1 00:00:07,670 --> 00:00:15,620 This is one of multiple videos discussing OSPF, D.R. and PDR or designated router and backup designated 2 00:00:15,620 --> 00:00:18,980 router elections and selection criteria. 3 00:00:19,310 --> 00:00:27,290 On Cisco, we have this document that says that DIERS and BTR elections are done via the hello protocol. 4 00:00:27,800 --> 00:00:32,750 Hello packets are exchanged via multicast packets on each segment. 5 00:00:33,350 --> 00:00:38,240 The router with the highest priority on a segment will become the DAR for that segment. 6 00:00:39,330 --> 00:00:41,630 The process is repeated for the BDR. 7 00:00:41,640 --> 00:00:46,050 In the case of a Thai, the rider with the highest rider ID will win. 8 00:00:46,770 --> 00:00:50,280 The default priority on an interface is one. 9 00:00:50,430 --> 00:00:57,360 If you set the priority to a zero, it means the router will not become a D.R. or a PDR. 10 00:00:58,310 --> 00:01:03,650 Remember that the D.R. and PDR concepts are pur multi access segment. 11 00:01:04,129 --> 00:01:10,160 Setting the OSPF property on an interface can be done using the IP OSPF Property Command. 12 00:01:10,700 --> 00:01:17,270 Now let's look at the segment that has router three, router to router one and router four connected 13 00:01:17,270 --> 00:01:17,990 to it. 14 00:01:18,840 --> 00:01:19,980 On this segment. 15 00:01:22,230 --> 00:01:24,630 Rather one has neighbor relationships. 16 00:01:25,340 --> 00:01:32,300 To write a to write a three and rather four note to the interface gigabit zero. 17 00:01:33,270 --> 00:01:41,040 The stop entry is for the segment on gigabit zero one, but we are concentrating on gigabit zero zero 18 00:01:41,040 --> 00:01:44,070 for the moment so we don't see a door. 19 00:01:44,100 --> 00:01:48,690 Show IP OSPF interface brief. 20 00:01:49,930 --> 00:01:55,720 What we do see is that the local Rada Rada one is the designated Rada. 21 00:01:56,050 --> 00:01:58,660 Now, that goes against what we've just read. 22 00:01:59,900 --> 00:02:05,810 The Rondo idea of the strata is lower than the other riders on the segment, and this is where real 23 00:02:05,810 --> 00:02:08,840 world comes into play versus theory. 24 00:02:08,990 --> 00:02:16,730 The reason why Rider One was elected as the designated rider is that in the previous video I configured 25 00:02:16,730 --> 00:02:20,390 Rider 1/1 before I configured the other riders. 26 00:02:20,540 --> 00:02:23,840 So Rider one was the only Radha on the segment. 27 00:02:25,080 --> 00:02:26,690 It was waiting for other routers. 28 00:02:26,700 --> 00:02:27,620 None appeared. 29 00:02:27,630 --> 00:02:33,270 So it became the D.R. when the other routers were added to the segment, and I added them in the order 30 00:02:33,270 --> 00:02:37,200 of router two, then router three, then router four. 31 00:02:37,830 --> 00:02:40,200 Router one was already the designated router. 32 00:02:40,530 --> 00:02:46,320 Once a designated rod is elected and a writer comes onto the segment with a higher priority or higher 33 00:02:46,320 --> 00:02:51,930 rod ID, it's not going to usurp or preempt the designated rider. 34 00:02:51,960 --> 00:03:00,510 The designated rider remains the designated router and hence the local router with a lower router ID, 35 00:03:00,540 --> 00:03:05,580 which we can see by using the command show IP OSPF interface. 36 00:03:09,630 --> 00:03:12,450 Is elected as the DA. 37 00:03:13,350 --> 00:03:15,960 On gigabit zero zero. 38 00:03:17,120 --> 00:03:19,430 Because it came online first. 39 00:03:20,630 --> 00:03:24,860 Notice it has an adjacency or neighbor relationship with three routers. 40 00:03:24,890 --> 00:03:31,460 Router two is the backup designated rata router three and root of four are druthers. 