Would you like to inspect the original subtitles? These are the user uploaded subtitles that are being translated: 1 00:00:00,000 --> 00:00:05,000 align:middle line:84% I’d like to show you an example of automatic summarization 2 00:00:05,000 --> 00:00:09,000 align:middle line:84% or auto summarization because it can cause confusion. 3 00:00:09,000 --> 00:00:12,000 align:middle line:84% Please note that the network on the left-hand side 4 00:00:12,000 --> 00:00:17,000 align:middle line:84% is a class A subnet 10.1.1.0/24 5 00:00:17,000 --> 00:00:23,000 align:middle line:84% this is a subnet of the classful class A network 10.0.0.0 6 00:00:23,000 --> 00:00:25,000 align:middle line:84% on the right-hand side in this topology 7 00:00:25,000 --> 00:00:32,000 align:middle line:84% we also have a class A subnet but between router 1 8 00:00:32,000 --> 00:00:37,000 align:middle line:84% and router 3 we have 2 class B subnets. 9 00:00:37,000 --> 00:00:41,000 align:middle line:84% Now classful routing protocols automatically summarize 10 00:00:41,000 --> 00:00:44,000 align:middle line:84% when going across classful boundaries. 11 00:00:44,000 --> 00:00:48,000 align:middle line:84% So when moving from a class A network to class C network 12 00:00:48,000 --> 00:00:51,000 align:middle line:84% or class B network in this example 13 00:00:51,000 --> 00:00:59,000 align:middle line:84% what happens is that router 1 will advertise network 10.1.1.0/24 14 00:00:59,000 --> 00:01:05,000 align:middle line:84% as 10.0.0.0/8 to router 2. 15 00:01:05,000 --> 00:01:09,000 align:middle line:84% In other words, it’s summarizing to a classful boundary 16 00:01:09,000 --> 00:01:14,000 align:middle line:84% in other words /8 because this is a class A network. 17 00:01:14,000 --> 00:01:20,000 align:middle line:84% Now in the same away router 3 will advertise network 10.1.2.0 18 00:01:20,000 --> 00:01:29,000 align:middle line:84% to a router 2 as 10.0.0.0/8 it is also automatically summarizing 19 00:01:29,000 --> 00:01:36,000 align:middle line:84% because there’s a class boundary on router 3 between a class B and a class A network. 20 00:01:36,000 --> 00:01:41,000 align:middle line:84% Now router 2 received the same network from both neighbors. 21 00:01:41,000 --> 00:01:46,000 align:middle line:84% Now router 2 believes that it has equal cost path 22 00:01:46,000 --> 00:01:53,000 align:middle line:84% to network 10.0.0.0 in other words, it can get to that network via router 1 23 00:01:53,000 --> 00:02:01,000 align:middle line:84% and router 3, it thinks that network 10.0.0.0 exist on the left hand-side 24 00:02:01,000 --> 00:02:04,000 align:middle line:84% as well as on the right hand side in this topology. 25 00:02:04,000 --> 00:02:08,000 align:middle line:84% So as an example, if you ping from router 2 26 00:02:08,000 --> 00:02:11,000 align:middle line:84% to IP address 10.1.1.1 27 00:02:11,000 --> 00:02:14,000 align:middle line:84% router 2 will see that it’s routing table 28 00:02:14,000 --> 00:02:19,000 align:middle line:84% it’s got 2 equal cost paths to get to that IP address 29 00:02:19,000 --> 00:02:23,000 align:middle line:84% because it’s part of network 10.0.0.0/8 30 00:02:23,000 --> 00:02:27,000 align:middle line:84% and what it's going to do is send 1 packet to router 1 31 00:02:27,000 --> 00:02:29,000 align:middle line:84% and then the next packet to router 3 32 00:02:29,000 --> 00:02:35,000 align:middle line:84% 1 packet to router 1, next packet to router 3 and so forth and so on. 33 00:02:35,000 --> 00:02:39,000 align:middle line:84% In other words, it’s going to send alternate packets 34 00:02:39,000 --> 00:02:41,000 align:middle line:84% to each neighboring router 35 00:02:41,000 --> 00:02:45,000 align:middle line:84% because it believes that it has equal cost paths to that IP address 36 00:02:45,000 --> 00:02:53,000 align:middle line:84% and can load balance across both F0/0 and F0/1. 37 00:02:53,000 --> 00:02:58,000 align:middle line:84% The result is that half your pings will succeed and half of them will fail. 4305