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These are the user uploaded subtitles that are being translated: 1 00:00:00,000 --> 00:00:02,000 align:middle line:84% So I want to teach you a trick 2 00:00:02,000 --> 00:00:06,000 align:middle line:84% now this doesn’t always apply, it only works in certain situations 3 00:00:06,000 --> 00:00:08,000 align:middle line:84% but it saves you a lot of a time 4 00:00:08,000 --> 00:00:12,000 align:middle line:84% if you remember back to your Binary, this bit is 128 5 00:00:12,000 --> 00:00:16,000 align:middle line:84% this bit is 64, this is 32, this is 16 6 00:00:16,000 --> 00:00:22,000 align:middle line:84% this is 8, this is 4, this is 2 and that is 1 7 00:00:22,000 --> 00:00:30,000 align:middle line:84% so 255 in decimal and an IP address would be an octet populated with binary 1's 8 00:00:30,000 --> 00:00:35,000 align:middle line:84% please refer back to the ICND 1 course if you can’t remember binary 9 00:00:35,000 --> 00:00:38,000 align:middle line:84% but hopefully, at the point, you're fairly comfortable with it. 10 00:00:38,000 --> 00:00:41,000 align:middle line:84% If you were given subnets where for instance 11 00:00:41,000 --> 00:00:45,000 align:middle line:84% the third octet was in the range 4 to 7 12 00:00:45,000 --> 00:00:49,000 align:middle line:84% in other words, from 4 to 1 less than 8 so 7 13 00:00:49,000 --> 00:00:52,000 align:middle line:84% you could summarize that automatically as 4 14 00:00:52,000 --> 00:01:02,000 align:middle line:84% so for example, let’s say you’re given 172.16.4.0/24 up to 172.16.7.0/24 15 00:01:02,000 --> 00:01:07,000 align:middle line:84% so notice in the third octet the range is from 4 to 7 16 00:01:07,000 --> 00:01:10,000 align:middle line:84% so in other words, from 4 to 1 less than 8 17 00:01:10,000 --> 00:01:16,000 align:middle line:84% you could immediately write the answer as 172.16.4.0 18 00:01:16,000 --> 00:01:21,000 align:middle line:84% now to work out the subnet mask you just remember that the first octet is 8 bits 19 00:01:21,000 --> 00:01:25,000 align:middle line:84% the second octet is 8 bits and that’s 16 20 00:01:25,000 --> 00:01:28,000 align:middle line:84% and then you need to work out where binary value of 4 is 21 00:01:28,000 --> 00:01:37,000 align:middle line:84% so let's count 1 2 3 4 5 6, so it’s in binary bits 6 22 00:01:37,000 --> 00:01:47,000 align:middle line:84% so 8 + 8 = 16 + 6 binary bits which we’ve not counted to see where 4 is 23 00:01:47,000 --> 00:01:52,000 align:middle line:84% gives you 22, so the mask would be 22 8 + 8 + 6 24 00:01:52,000 --> 00:01:55,000 align:middle line:84% and it’s a simple as that 25 00:01:55,000 --> 00:01:58,000 align:middle line:84% to work out the answers to a question likes this 26 00:01:58,000 --> 00:02:01,000 align:middle line:84% by the same token if you were given an example 27 00:02:01,000 --> 00:02:04,000 align:middle line:84% where the values was from 8 to 15 28 00:02:04,000 --> 00:02:08,000 align:middle line:84% in other words 8 to 1 less than 16 29 00:02:08,000 --> 00:02:10,000 align:middle line:84% you could summarize that immediately as 8. 30 00:02:10,000 --> 00:02:20,000 align:middle line:84% So let’s say for example it was 10.8.0.0/16 up to 10.15.0.0/16 31 00:02:20,000 --> 00:02:24,000 align:middle line:84% in other words, from 8 to 1 less than 16 32 00:02:24,000 --> 00:02:30,000 align:middle line:84% you could summarize it automatically as 10.8.0.0 33 00:02:30,000 --> 00:02:35,000 align:middle line:84% so in other words, were saying if it's from this binary value 8 34 00:02:35,000 --> 00:02:37,000 align:middle line:84% up to 1 less than the next binary value 35 00:02:37,000 --> 00:02:41,000 align:middle line:84% you just summarize it down to this binary value of 8 36 00:02:41,000 --> 00:02:44,000 align:middle line:84% finally, to work out the subnet mask you need to remember 37 00:02:44,000 --> 00:02:49,000 align:middle line:84% that the first octet is 8 bits and then work out where 8 is 38 00:02:49,000 --> 00:02:53,000 align:middle