2. Scenario
ISP Network
ISP
ISP has static routes for
172.16.0.0/16 and RTA has a static default
192.168.1.0/24 to RTA route 0.0.0.0/0 to ISP
Central Office Remote Office
172.16.1.0/24
RTA
172.16.4.0/24
172.16.2.0/24
172.16.3.0/24
172.16.5.0/24
192.168.1.0/24
RTB RTC
Customer Network
172.16.0.0/16 and 192.168.1.0/24
Rick Graziani graziani@cabrillo.edu 2
3. Situation Normal
RouterA#show ip route
172.16.0.0/24 is subnetted, 3 subnets
C 172.16.1.0 is directly connected, Ethernet0
C 172.16.2.0 is directly connected, Ethernet1
R 172.16.3.0 [120/1] via 172.16.2.1, 00:00:20, Ethernet1
R 172.16.4.0 [120/2] via 172.16.2.1, 00:00:20, Ethernet1
S* 0.0.0.0/0 is directly connected, Serial1
Rick Graziani graziani@cabrillo.edu 3
4. Link Down
X
Rick Graziani graziani@cabrillo.edu 4
5. Link Down
X
RouterA#show ip route
172.16.0.0/24 is subnetted, 3 subnets
C 172.16.1.0 is directly connected, Ethernet0
C 172.16.2.0 is directly connected, Ethernet1
S* 0.0.0.0/0 is directly connected, Serial1
Rick Graziani graziani@cabrillo.edu 5
6. Link down – Solution
#1: no ip classless X
RouterA#show ip route
172.16.0.0/24 is subnetted, 3 subnets
C 172.16.1.0 is directly connected, Ethernet0
C 172.16.2.0 is directly connected, Ethernet1
S* 0.0.0.0/0 is directly connected, Serial1
Rick Graziani graziani@cabrillo.edu 6
7. Link down – Solution
#2: Discard Route
X
RTA(config)# ip route 172.16.0.0 255.255.0.0 null0
Rick Graziani graziani@cabrillo.edu 7
8. Link down – Solution
#2: Discard Route
X
RTA(config)# ip route 172.16.0.0 255.255.0.0 null0
RouterA#show ip route
172.16.0.0/16 is subnetted, 3 subnets, 2 masks
S 172.16.0.0/16 is directly connected, Null0
C 172.16.1.0/24 is directly connected, Ethernet0
C 172.16.2.0/24 is directly connected, Ethernet1
S* 0.0.0.0/0 is directly connected, Serial1
Rick Graziani graziani@cabrillo.edu 8
9. Link down – Solution #2: Discard Route
RouterA#show ip route
172.16.0.0/16 is subnetted, 3 subnets, 2 masks
S 172.16.0.0/16 is directly connected, Null0
C 172.16.1.0/24 is directly connected, Ethernet0
C 172.16.2.0/24 is directly connected, Ethernet1
S* 0.0.0.0/0 is directly connected, Serial1
• When all links are up…
RouterA#show ip route
172.16.0.0/16 is subnetted, 5 subnets, 2 masks
S 172.16.0.0/16 is directly connected, Null0
C 172.16.1.0/24 is directly connected, Ethernet0
C 172.16.2.0/24 is directly connected, Ethernet1
R 172.16.3.0/24 [120/1] via 172.16.2.1, 00:00:20, Ethernet1
R 172.16.4.0/24 [120/2] via 172.16.2.1, 00:00:20, Ethernet1
S* 0.0.0.0/0 is directly connected, Serial1
Rick Graziani graziani@cabrillo.edu 9
