1 00:00:01,920 --> 00:00:05,850 And then lastly, the stub area flag needs to be the same. 2 00:00:06,120 --> 00:00:12,390 The stub area flag denotes whether this is a stub area or a normal area. 3 00:00:12,720 --> 00:00:16,440 We'll talk more about stub areas once again in later slides. 4 00:00:17,040 --> 00:00:21,330 Now let's talk about designated routers and backup designated routers. 5 00:00:22,590 --> 00:00:23,490 In this topology. 6 00:00:23,490 --> 00:00:26,580 I have six routers connected to the same Ethernet segment. 7 00:00:27,630 --> 00:00:34,110 So assume that these six routers are connected to a switch or a hub, all sharing the same Ethernet 8 00:00:34,110 --> 00:00:34,860 segment. 9 00:00:35,690 --> 00:00:43,310 Designated Rodders orders are used on broadcast multi axis environments such as Ethernet and certain 10 00:00:43,310 --> 00:00:48,140 when implementations such as non broadcast multi access environments in frame relay. 11 00:00:49,250 --> 00:00:51,770 So to explain why we have a designated router. 12 00:00:52,930 --> 00:00:53,980 Let's assume. 13 00:00:54,790 --> 00:00:57,940 That this network ten 110 is connected to right one. 14 00:00:59,050 --> 00:01:04,750 And the six routers are connected to this Ethernet segment and assume that this network goes down. 15 00:01:05,550 --> 00:01:11,010 So let's assume for the moment that there's no designated rider on this Ethernet segment, and hopefully 16 00:01:11,010 --> 00:01:14,490 you'll quickly see why there's a requirement for a designated rudder. 17 00:01:15,320 --> 00:01:22,100 Without a designated rado, all of these riders would have a full adjacency in a full adjacency also 18 00:01:22,100 --> 00:01:23,600 exchanged between writers. 19 00:01:24,530 --> 00:01:31,610 So in this example are one needs to notify the other routers using a link state update that there's 20 00:01:31,610 --> 00:01:34,160 been a change in the network topology. 21 00:01:34,820 --> 00:01:43,250 So our one will send an update to our three sent an update to our two to or four to or five to or six 22 00:01:43,250 --> 00:01:49,850 notifying all of the routers that there's been a change in the topology or to when receiving that update 23 00:01:49,850 --> 00:01:55,970 from R one in this case, because there's no designated router has a full relationship with all other 24 00:01:55,970 --> 00:01:56,510 routers. 25 00:01:56,510 --> 00:02:00,890 So it sends an update to all of its neighbors to notify them that there's a problem. 26 00:02:01,370 --> 00:02:05,840 The same will happen on our three or three received an update from R one. 27 00:02:06,080 --> 00:02:12,020 So it notifies all of its neighbors that there's been a change in the topology or for we'll do the same 28 00:02:12,020 --> 00:02:12,500 thing. 29 00:02:13,290 --> 00:02:15,270 It received an update from our one. 30 00:02:15,780 --> 00:02:18,150 So it sends an update to all of its neighbors. 31 00:02:18,150 --> 00:02:20,000 And I'm sure you're getting the picture now. 32 00:02:20,010 --> 00:02:24,660 Our five received that update from our one, so it sends an update to all of its neighbors. 33 00:02:25,230 --> 00:02:28,620 And lastly, our six sends an update to all of its neighbors. 34 00:02:29,010 --> 00:02:31,260 So there's a lot of duplicate traffic. 35 00:02:31,970 --> 00:02:38,330 When a single network goes down and these six routers have a full adjacency with one another. 36 00:02:38,750 --> 00:02:44,420 So rather than doing that, a designated router is selected on this specific segment. 37 00:02:44,960 --> 00:02:49,610 So let's assume that our two was elected as a designated writer. 38 00:02:49,970 --> 00:02:52,970 Designated routers are selected on two criteria. 39 00:02:53,120 --> 00:02:55,250 The first one is highest priority. 40 00:02:55,730 --> 00:02:58,400 You can specify the priority on an interface. 41 00:02:58,460 --> 00:03:01,700 The default priority is one zero. 42 00:03:01,700 --> 00:03:06,200 Excludes a router from becoming a designated router or backup designated router. 43 00:03:06,230 --> 00:03:09,410 The values for the priority are from 1 to 255. 44 00:03:09,950 --> 00:03:12,750 So the first criteria is highest priority. 45 00:03:12,770 --> 00:03:18,470 If the priorities are the same, then the rider with the highest rider ID is elected as the designated 46 00:03:18,470 --> 00:03:19,850 rider for that segment. 47 00:03:20,240 --> 00:03:23,840 So in this example, we've elected R2 as the designated rider. 48 00:03:24,050 --> 00:03:27,020 And let's assume again that this network goes down. 49 00:03:28,160 --> 00:03:33,710 But what happens now is our one sends an update only to the designated writers. 