line:84% so 8 is 1 2 3 4 5 39 00:02:53,000 --> 00:02:56,000 align:middle line:84% so 8 + 5 will give you 13 40 00:02:56,000 --> 00:03:01,000 align:middle line:84% 8 binary bits + 5 binary bits gives you 13 41 00:03:01,000 --> 00:03:06,000 align:middle line:84% so the mask is 13, by the same token 16 to 31 42 00:03:06,000 --> 00:03:10,000 align:middle line:84% so 1 less than 32 can be summarized to 16 43 00:03:10,000 --> 00:03:15,000 align:middle line:84% 32 to 63 in other words, 1 less than 64 44 00:03:15,000 --> 00:03:18,000 align:middle line:84% so 32 to 63 can be summarize to 32 45 00:03:18,000 --> 00:03:22,000 align:middle line:84% 64 to 1 less than 128 in other words 127 46 00:03:22,000 --> 00:03:26,000 align:middle line:84% so 64 to 127 can be summarize as 64 47 00:03:26,000 --> 00:03:31,000 align:middle line:84% now I’ve already shown you those examples by working it out in binary 48 00:03:31,000 --> 00:03:36,000 align:middle line:84% just to remind you 64 up to 127 49 00:03:36,000 --> 00:03:43,000 align:middle line:84% we work out in binary and work out the answer as 172.16.64.0 50 00:03:43,000 --> 00:03:48,000 align:middle line:84% so once again, 64 to 127 can be summarized as 64 51 00:03:48,000 --> 00:03:51,000 align:middle line:84% and then you count the number of common bits 52 00:03:51,000 --> 00:03:58,000 align:middle line:84% so 8 + 8 + 2 because 64 is in the second binary bit position 53 00:03:58,000 --> 00:04:00,000 align:middle line:84% giving you a total of 18 54 00:04:00,000 --> 00:04:02,000 align:middle line:84% so, therefore, you can work out this answer 55 00:04:02,000 --> 00:04:05,000 align:middle line:84% in a matter of seconds rather than minutes 56 00:04:05,000 --> 00:04:12,000 align:middle line:84% this example with 172.16.32.0 up to 172.16.63.0 57 00:04:12,000 --> 00:04:14,000 align:middle line:84% can quickly and easily be summarized as 172.16.32.0 58 00:04:14,000 --> 00:04:19,000 align:middle line:84% 19 bits are in common and the way we work that out 59 00:04:19,000 --> 00:04:23,000 align:middle line:84% is 8 bits in the first octet + 8 bits in the second octet 60 00:04:23,000 --> 00:04:29,000 align:middle line:84% is 16 + 32 is in the third binary bit position 61 00:04:29,000 --> 00:04:32,000 align:middle line:84% so 3 bits gives you a total of 19 62 00:04:32,000 --> 00:04:36,000 align:middle line:84% so I’m hoping this trick will save you quite a bit of time 63 00:04:36,000 --> 00:04:39,000 align:middle line:84% when working out summarization please be careful though 64 00:04:39,000 --> 00:04:45,000 align:middle line:84% if you are given an example of let say 16 to 35 65 00:04:45,000 --> 00:04:47,000 align:middle line:84% you're going to have to split up your summary 66 00:04:47,000 --> 00:04:52,000 align:middle line:84% the 16 to 31 subnets can easily summarize very quickly 67 00:04:52,000 --> 00:04:55,000 align:middle line:84% but if the question asks you to summarize subnets 68 00:04:55,000 --> 00:04:58,000 align:middle line:84% that go across this bit boundaries 69 00:04:58,000 --> 00:05:00,000 align:middle line:84% then you would have to work it out in binary 70 00:05:00,000 --> 00:05:02,000 align:middle line:84% but this will hopefully save you a bit of time 71 00:05:02,000 --> 00:05:04,000 align:middle line:84% also be careful if you're given an example 72 00:05:04,000 --> 00:05:08,000 align:middle line:84% where you're asked to summarize from 16 to let say 19 73 00:05:08,000 --> 00:05:11,000 align:middle line:84% and you use this example that I’ve explain 74 00:05:11,000 --> 00:05:15,000 align:middle line:84% you’ll be summarizing more than just those subnets 75 00:05:15,000 --> 00:05:18,000 align:middle line:84% so it will be better, in that case, to do it in binary 76 00:05:18,000 --> 00:05:21,000 align:middle line:84% So what are the advantages of VLSM and summarization? 