41 00:03:32,240 --> 00:03:42,050 Show IP, OSPF and neighbor router two is a BTR router three out of four or D or others on gigabit zero 42 00:03:42,050 --> 00:03:42,620 zero. 43 00:03:43,330 --> 00:03:46,810 So what happens if we go into gigabit zero zero? 44 00:03:47,850 --> 00:03:52,200 And we top ip ospf priority zero. 45 00:03:54,410 --> 00:03:57,260 Show ip ospf. 46 00:03:58,150 --> 00:03:59,140 Interface. 47 00:04:00,740 --> 00:04:02,270 What you'll notice is. 48 00:04:03,440 --> 00:04:08,960 The rata state on gigabyte zero zero has changed to dire other. 49 00:04:10,290 --> 00:04:16,769 It's now formed a full relationship to rather to so show IP OSPF neighbor. 50 00:04:17,610 --> 00:04:25,320 Rather one has a full relationship to brother two as well as rather for the reason why rather to became 51 00:04:25,320 --> 00:04:26,340 the D.R is. 52 00:04:26,340 --> 00:04:29,760 It was already the PDR and then. 53 00:04:30,910 --> 00:04:33,760 Out of Route three and Route A four. 54 00:04:34,180 --> 00:04:39,220 Route a four became the BTR because it had a higher router ID. 55 00:04:39,790 --> 00:04:46,840 So the theory is as follows It with the highest priority becomes designated rudder on a segment. 56 00:04:46,840 --> 00:04:48,540 That's not entirely true. 57 00:04:48,550 --> 00:04:55,510 If one rider comes onto the segment first before other riders, that rider will be the designated rider. 58 00:04:55,780 --> 00:05:02,470 But if they all come online at the same time, it's based on highest priority, then highest rate ID, 59 00:05:02,770 --> 00:05:08,560 then highest loopback interface IP address, then highest configured physical interface address. 60 00:05:09,160 --> 00:05:15,400 And in this case, Rider four became the backup designated rider because it had the highest IP address. 61 00:05:15,730 --> 00:05:23,290 So what I'm going to do now is reboot the switch so that you can see the full process of how the exam 62 00:05:24,010 --> 00:05:26,380 explains how rider IDs are elected. 63 00:05:27,300 --> 00:05:29,430 So first thing I'll do is change. 64 00:05:30,270 --> 00:05:31,320 The priority. 65 00:05:32,570 --> 00:05:35,060 Overall a one back to one. 66 00:05:35,790 --> 00:05:36,990 On router two. 67 00:05:37,530 --> 00:05:41,330 Let's confirm the relationships. 68 00:05:41,460 --> 00:05:43,560 Show ip ospf neighbor. 69 00:05:44,780 --> 00:05:45,770 Rather one is a. 70 00:05:46,160 --> 00:05:48,150 The root of three is a draw. 71 00:05:48,170 --> 00:05:51,530 The root of four is a backup designated rider. 72 00:05:52,410 --> 00:05:56,610 And the local Rada is a designated Rada. 73 00:05:57,600 --> 00:06:03,660 In the output chair we can see that we're out of four is the backup designated rider router three and 74 00:06:03,660 --> 00:06:05,850 rider one or druthers. 75 00:06:06,360 --> 00:06:13,620 So let's see what happens when we shut down interfaces on. 76 00:06:14,630 --> 00:06:21,380 The switch, so shut down a range of interfaces, gigabit 0 to 3. 77 00:06:21,890 --> 00:06:23,990 So I've shut those interfaces down. 78 00:06:25,000 --> 00:06:27,790 On router one show ip ospf neighbor. 79 00:06:27,880 --> 00:06:33,640 Now it may take it a while in Genesis three to realize that the interfaces have gone down. 80 00:06:34,090 --> 00:06:40,780 So what I'm going to do at this point is pause the video, and what we should see is that the states 81 00:06:40,780 --> 00:06:41,740 change. 82 00:06:42,590 --> 00:06:47,660 So after a while, the date timer expires for the neighbor relationships. 83 00:06:48,080 --> 00:06:51,080 Previously we had these neighbor relationships. 84 00:06:52,390 --> 00:06:55,960 And now those neighbor relationships have disappeared. 