1 00:00:11,100 --> 00:00:16,110 In this video, we are troubleshooting network connectivity problems. 2 00:00:16,239 --> 00:00:24,280 We've been told in this scenario that router 1 which is a VIRL router running in GNS3 is not able 3 00:00:24,280 --> 00:00:31,090 to ping the loopback of VIRL router 2 with the loopback of quadruple 4 4 00:00:31,230 --> 00:00:32,259 and that's true. 5 00:00:33,440 --> 00:00:44,540 Show IP route shows us that we only have a connected and local network in the routing table, show IP 6 00:00:44,540 --> 00:00:53,060 protocols shows us that we are running the OSPF routing protocol and it's enabled on all interfaces 7 00:00:53,070 --> 00:01:00,260 on the router. Show IP OSPF neighbor shows us that we have a full relationship with a neighboring router 8 00:01:00,650 --> 00:01:10,510 with IP address 10 121 which is this router 1 router. So on router 1, show IP OSPF 9 00:01:10,570 --> 00:01:19,730 neighbor we only have a neighbor relationship to 10.1.1.1 the router on the left 10 00:01:19,750 --> 00:01:20,860 on interface fast 11 00:01:20,880 --> 00:01:33,130 Ethernet 00. Show IP protocols OSPF is enabled on this router and is enabled on all interfaces, show IP 12 00:01:33,130 --> 00:01:41,950 OSPF interface brief. We have OSPF enabled on the serial interface and the FastEthernet interface, can 13 00:01:41,950 --> 00:01:43,840 you see the problem? 14 00:01:43,840 --> 00:01:47,440 Notice the state of the serial interface is down. 15 00:01:47,560 --> 00:01:56,650 So show IP interface brief we can see that the serial interface is up, down. 16 00:01:56,730 --> 00:01:59,800 Something wrong with the serial interface. 17 00:02:00,120 --> 00:02:04,090 Show run interface serial 2/0, 18 00:02:04,260 --> 00:02:11,550 shows us that an IP address is configured on the serial interface but the interface is no shut. 19 00:02:11,550 --> 00:02:14,110 In other words it's administratively up 20 00:02:14,250 --> 00:02:15,410 nothing else is shown. 21 00:02:15,420 --> 00:02:18,630 So show interface serial 2/0 22 00:02:18,690 --> 00:02:25,800 we can see that the interfaces up down, we can see the encapsulation keepalive is set to 10 seconds. 23 00:02:28,620 --> 00:02:37,880 Let's have a look at router 2, show IP OSPF interface brief we can see once again that OSPF is enabled on the serial interface 24 00:02:38,310 --> 00:02:47,350 but the neighbor relationship is down, show IP interface brief interface is up, down. 25 00:02:47,370 --> 00:02:57,080 So this interface has not been administratively shut down, show interface serial 2/0. 26 00:02:57,190 --> 00:02:59,200 Can you see the problem? 27 00:02:59,200 --> 00:03:01,690 What's wrong in this output? 28 00:03:03,550 --> 00:03:14,400 Notice here, encapsulation HDLC on this side encapsulation PPP, show run interface serial 2/0. 29 00:03:14,650 --> 00:03:19,850 We can see that this side is being set to use PPP encapsulation. 30 00:03:20,350 --> 00:03:31,870 So we need to either set it to use HDLC which is the default on Cisco serial interfaces or we need 31 00:03:31,870 --> 00:03:42,520 to set the encapsulation to PPP on both sides. So set the encapsulation to HDLC show IP interface 32 00:03:42,850 --> 00:03:43,480 brief 33 00:03:46,540 --> 00:03:55,180 interfaces up, down but notice that it's just come up and notice the OSPF relationship has gone to full. 34 00:03:56,160 --> 00:04:07,020 So on router 1 show IP OSPF neighbor notice we now have two neighbor relationships, one to router 2 across 35 00:04:07,080 --> 00:04:16,720 the several interface and one to this IOS V router on the left using the FastEthernet interface. 36 00:04:16,779 --> 00:04:18,240 So that looks a lot better. 37 00:04:18,450 --> 00:04:22,650 Let's have a look at the routing table of router 1 which is VIRL 38 00:04:22,690 --> 00:04:24,390 IOS V router. 39 00:04:24,390 --> 00:04:30,330 We now see the loopback quadruple 4 in the routing table. 40 00:04:30,450 --> 00:04:32,560 So can we ping that loopback? 41 00:04:32,580 --> 00:04:34,250 Yes, we can. 42 00:04:34,370 --> 00:04:45,140 We're gonna disable IP domain lookup to speed up tracing, so trace quadruple 4. 43 00:04:45,320 --> 00:04:51,620 The trace shows us the path that goes to this router 1 with serial interface to router 2 with 44 00:04:51,640 --> 00:05:01,500 a serial interface and then on to the IOS V router with the loopback quadruple 4. Now to prove 45 00:05:01,500 --> 00:05:13,630 that I could do a debug, so debug IP ICMP do a ping and notice we see the output on the screen of this router. 46 00:05:13,920 --> 00:05:21,970 So be careful when asked to troubleshoot a routing protocol such as OSPF in the CCNA exam. 47 00:05:22,080 --> 00:05:29,010 The issue could be unrelated to OSPF the issue could be an interface that's been misconfigured. In this 48 00:05:29,010 --> 00:05:32,210 example the encapsulation was incorrect. 49 00:05:32,280 --> 00:05:38,580 You have to have either HDLC on both sides or PPP on both sides of a serial link. 50 00:05:38,730 --> 00:05:42,090 A mismatch caused the interface to go up down. 51 00:05:42,150 --> 00:05:48,240 In other words the interface was administratively up but the line protocol was down which caused an 52 00:05:48,240 --> 00:05:53,520 issue with route exchange and advertisement which meant that we couldn't send traffic from the left 53 00:05:53,520 --> 00:05:57,520 end side of the network to the right end side of the network. 54 00:05:57,780 --> 00:06:03,100 If you enjoyed the video please like it and please subscribe to my YouTube channel. 55 00:06:03,180 --> 00:06:04,700 I wish you all the very best.