77 00:05:21,000 --> 00:05:25,000 align:middle line:84% We get more efficient use of the IP address space 78 00:05:25,000 --> 00:05:28,000 align:middle line:84% so rather than for instance having to use a /24 mask 79 00:05:28,000 --> 00:05:32,000 align:middle line:84% on a serial link which consumes 254 host addresses 80 00:05:32,000 --> 00:05:37,000 align:middle line:84% we can use a /30 mask which only needs to, there are fewer updates 81 00:05:37,000 --> 00:05:39,000 align:middle line:84% because we can hide network changes 82 00:05:39,000 --> 00:05:42,000 align:middle line:84% or topology changes by sending a summary root 83 00:05:42,000 --> 00:05:46,000 align:middle line:84% rather than individual networks or subnets to other devices 84 00:05:46,000 --> 00:05:50,000 align:middle line:84% it also allows us to implement hierarchical levels 85 00:05:50,000 --> 00:05:55,000 align:middle line:84% for better route summarization, so in the real world VLSM and route summarization 86 00:05:55,000 --> 00:06:01,000 align:middle line:84% are used very heavily to conserve IP addresses and reduce routing table sizes 87 00:06:01,000 --> 00:06:06,000 align:middle line:84% so here’s an example of address hiding and topology change hiding 88 00:06:06,000 --> 00:06:10,000 align:middle line:84% the router on the right-hand side only receives 1 route 89 00:06:10,000 --> 00:06:14,000 align:middle line:84% from the route from the left-hand side 10.1.0.0/16 90 00:06:14,000 --> 00:06:21,000 align:middle line:84% so if a more specific subnet like 10.1.12.0/24 went down 91 00:06:21,000 --> 00:06:24,000 align:middle line:84% the router on the right-hand side is oblivious to that fact 92 00:06:24,000 --> 00:06:30,000 align:middle line:84% because it only has 10.1.0.0/16 in its routing table 93 00:06:30,000 --> 00:06:35,000 align:middle line:84% and that’s all that's been advertised to it that route state has not changed 94 00:06:35,000 --> 00:06:37,000 align:middle line:84% and thus the router on the right-hand side 95 00:06:37,000 --> 00:06:41,000 align:middle line:84% does not have to reprocess or re-compute its routing table 96 00:06:41,000 --> 00:06:47,000 align:middle line:84% it is oblivious to the fact that this subnet 10.1.12.0 has gone down 97 00:06:47,000 --> 00:06:53,000 align:middle line:84% because all it sees is the super net or summary of 10.1.0.0/16 98 00:06:53,000 --> 00:06:57,000 align:middle line:84% thus there are major advantages to implementing summarization 99 00:06:57,000 --> 00:07:00,000 align:middle line:84% including topology change hiding 100 00:07:00,000 --> 00:07:04,000 align:middle line:84% however, it’s important that you realize that there’s a difference 101 00:07:04,000 --> 00:07:06,000 align:middle line:84% between what are called classful routing protocols 102 00:07:06,000 --> 00:07:08,000 align:middle line:84% and classles routing protocols 103 00:07:08,000 --> 00:07:13,000 align:middle line:84% classful routing protocols do not include the subnet mask 104 00:07:13,000 --> 00:07:18,000 align:middle line:84% when advertising the network, that means other devices do not know 105 00:07:18,000 --> 00:07:20,000 align:middle line:84% what subnet mask is being used 106 00:07:20,000 --> 00:07:24,000 align:middle line:84% so router assumes and we all know how bad it is to assume 107 00:07:24,000 --> 00:07:27,000 align:middle line:84% but they assume that within the same network 108 00:07:27,000 --> 00:07:29,000 align:middle line:84% there is consistency of the subnet mask 109 00:07:29,000 --> 00:07:32,000 align:middle line:84% in other words, everyone within the same network 110 00:07:32,000 --> 00:07:36,000 align:middle line:84% is using the same subnet mask as everyone else 111 00:07:36,000 --> 00:07:39,000 align:middle line:84% so in other words, when a router's received on an interface 112 00:07:39,000 --> 00:07:42,000 align:middle line:84% the subnet mask for the received route is implied 113 00:07:42,000 --> 00:07:45,000 align:middle line:84% by the subnet mask on the local interface 114 00:07:45,000 --> 00:07:50,000 align:middle line:84% as the router does not know what subnet mask was used by the other routers 115 00:07:50,000 --> 00:07:53,000 align:middle line:84% so it assumes that they are using the same subnet mask as itself. 