1 00:00:08,260 --> 00:00:09,280 Welcome to the vlog. 2 00:00:11,450 --> 00:00:17,810 In this video, we're going to look at why specific routes were chosen rather than others in a network 3 00:00:17,810 --> 00:00:19,130 running Cisco routers. 4 00:00:30,110 --> 00:00:35,630 So in this topology, I have four routers running within gns3. Let's have a look at router 1 5 00:00:37,470 --> 00:00:43,530 and use the command show IP route to view the routing table. So various routes have been learnt. 6 00:00:43,980 --> 00:00:48,660 the one I want to point out is this one. In the routing table of router 1 7 00:00:49,320 --> 00:00:53,190 this network has been added by EIGRP. 8 00:00:54,880 --> 00:01:00,580 D indicates EIGRP as the routing protocol that added to the route. 9 00:01:01,730 --> 00:01:09,080 So we've got multiple routes but this route is the one of interest, so we should be able to ping 10 00:01:09,890 --> 00:01:12,040 4.4.4.4? which we can. 11 00:01:12,380 --> 00:01:14,360 That's the loopback of router 4. 12 00:01:16,360 --> 00:01:19,060 To get to that network, we go via 13 00:01:20,170 --> 00:01:28,480 10.1.2.2 and then 10.142, in other words, it's going via this link 14 00:01:29,350 --> 00:01:33,460 and then this link to get to the back of for. 15 00:01:34,850 --> 00:01:40,460 Let's have a look at which routing protocols are running on the router, show IP protocols shows us that 16 00:01:40,460 --> 00:01:41,870 OSPF is enabled. 17 00:01:43,790 --> 00:01:46,610 We are running OSPF on all interfaces on the router. 18 00:01:48,520 --> 00:01:51,370 EIGRP has also enabled on this router 19 00:01:53,700 --> 00:01:54,330 and so is rip. 20 00:01:55,420 --> 00:02:01,690 So router 1 has three routing protocols enabled now typically you won't do that in the real world 21 00:02:01,960 --> 00:02:04,090 but we're doing that here to prove a point. 22 00:02:04,840 --> 00:02:12,330 So let's look at the OSPF database so show IP OSPF database in the OSPF database. 23 00:02:12,370 --> 00:02:15,810 We have a runner ID for router 4. 24 00:02:16,390 --> 00:02:20,020 In other words, that route exists in the OSPF database. 25 00:02:20,530 --> 00:02:26,770 So why is OSPF not chosen for this route? 26 00:02:27,700 --> 00:02:32,430 OSPF is being used for this route but it's not being used for this route. 27 00:02:32,470 --> 00:02:34,420 Why is that? So 28 00:02:34,420 --> 00:02:36,850 to help troubleshoot we can use the following commands 29 00:02:36,850 --> 00:02:40,360 show IP OSPF interface brief. 30 00:02:41,520 --> 00:02:50,310 OSPF is enabled on both interfaces of router 1, show IP OSPF neighbor, but it only has a neighbor 31 00:02:50,310 --> 00:03:01,080 relationship to router 2. Show IP EIGRP neighbor, EIGRP only has a neighbor relationship to router 3. 32 00:03:04,010 --> 00:03:14,900 Now, in this topology, I didn't enable EIGRP on router 2, router 2 only has OSPF enabled. On router 33 00:03:14,910 --> 00:03:28,220 3 in this topology, we only have EIGRP and rip enabled but on router 2 it is learning about 34 00:03:28,220 --> 00:03:36,920 the network 4.4.4.4 through OSPF from router 4 so router 2 is able to ping that loop 35 00:03:36,920 --> 00:03:37,310 back. 36 00:03:37,970 --> 00:03:46,940 But on router 1, the network is not shown in the routing table and that's because of administrative 37 00:03:46,940 --> 00:03:47,510 distance. 38 00:03:48,110 --> 00:03:51,020 EIGRP has an administrative distance of 90. 39 00:03:51,440 --> 00:03:54,250 OSPF has an administrative distance of 110. 40 00:03:54,710 --> 00:04:00,740 In other words, if a router such as router 1 learns about a network such as 4.4.4. 41 00:04:00,740 --> 00:04:10,160 4 from both an OSPF router and an EIGRP router. The EIGRP router is deemed to be more reliable. 42 00:04:10,700 --> 00:04:12,230 It's more believable. 43 00:04:12,620 --> 00:04:16,100 The lower the administrative distance, the more believable a route. 44 00:04:16,820 --> 00:04:23,000 In this example, OSPF has a higher admin distance when compared to EIGRP. 45 00:04:24,010 --> 00:04:31,690 So to get to this network, EIGRP is believed because it has a lower administrative distance when 46 00:04:31,690 --> 00:04:40,930 compared to OSPF, so for both the loopbacks of router 3 and router 4 EIGRP is used but OSPF is used for 47 00:04:40,930 --> 00:04:42,090 the loopback of router 2 48 00:04:42,910 --> 00:04:46,900 So let's concentrate on this network, EIGRP is used at the moment. 49 00:04:48,100 --> 00:04:49,990 So on router 3 50 00:04:51,780 --> 00:04:59,760 this is the configuration of EIGRP if I shut it down the link on router 3, so I'm gonna shut 51 00:04:59,760 --> 00:05:00,870 down this port. 52 00:05:01,770 --> 00:05:08,150 That means Rawda three will no longer advertise network photophobia at to write a one. 53 00:05:09,000 --> 00:05:15,360 What happens now and what you'll notice is the entry is now replaced with an upfront. 54 00:05:16,970 --> 00:05:21,260 and if we trace to quadruple four it's going via router 2. 55 00:05:22,740 --> 00:05:29,820 If I no shut that a link on router 3 the links should come up and EIGRP should advertise 56 00:05:29,820 --> 00:05:37,260 the route once again on router 1 the route is replaced with an EIGRP entry rather than an OSPF 57 00:05:37,290 --> 00:05:43,950 entry because EIGRP has a lower admin distance than OSPF does. 58 00:05:44,550 --> 00:05:48,560 So EIGRP is chosen rather than OSPF. 59 00:05:49,140 --> 00:05:53,790 So just to prove the point again I'll shut the interface this is the interface on router 3. 60 00:05:55,510 --> 00:06:05,310 Before I did that an EIGRP route was used, but now an OSPF route is used tracing to that loopback, it's 61 00:06:05,920 --> 00:06:07,690 now going via OSPF. 62 00:06:08,730 --> 00:06:09,990 No shut the link. 63 00:06:11,540 --> 00:06:15,120 Do a trace again, still hasn't switched over yet. 64 00:06:15,470 --> 00:06:16,400 Let's try again. 65 00:06:17,240 --> 00:06:22,210 Notice it went from that to this, so a different path was taken. 66 00:06:22,670 --> 00:06:30,950 So because both EIGRP and OSPF are advertising the same network, there's a conflict and only 67 00:06:30,950 --> 00:06:38,030 one entry can be put into the routing table because EIGRP has a lower administrative distance than OSPF. 68 00:06:38,300 --> 00:06:42,800 EIGRP by default has an admin distance of 90 for internal EIGRP routes, 69 00:06:42,800 --> 00:06:49,070 OSPF has an admin distance of 110 and because of that EIGRP was chosen. 70 00:06:49,730 --> 00:06:53,570 I'll cover other scenarios based on the topology and separate videos. 71 00:06:54,320 --> 00:06:55,450 I hope you enjoyed the video. 72 00:06:55,880 --> 00:06:59,950 If it's been of benefit to you, please like it and please subscribe. 73 00:07:00,590 --> 00:07:02,120 I want to wish you all the very best.