Routing Table Question

[Craddockc]

Community,

I am studying for the CCNP ROUTE exam and noticed something interesting in the routing table for one of the routers in my GNS3 Lab that I have never noticed before:

When the router didnt have a locally connected 10.0.0.0/8 network it presented the routes for the 10.0.0.0 network like this:

10.0.0.0/24 is subnetted, 4 subnets
D 10.9.0.0 [90/409600] via 199.0.0.1, 02:12:18, Ethernet0/1
D 10.10.0.0 [90/409600] via 199.0.0.1, 02:12:18, Ethernet0/1
D 10.11.0.0 [90/409600] via 199.0.0.9, 02:12:11, Ethernet0/2
D 10.12.0.0 [90/409600] via 199.0.0.9, 02:12:11, Ethernet0/2

However, when I add a loopback interface in that major network to that router, the RT now looks like this:

10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
D 10.9.0.0/24 [90/409600] via 199.0.0.1, 02:15:04, Ethernet0/1
D 10.10.0.0/24 [90/409600] via 199.0.0.1, 02:15:04, Ethernet0/1
D 10.11.0.0/24 [90/409600] via 199.0.0.9, 02:14:57, Ethernet0/2
D 10.12.0.0/24 [90/409600] via 199.0.0.9, 02:14:57, Ethernet0/2
C 10.20.0.0/24 is directly connected, Loopback1
L 10.20.0.1/32 is directly connected, Loopback1

Im noticing it now says 10.0.0.0/8 is variably subnetted and now provides the subnet mask with each individual prefix. Where when the router didnt have that subnet locally connected it didnt look like that. Can someone please help me understand this?

Thanks. 

[Meheretab Mengistu · Accepted solution]

Hi,

10.0.0.0/24 is subnetted, 4 subnets
D 10.9.0.0 [90/409600] via 199.0.0.1, 02:12:18, Ethernet0/1
D 10.10.0.0 [90/409600] via 199.0.0.1, 02:12:18, Ethernet0/1
D 10.11.0.0 [90/409600] via 199.0.0.9, 02:12:11, Ethernet0/2
D 10.12.0.0 [90/409600] via 199.0.0.9, 02:12:11, Ethernet0/2

And, 

10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
D 10.9.0.0/24 [90/409600] via 199.0.0.1, 02:15:04, Ethernet0/1
D 10.10.0.0/24 [90/409600] via 199.0.0.1, 02:15:04, Ethernet0/1
D 10.11.0.0/24 [90/409600] via 199.0.0.9, 02:14:57, Ethernet0/2
D 10.12.0.0/24 [90/409600] via 199.0.0.9, 02:14:57, Ethernet0/2
C 10.20.0.0/24 is directly connected, Loopback1
L 10.20.0.1/32 is directly connected, Loopback1

It is changed when you start using more than one subnet mask.

HTH,

Meheretab

[paul driver]

Hello

10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
These are called "parent/level 1 routes"  which are create automatically when a route with a higher subnetmask of the routes default subnetmask class is added to the route table which doesn't have an exit interface (loopbacks), if you notice it doesn't even have a route identification next to it or even a interface.

C 10.20.0.0/24 is directly connected, Loopback1
These are called "child route/level2 route" which are automatically created and have a subnetmask the same or higher than it default class subnetmask and is a "child" of a parent route..

D 10.9.0.0/24 [90/409600] via 199.0.0.1, 02:15:04, Ethernet0/1
These are called "network/ultimate route" these have a next-hop or exit interface

L 10.20.0.1/32 is directly connected, Loopback1
These are called a "local route" which are routes automatically created showing the actual ip address of an configured active interface.

[Craddockc]

Thank you both for the great explanations! I appreciate it!