85 00:06:55,990 --> 00:07:02,980 The only neighbor relationship that writer one has is to write a for using this Ethernet link. 86 00:07:05,550 --> 00:07:09,780 So to ensure that it doesn't get confusing, I'll shut that link down as well. 87 00:07:10,110 --> 00:07:15,360 So we have no OSPF neighbor relationships on router one at all. 88 00:07:16,240 --> 00:07:19,330 The switch has all of its interfaces shut down once again. 89 00:07:19,600 --> 00:07:23,860 So let's have a look on router to show ip ospf neighbor. 90 00:07:23,900 --> 00:07:26,020 No neighbor relationships and router to. 91 00:07:27,000 --> 00:07:34,230 Rather three show ip ospf neighbor no neighbor relationships rather for show ip ospf neighbor. 92 00:07:34,410 --> 00:07:41,100 It only has a neighbor relationship on the serial interface, not on the ethernet interface. 93 00:07:41,370 --> 00:07:47,100 So router four has a neighbor relationship to right of five, but no other neighbor relationships. 94 00:07:48,270 --> 00:07:49,620 So on the switch. 95 00:07:49,620 --> 00:07:50,520 No, shut. 96 00:07:52,500 --> 00:07:56,220 We've enabled all the interfaces on the switch. 97 00:07:56,680 --> 00:07:58,140 Some rather one. 98 00:08:00,070 --> 00:08:04,540 After a period of time, we should see that neighbor relationships are formed. 99 00:08:04,930 --> 00:08:09,130 We can do a ping to check if the interfaces have come up. 100 00:08:15,050 --> 00:08:17,270 Interfaces are starting to come up on the switch. 101 00:08:18,080 --> 00:08:20,210 Pings are now succeeding. 102 00:08:21,020 --> 00:08:23,840 And now we see neighbor relationships. 103 00:08:25,030 --> 00:08:28,330 Three neighbor relationships have been formed on rather one such show. 104 00:08:28,360 --> 00:08:30,430 IP ospf neighbor. 105 00:08:30,880 --> 00:08:35,710 Rather one has a two way relationship to router two. 106 00:08:36,309 --> 00:08:44,140 Router four has become the designated router and router three has become the backup designated router. 107 00:08:44,770 --> 00:08:51,880 The reason why router four became the designated router is because it has the highest IP address on 108 00:08:51,880 --> 00:08:53,920 any physical interface. 109 00:08:54,680 --> 00:08:57,170 This interface has an IP address of 10.11. 110 00:09:01,250 --> 00:09:05,810 This interface on the other two is ten one one, two. 111 00:09:08,420 --> 00:09:11,480 This interface on router three is ten 113. 112 00:09:13,040 --> 00:09:15,860 This to face and write a four is ten and 114. 113 00:09:16,220 --> 00:09:21,110 But in addition it has an IP address on the serial interface of ten 1 to 1. 114 00:09:21,380 --> 00:09:24,860 So because it has the highest IP address of any physical interface. 115 00:09:31,000 --> 00:09:32,980 It becomes the D.R.. 116 00:09:34,820 --> 00:09:36,590 With this IP address. 117 00:09:38,230 --> 00:09:46,030 The reason why it didn't use this IP address is because this interface was added after the OSPF process 118 00:09:46,030 --> 00:09:49,870 had run, so the router ID had already been selected. 119 00:09:51,280 --> 00:09:54,430 These two interfaces were up when OSPF started. 120 00:09:54,580 --> 00:09:57,970 This interface wasn't up when OSPF was started. 121 00:09:58,300 --> 00:10:02,200 So the router ID that was chosen was ten 1 to 1. 122 00:10:02,410 --> 00:10:10,000 So that proves that the router with the highest IP address of any physical interface will become the 123 00:10:10,000 --> 00:10:13,630 designated rider on a segment when all the devices come up. 124 00:10:14,440 --> 00:10:19,390 Let's see if we can change that by manipulating priorities. 125 00:10:21,650 --> 00:10:24,380 I hope you found the video useful. 126 00:10:24,470 --> 00:10:29,030 If you enjoyed it, please like it and please subscribe to my YouTube channel. 127 00:10:29,210 --> 00:10:30,950 I wish you all the very best.