telling you about two groups. 160 00:13:13,320 --> 00:13:17,600 He says number one, the 238 group. 161 00:13:17,600 --> 00:13:20,900 This is a positive group prefix. 162 00:13:20,900 --> 00:13:23,500 This is his way of saying, Hey, mapping agent. 163 00:13:23,500 --> 00:13:27,480 I would like to be the RP for everything beginning with 238. 164 00:13:27,480 --> 00:13:30,420 So hopefully you'll elect me. 165 00:13:30,420 --> 00:13:36,620 But down down here, 239, we have a negative group prefix. 166 00:13:36,620 --> 00:13:41,080 Now you might just think, well, why is he even had that in there at all? 167 00:13:41,080 --> 00:13:44,760 I mean, if he doesn't want to be the RP for 239, why not just exclude 168 00:13:44,760 --> 00:13:47,680 that and just included 238? 169 00:13:47,680 --> 00:13:50,880 And anything that he doesn't include, he'll just be implicitly assumed 170 00:13:50,880 --> 00:13:54,320 that he doesn't want to be the RP for the rest of that stuff. 171 00:13:54,320 --> 00:13:57,120 Well, that's kind of the way that's working. 172 00:13:57,120 --> 00:14:01,980 For example, in this particular case, the no routers in my domain will 173 00:14:01,980 --> 00:14:07,640 ever use this guy for 237, 236, 224, 225. 174 00:14:07,640 --> 00:14:11,100 They will only learn that he wants to be the RP for everything beginning 175 00:14:11,100 --> 00:14:15,760 with 238. So why is he including this negative group prefix? 176 00:14:15,760 --> 00:14:19,000 This is actually kind of interesting what happens here. 177 00:14:19,000 --> 00:14:24,980 What this is saying is he saying, Hey, everybody, everywhere, all PIM 178 00:14:24,980 --> 00:14:30,920 routers, the 239 prefix must be used in dense mode. 179 00:14:30,920 --> 00:14:35,420 We will not have any RP for 239. 180 00:14:35,420 --> 00:14:41,980 So if you ever get an actual multicast dream going to 239 dot something, 181 00:14:41,980 --> 00:14:46,900 flood it via dense mode, don't look for an RP because there's no RP that's 182 00:14:46,900 --> 00:14:50,820 servicing it. Now, you might be thinking yourself, well, okay, but what 183 00:14:50,820 --> 00:14:53,740 if I configure this command on another router? 184 00:14:53,740 --> 00:14:58,500 And on that router, I say, you can be the RP for the 239 group. 185 00:14:58,500 --> 00:15:00,000 Will that override this? 186 00:15:00,000 --> 00:15:02,260 No, it will not. 187 00:15:02,260 --> 00:15:04,860 This overrides that. 188 00:15:04,860 --> 00:15:15,500 So if a router sends out a network, it basically forces the entire domain 189 00:15:15,500 --> 00:15:20,040 to use dense mode for this particular group. 190 00:15:20,040 --> 00:15:21,800 Now, why you'd want to do that? 191 00:15:21,800 --> 00:15:27,740 I don't know, but I just want to point out that's how that works. 192 00:15:27,740 --> 00:15:31,920 So be very careful if you're using access lists along with your RP announced 193 00:15:31,920 --> 00:15:35,600 command, like I did right here. 194 00:15:35,600 --> 00:15:40,560 Be very careful if you've got any deny statements in that access list, 195 00:15:40,560 --> 00:15:43,260 because that's what's going to happen. 196 00:15:43,260 --> 00:15:46,380 That particular group will now be forced to use dense mode and you might 197 00:15:46,380 --> 00:15:50,500 end up with flooding that you did not intend. 198 00:15:50,500 --> 00:15:55,940 Now, John points out that 239, according to the Internet Assigned Numbers 199 00:15:55,940 --> 00:15:58,840 Authority, was the range for source specific multicast. 200 00:15:58,840 --> 00:16:03,420 That's true. But remember, these are all recommendations, right? 201 00:16:03,420 --> 00:16:07,440 A router will not say, oh, 239, I'm only going to accept that if that 202 00:16:07,440 --> 00:16:09,280 source specific multicast. 203 00:16:09,280 --> 00:16:12,480 That's just a recommendation by the Internet Assigned Numbers Authority. 204 00:16:12,480 --> 00:16:16,900 Nothing preventing you from using IGMP version 2, not doing source specific 205 00:16:16,900 --> 00:16:22,200 multicast and still using 239.something as your multicast streams. 206 00:16:22,200 --> 00:16:23,240 That's all just recommendations.