116 00:07:53,000 --> 00:07:58,000 align:middle line:84% routes will automatically be summarized when going across a classful boundary 117 00:07:58,000 --> 00:08:02,000 align:middle line:84% so summary routes are exchanged when crossing a classful boundary 118 00:08:02,000 --> 00:08:06,000 align:middle line:84% in other words, as an example when going from a 10 network 119 00:08:06,000 --> 00:08:12,000 align:middle line:84% to a 192.168 network or from 10 to 11 and so forth and so on 120 00:08:12,000 --> 00:08:14,000 align:middle line:84% examples of classful routing protocols 121 00:08:14,000 --> 00:08:17,000 align:middle line:84% includes RIP version 1 and IGRP 122 00:08:17,000 --> 00:08:21,000 align:middle line:84% IGRP is no longer supported on the Cisco IOS 123 00:08:21,000 --> 00:08:24,000 align:middle line:84% and RIP version 1 shouldn’t be used in today’s networks 124 00:08:24,000 --> 00:08:27,000 align:middle line:84% but just for completeness, it's mentioned here. 125 00:08:27,000 --> 00:08:32,000 align:middle line:84% Classless routing protocols do include the subnet mask 126 00:08:32,000 --> 00:08:34,000 align:middle line:84% with the network in routing advertisements 127 00:08:34,000 --> 00:08:38,000 align:middle line:84% in other words, classless routing protocols advertise 128 00:08:38,000 --> 00:08:45,000 align:middle line:84% not just the network like 10.1.1.0 but also the associated mask like /24 129 00:08:45,000 --> 00:08:49,000 align:middle line:84% because the subnet mask is included in the routing updates 130 00:08:49,000 --> 00:08:54,000 align:middle line:84% classless routing protocols support Variable Length Subnet Mask or VLSM 131 00:08:54,000 --> 00:08:57,000 align:middle line:84% summary routes can be manually configured 132 00:08:57,000 --> 00:09:00,000 align:middle line:84% so unlike in classful routing protocols 133 00:09:00,000 --> 00:09:04,000 align:middle line:84% where automatic summarization takes place across classful boundaries 134 00:09:04,000 --> 00:09:09,000 align:middle line:84% in classless routing protocols summarization in some cases, for example 135 00:09:09,000 --> 00:09:14,000 align:middle line:84% with EIGRP can be configured on any interface anywhere in the network 136 00:09:14,000 --> 00:09:18,000 align:middle line:84% examples of classless routing protocols include 137 00:09:18,000 --> 00:09:22,000 align:middle line:84% RIP version 2, EIGRP, OSPF and ISIS 138 00:09:22,000 --> 00:09:28,000 align:middle line:84% in this course, we'll concentrate mainly on RIP v2, EIGRP and OSPF 139 00:09:28,000 --> 00:09:32,000 align:middle line:84% but just be aware that there are other routing protocols out there 140 00:09:32,000 --> 00:09:40,000 align:middle line:84% be careful EIGRP and RIP v2 act as classful routing protocols by default 141 00:09:40,000 --> 00:09:44,000 align:middle line:84% you need to use the command no auto summary within the routing process 142 00:09:44,000 --> 00:09:46,000 align:middle line:84% to disable this default behavior 143 00:09:46,000 --> 00:09:50,000 align:middle line:84% so that they act like a classless routing protocol. 144 00:09:50,000 --> 00:09:54,000 align:middle line:84% So let’s look at some of the issues regarding discontiguous networks 145 00:09:54,000 --> 00:09:58,000 align:middle line:84% or discontiguous subnets, the router on the left 146 00:09:58,000 --> 00:10:02,000 align:middle line:84% has a network of 10.1.1.0/24 connected to it 147 00:10:02,000 --> 00:10:07,000 align:middle line:84% this if you remember is a class A subnet, the router on the right 148 00:10:07,000 --> 00:10:13,000 align:middle line:84% has a subnet of 10.1.2.0/24 connected to it also a class A subnet 149 00:10:13,000 --> 00:10:16,000 align:middle line:84% they are both connected to the router at the top 150 00:10:16,000 --> 00:10:22,000 align:middle line:84% with class C addresses of 192.168.1.0 and 192.168.2.0 151 00:10:22,000 --> 00:10:28,000 align:middle line:84% so please note we are going