Classful Routing Protocol Concepts

• Classful routing protocols do not include subnet mask information in their routing updates.
• A router sends the entire subnet address when an update packet involves a subnet of the same classful network as the IP address of the transmitting interface.
• If sending an update about a subnet of a network across an interface belonging to a different network, the router will send the classful summary route. This is called autosummarization across the network boundary.

Example:

Dynagen configuration:

autostart = False
ghostios = true
sparsemem = true

[localhost]

[[7200]]
image = C7200.BIN
# On Linux / Unix use forward slashes:
# image = /opt/7200-images/c7200-jk9o3s-mz.124-7a.image
npe = npe-400
ram = 160

[[ROUTER A]]
S1/0 = B s1/0
model = 7200
console = 2001
idlepc = 0x6082d7a0

[[router B]]
s1/1 = C s1/0
model = 7200
console = 2002
idlepc = 0x607016a0

[[router C]]
model = 7200
console = 2003
idlepc = 0x607016a0

Router A Config:

interface FastEthernet0/0
 ip address 10.1.0.1 255.255.0.0
 duplex half
 no keepalive
!
interface Serial1/0
 ip address 10.2.0.1 255.255.0.0
 serial restart-delay 0
!
router rip
 network 10.0.0.0
!
ip classless

Router B Config:

interface FastEthernet0/0
 no ip address
 shutdown
 duplex half
!
interface Serial1/0
 ip address 10.2.0.2 255.255.0.0
 serial restart-delay 0
!
interface Serial1/1
 ip address 172.16.2.2 255.255.255.0
 serial restart-delay 0
!
router rip
 network 10.0.0.0
 network 172.16.0.0
!
ip classless

Router C Config:

interface FastEthernet0/0
 ip address 172.16.1.1 255.255.255.0
 duplex half
 no keepalive
!
interface Serial1/0
 ip address 172.16.2.1 255.255.255.0
 serial restart-delay 0
!
router rip
 network 172.16.0.0
!
ip classless

Router B: show ip route
Gateway of last resort is not set

     172.16.0.0/24 is subnetted, 2 subnets
R       172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial1/1
C       172.16.2.0 is directly connected, Serial1/1
     10.0.0.0/16 is subnetted, 2 subnets
C       10.2.0.0 is directly connected, Serial1/0
R       10.1.0.0 [120/1] via 10.2.0.1, 00:00:21, Serial1/0

• In the output above, Router A advertises the 10.1.0.0 subnet to router B because the interface connecting them belongs to the same major classful 10.0.0.0 network. When router B receives the update packet, it assumes that the 10.1.0.0 subnet uses the same 16-bit mask as the one used on its 10.2.0.0 subnet.
• Similarly, Router C advertises the 172.16.1.0 subnet to router B because the interface connecting them belongs to the same major classful 172.16.0.0 network. Therefore, router B’s routing table has information about all the subnets that are in use in the network.

Router A: show ip route
Gateway of last resort is not set

R    172.16.0.0/16 [120/1] via 10.2.0.2, 00:00:16, Serial1/0
     10.0.0.0/16 is subnetted, 2 subnets
C       10.2.0.0 is directly connected, Serial1/0
C       10.1.0.0 is directly connected, FastEthernet0/0

• In the output above however, router B summarizes the 172.16.1.0 and 172.16.2.0 subnets to 172.16.0.0 before sending them to router A. Therefore, router A’s routing table contains summary information about only the 172.16.0.0 network.

Router C: show ip route
Gateway of last resort is not set

172.16.0.0/24 is subnetted, 2 subnets
C       172.16.1.0 is directly connected, FastEthernet0/0
C       172.16.2.0 is directly connected, Serial1/0
R    10.0.0.0/8 [120/1] via 172.16.2.2, 00:00:02, Serial1/0 

• Similarly above, router B summarizes the 10.1.0.0 and 10.2.0.0 subnets to 10.0.0.0 before sending the routing information to router C. This summarization occurs because the update crosses a major network boundary. The update goes from a subnet of network 10.0.0.0, subnet 10.2.0.0, to a subnet of another major network, network 172.16.0.0. Router C’s routing table contains summary information about only the 10.0.0.0 network.