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I’d like to show you an example of automatic summarization

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or auto summarization because it can cause confusion.

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Please note that the network on the left-hand side

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is a class A subnet 10.1.1.0/24

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this is a subnet of the classful class A network 10.0.0.0

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on the right-hand side in this topology

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we also have a class A subnet but between router 1

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and router 3 we have 2 class B subnets.

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Now classful routing protocols automatically summarize

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when going across classful boundaries.

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So when moving from a class A network to class C network

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or class B network in this example

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what happens is that router 1 will advertise network 10.1.1.0/24

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as 10.0.0.0/8 to router 2.

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In other words, it’s summarizing to a classful boundary

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in other words /8 because this is a class A network.

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Now in the same away router 3 will advertise network 10.1.2.0

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to a router 2 as 10.0.0.0/8 it is also automatically summarizing

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because there’s a class boundary on router 3 between a class B and a class A network.

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Now router 2 received the same network from both neighbors.

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Now router 2 believes that it has equal cost path

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to network 10.0.0.0 in other words, it can get to that network via router 1

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and router 3, it thinks that network 10.0.0.0 exist on the left hand-side

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as well as on the right hand side in this topology.

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So as an example, if you ping from router 2

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to IP address 10.1.1.1

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router 2 will see that it’s routing table

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it’s got 2 equal cost paths to get to that IP address

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because it’s part of network 10.0.0.0/8

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and what it's going to do is send 1 packet to router 1

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and then the next packet to router 3

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1 packet to router 1, next packet to router 3 and so forth and so on.

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In other words, it’s going to send alternate packets

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to each neighboring router

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because it believes that it has equal cost paths to that IP address

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and can load balance across both F0/0 and F0/1.

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The result is that half your pings will succeed and half of them will fail.