from a class A, to class C, to class A subnet 152 00:10:28,000 --> 00:10:30,000 align:middle line:84% when traversing these routers 153 00:10:30,000 --> 00:10:37,000 align:middle line:84% the problem here is classful routing protocols like RIP v1 and IGRP 154 00:10:37,000 --> 00:10:42,000 align:middle line:84% will automatically summarize this subnets their classful network 155 00:10:42,000 --> 00:10:48,000 align:middle line:84% so 10.1.2.0 Will automatically be summarize as 10.0.0.0 156 00:10:48,000 --> 00:10:52,000 align:middle line:84% the same will take place here, on this router 10.1.1.0 157 00:10:52,000 --> 00:10:55,000 align:middle line:84% will automatically be summarize to 10.0.0.0 158 00:10:55,000 --> 00:10:58,000 align:middle line:84% this causes an issue for the router in the middle 159 00:10:58,000 --> 00:11:01,000 align:middle line:84% because when it wants to go to 10.1.1.0 160 00:11:01,000 --> 00:11:06,000 align:middle line:84% it believes it can send traffic to the left, as well as to the right 161 00:11:06,000 --> 00:11:10,000 align:middle line:84% because it's receiving the same route from multiple routers 162 00:11:10,000 --> 00:11:15,000 align:middle line:84% If this router was pinging a device over here 10.1.1 163 00:11:15,000 --> 00:11:20,000 align:middle line:84% it would only be a 50% success rate because half of the packets 164 00:11:20,000 --> 00:11:23,000 align:middle line:84% will be sent to this network on the right-hand side 165 00:11:23,000 --> 00:11:27,000 align:middle line:84% be careful of routing protocols like EIGRP and RIP v2 166 00:11:27,000 --> 00:11:31,000 align:middle line:84% even though they are classless they act as classful 167 00:11:31,000 --> 00:11:35,000 align:middle line:84% and thus have the same issue, where they automatically summarize 168 00:11:35,000 --> 00:11:39,000 align:middle line:84% a classful boundaries, don't forget to use the command 169 00:11:39,000 --> 00:11:45,000 align:middle line:84% no auto summary under the routing process to disable this behavior 170 00:11:45,000 --> 00:11:49,000 align:middle line:84% Once you've typed that command, the routers will not summarize the networks 171 00:11:49,000 --> 00:11:52,000 align:middle line:84% and they will be advertised in EIGRP in RIP v2 172 00:11:52,000 --> 00:11:58,000 align:middle line:84% as 10.1.1.0/24 as well as 10.1.2.0/24 173 00:11:58,000 --> 00:12:01,000 align:middle line:84% so the router in the middle will be able to correctly route 174 00:12:01,000 --> 00:12:06,000 align:middle line:84% to the various networks OSPF does not have this issue 175 00:12:06,000 --> 00:12:09,000 align:middle line:84% because OSPF does not automatically summarize 176 00:12:09,000 --> 00:12:12,000 align:middle line:84% you have to manually summarize networks. 177 00:12:12,000 --> 00:12:16,000 align:middle line:84% So when does automatic summarization does takes place? 178 00:12:16,000 --> 00:12:23,000 align:middle line:84% well it only affects this routing protocols RIP v2, EIGRP, RIP v1 and IGRP 179 00:12:23,000 --> 00:12:26,000 align:middle line:84% it occurs when you move across classful boundaries 180 00:12:26,000 --> 00:12:30,000 align:middle line:84% in other words, when a subnet is advertised from a class A to class B 181 00:12:30,000 --> 00:12:34,000 align:middle line:84% or B to C or any one of these combinations 182 00:12:34,000 --> 00:12:38,000 align:middle line:84% in other words, when a router has 1 interface in a class A network for example 183 00:12:38,000 --> 00:12:40,000 align:middle line:84% and another interface in a class B network 184 00:12:40,000 --> 00:12:45,000 align:middle line:84% and that advertisement crosses that classful boundary going from A to B 185 00:12:45,000 --> 00:12:48,000 align:middle line:84% the network will automatically be summarized 186 00:12:48,000 --> 00:12:51,000 align:middle line:84% another one that people forget is when you are moving 187 00:12:51,000 --> 00:12:56,000 align:middle line:84% across major network boundaries, automatic summarization will