I’d like to show you an example of automatic summarization
or auto summarization because it can cause confusion.
Please note that the network on the left-hand side
is a class A subnet 10.1.1.0/24
this is a subnet of the classful class A network 10.0.0.0
on the right-hand side in this topology
we also have a class A subnet but between router 1
and router 3 we have 2 class B subnets.
Now classful routing protocols automatically summarize
when going across classful boundaries.
So when moving from a class A network to class C network
or class B network in this example
what happens is that router 1 will advertise network 10.1.1.0/24
as 10.0.0.0/8 to router 2.
In other words, it’s summarizing to a classful boundary
in other words /8 because this is a class A network.
Now in the same away router 3 will advertise network 10.1.2.0
to a router 2 as 10.0.0.0/8 it is also automatically summarizing
because there’s a class boundary on router 3 between a class B and a class A network.
Now router 2 received the same network from both neighbors.
Now router 2 believes that it has equal cost path
to network 10.0.0.0 in other words, it can get to that network via router 1
and router 3, it thinks that network 10.0.0.0 exist on the left hand-side
as well as on the right hand side in this topology.
So as an example, if you ping from router 2
to IP address 10.1.1.1
router 2 will see that it’s routing table
it’s got 2 equal cost paths to get to that IP address
because it’s part of network 10.0.0.0/8
and what it's going to do is send 1 packet to router 1
and then the next packet to router 3
1 packet to router 1, next packet to router 3 and so forth and so on.
In other words, it’s going to send alternate packets
to each neighboring router
because it believes that it has equal cost paths to that IP address
and can load balance across both F0/0 and F0/1.
The result is that half your pings will succeed and half of them will fail.