50 00:03:34,470 --> 00:03:39,450 Designated Rodders are listening on this multicast address to 24006. 51 00:03:40,050 --> 00:03:42,540 Other riders are not listening to that multicast address. 52 00:03:42,540 --> 00:03:47,670 So from an OSPF point of view, they do not receive or see that update. 53 00:03:47,970 --> 00:03:52,080 Only the designated rider receives that multicast update. 54 00:03:53,090 --> 00:03:55,400 Now multicast isn't covered in this course. 55 00:03:56,150 --> 00:03:58,430 But briefly, if this infrastructure was a hub. 56 00:03:58,430 --> 00:04:04,820 So the riders were connected via hub at layer one, that multicast would go to all the routers. 57 00:04:05,180 --> 00:04:11,540 However, only certain routers are listening or accepting that multicast, so only certain routers have 58 00:04:11,540 --> 00:04:13,400 subscribed to that multicast. 59 00:04:14,090 --> 00:04:22,280 In this case only designated rodders are listening for and accepting multi costs to address 224006. 60 00:04:22,820 --> 00:04:30,080 So the other orders at layer two will drop this update OSPF residing at layer four and the OSA model 61 00:04:30,080 --> 00:04:33,530 will not see this update on the other routers. 62 00:04:33,530 --> 00:04:38,870 So routers three, four, five and six from an OSPF point of view at layer four will not receive the 63 00:04:38,870 --> 00:04:39,530 update. 64 00:04:39,560 --> 00:04:41,600 Only router two will receive the update. 65 00:04:42,630 --> 00:04:44,040 So logically what happens? 66 00:04:44,040 --> 00:04:45,330 The link goes down. 67 00:04:45,420 --> 00:04:51,270 Rather one is updating rather to the designated router, but sending a multicast to this address only 68 00:04:51,270 --> 00:04:52,680 router to the designated routers. 69 00:04:52,680 --> 00:04:59,910 Receiving the multicast router two then sends an update to all the other routers on this multicast address 70 00:04:59,910 --> 00:05:02,100 224005. 71 00:05:02,160 --> 00:05:08,880 All OSPF routers are listening to this multicast address so they will receive the update router. 72 00:05:08,880 --> 00:05:14,250 One would receive the update but wouldn't process it because its OSPF topology table is already up to 73 00:05:14,250 --> 00:05:14,790 date. 74 00:05:14,940 --> 00:05:21,960 So logically what happens is the update goes from R1 to R2 or T sends that update to all the other routers. 75 00:05:22,350 --> 00:05:28,140 They process the update and therefore the topology database is updated with the new information that 76 00:05:28,140 --> 00:05:30,120 this network has gone down. 77 00:05:30,750 --> 00:05:38,160 As you can see here, it's much more efficient to use a designated router than to allow full adjacencies 78 00:05:38,160 --> 00:05:41,820 between all routers and have all those duplicate updates. 79 00:05:42,990 --> 00:05:49,050 It's important to realize that only the designated rider and backup designated rider will have full 80 00:05:49,050 --> 00:05:52,230 relationships to all the other artists. 81 00:05:52,950 --> 00:05:59,100 So for instance, root of four and Root of five will only have a state known as two way. 82 00:05:59,610 --> 00:06:05,640 In two way they know about each other, but no updates will be exchanged between the riders. 83 00:06:05,670 --> 00:06:08,940 So in other words, our phone or five will not update each other. 84 00:06:08,940 --> 00:06:12,510 Neither will or five and or six and so forth and so on. 85 00:06:12,810 --> 00:06:19,350 All riders will only update the designated rider and back up designated rider with changes in the topology 86 00:06:19,740 --> 00:06:22,770 so they have a full relationship to the designated rider. 87 00:06:23,340 --> 00:06:28,740 This allows for the saving of updates and duplicate traffic on a single segment. 88 00:06:30,030 --> 00:06:37,200 Once again, it's important to realize that writers on the segment will only form full relationships 89 00:06:37,200 --> 00:06:40,260 with designated writers and backup designated writers. 90 00:06:40,290 --> 00:06:42,750 Now, in this example, I've only got a designated writer. 91 00:06:43,080 --> 00:06:48,810 The issue with only having a designated writer is that if this writer goes down, updates will not be 92 00:06:48,810 --> 00:06:50,250 sent and received properly. 93 00:06:50,280 --> 00:06:56,550 So on a segment, a designated writer will be elected, and normally a backup designated writer would 94 00:06:56,550 --> 00:06:57,840 also be elected. 95 00:06:57,990 --> 00:07:01,590 So you'd have both a designated writer and a backup designated writer. 96 00:07:02,130 --> 00:07:06,810 The PDR will become the D.R. if the D.R. fails.