also take place 188 00:12:56,000 --> 00:13:00,000 align:middle line:84% in other words, if you go from a 10 network to an 11 network 189 00:13:00,000 --> 00:13:04,000 align:middle line:84% or to a 12 network automatic summarization will take place 190 00:13:04,000 --> 00:13:10,000 align:middle line:84% notice the major network 10 has changed to 11 or to 12 191 00:13:10,000 --> 00:13:12,000 align:middle line:84% these are all class A networks 192 00:13:12,000 --> 00:13:16,000 align:middle line:84% but you are moving across a major network boundary 193 00:13:16,000 --> 00:13:19,000 align:middle line:84% so if 1 interface on a router is in the 10 network 194 00:13:19,000 --> 00:13:23,000 align:middle line:84% and another interface on a router is in the 11 network 195 00:13:23,000 --> 00:13:25,000 align:middle line:84% there will be automatic summarization. 196 00:13:25,000 --> 00:13:31,000 align:middle line:84% Remember on EIGRP and RIP v2 to type the command no auto-summary 197 00:13:31,000 --> 00:13:34,000 align:middle line:84% because even though they are classless routing protocols 198 00:13:34,000 --> 00:13:36,000 align:middle line:84% they act as classful routing protocols 199 00:13:36,000 --> 00:13:39,000 align:middle line:84% when it comes to automatic summarization 200 00:13:39,000 --> 00:13:43,000 align:middle line:84% now here’s another situation that causes a lot of confusion 201 00:13:43,000 --> 00:13:46,000 align:middle line:84% in ICND 1 you learned about administrative distance 202 00:13:46,000 --> 00:13:48,000 align:middle line:84% and you learned that the lower the administrative distance 203 00:13:48,000 --> 00:13:55,000 align:middle line:84% the more preferable a route is, the administrative distance of RIP v2 is 120 204 00:13:55,000 --> 00:13:58,000 align:middle line:84% the administrative distance of OSPF is 110 205 00:13:58,000 --> 00:14:01,000 align:middle line:84% the administrative distance of EIGRP is 90. 206 00:14:01,000 --> 00:14:07,000 align:middle line:84% So let's assume router 1, router 2 and router 3 have networks in the 10 range 207 00:14:07,000 --> 00:14:13,000 align:middle line:84% connected to them, they are advertising various routes to router 4. 208 00:14:13,000 --> 00:14:17,000 align:middle line:84% So RIP v2 is advertising 10.1.1.0/27 209 00:14:17,000 --> 00:14:25,000 align:middle line:84% OSPF is advertising 10.1.0.0/16 EIGRP is advertising 10.0.0.0/8 210 00:14:25,000 --> 00:14:31,000 align:middle line:84% so router 4 is receiving multiple advertisements in the 10 range 211 00:14:31,000 --> 00:14:36,000 align:middle line:84% but if on router 4 you type the command ping 10.1.1.1 212 00:14:36,000 --> 00:14:41,000 align:middle line:84% which way will a traffic flow, will it go to router 3 213 00:14:41,000 --> 00:14:44,000 align:middle line:84% or will it go to router 2 or will it go to router 1? 214 00:14:44,000 --> 00:14:50,000 align:middle line:84% Now remember EIGRP has a lower administrative distance than OSPF 215 00:14:50,000 --> 00:14:52,000 align:middle line:84% which has a lower administrative distance than RIP 216 00:14:52,000 --> 00:14:56,000 align:middle line:84% but please note administrative distance only comes into play 217 00:14:56,000 --> 00:14:59,000 align:middle line:84% when the same prefix is advertised 218 00:14:59,000 --> 00:15:04,000 align:middle line:84% a prefix is not just the network it's the network and the mask 219 00:15:04,000 --> 00:15:08,000 align:middle line:84% router 4 will see this prefixes 10.1.1.0/27 220 00:15:08,000 --> 00:15:16,000 align:middle line:84% 10.1.0.0/16 and 10.0.0.0/8 as separate prefixes 221 00:15:16,000 --> 00:15:20,000 align:middle line:84% these 3 routes will appear in the routing table with router 4 222 00:15:20,000 --> 00:15:24,000 align:middle line:84% and router 4 will make its decision on the best match. 223 00:15:24,000 --> 00:15:30,000 align:middle line:84% 10.1.1.0/27 is the best match out of these 3 routes. 224 00:15:30,000 --> 00:15:37,000 align:middle line:84% 27 is the most specific, so the most specific or best match will be used 225 00:15:37,000 --> 00:15:39,000 align:middle line:84% and not the administrative distance 226 00:15:39,000 --> 00:15:42,000 align:middle line:84% the administrative distance would only be used 227 00:15:42,000 --> 00:15:47,000 align:middle line:84% if the same route was advertised by multiple routing protocols 228 00:15:47,000 --> 00:15:51,000 align:middle line:84% so in this case, the ping to 10.1.1.1 229 00:15:51,000 --> 00:15:55,000 align:middle line:84% will go to router 1 and not router 2 or router 3 230 00:15:55,000 --> 00:16:00,000 align:middle line:84% however, in this example, notice the same prefix is advertised 231 00:16:00,000 --> 00:16:08,000 align:middle line:84% by the 3 routers 10.0.0.0/8 is advertised by RIP, OSPF and EIGRP 232 00:16:08,000 --> 00:16:12,000 align:middle line:84% in this case only 1 route can be put into the routing table 233 00:16:12,000 --> 00:16:16,000 align:middle line:84% and the choice is done via administrative distance 234 00:16:16,000 --> 00:16:19,000 align:middle line:84% EIGRP having the lowest administrative distance 235 00:16:19,000 --> 00:16:21,000 align:middle line:84% will have its route inserted into a routing table 236 00:16:21,000 --> 00:16:26,000 align:middle line:84% and the ping from router 4 will now go to router 3 237 00:16:26,000 --> 00:16:33,000 align:middle line:84% to sum this up, in this example, there are 3 separate prefixes 238 00:16:33,000 --> 00:16:37,000 align:middle line:84% the router does not see this as the same network 239 00:16:37,000 --> 00:16:40,000 align:middle line:84% it sees them as 3 separate prefixes or subnets 240 00:16:40,000 --> 00:16:43,000 align:middle line:84% all 3 will be put into the routing table 241 00:16:43,000 --> 00:16:48,000 align:middle line:84% and a decision will be made on the best match or longest prefix 242 00:16:48,000 --> 00:16:53,000 align:middle line:84% in this case, 27 is longer than 16, just longer than 8 243 00:16:53,000 --> 00:17:00,000 align:middle line:84% so the RIP v2 route will be chosen, however, where the route is the same route. 244 00:17:00,000 --> 00:17:02,000 align:middle line:84% So in this example 10.0.0.0/8 245 00:17:02,000 --> 00:17:08,000 align:middle line:84% the choice will be made on administrative distance with EIGRP winning 246 00:17:08,000 --> 00:17:11,000 align:middle line:84% because it has the lowest administrative distance 247 00:17:11,000 --> 00:17:15,000 align:middle line:84% please don’t forget this, a lot of engineers make the mistake 248 00:17:15,000 --> 00:17:18,000 align:middle line:84% of assuming that administrative distance is the way choices are made 249 00:17:18,000 --> 00:17:20,000 align:middle line:84% for choosing the best route 250 00:17:20,000 --> 00:17:24,000 align:middle line:84% administrative distance is only chosen as a tie breaker 251 00:17:24,000 --> 00:17:27,000 align:middle line:84% when the same route or prefix is attempted 252 00:17:27,000 --> 00:17:31,000 align:middle line:84% to be put into the routing table by multiple routing protocols 253 00:17:31,000 --> 00:17:34,000 align:middle line:84% So what have we covered? 254 00:17:34,000 --> 00:17:37,000 align:middle line:84% we look at Variable Length Subnet Mask or VLSM 255 00:17:37,000 --> 00:17:41,000 align:middle line:84% we discuss CIDR or Classless Inter-Domain Routing 256 00:17:41,000 --> 00:17:44,000 align:middle line:84% we talked about summarization and the advantages of summarization 257 00:17:44,000 --> 00:17:48,000 align:middle line:84% I showed you examples of how to work out summarized routes 258 00:17:48,000 --> 00:17:51,000 align:middle line:84% I showed you routing choices and how routers will make a choice 259 00:17:51,000 --> 00:17:55,000 align:middle line:84% firstly on most specific match and then secondly on administrative distance 260 00:17:55,000 --> 00:17:59,000 align:middle line:84% and then I showed you some issues regarding discontiguous networks. 261 00:17:59,000 --> 00:18:04,000 align:middle line:84% Thank you for watching! 30814