Prerequisites: CCNP level skills.
Topology
Task List
Step 1
In R1 configure two additional loopback interfaces with IP addresses 192.168.1.1/28 and 192.168.1.17/28. Advertise these IP addresses as OSPF external prefixes. 192.168.1.0/28 should show as E1, 192.168.1.16/28 should show as E2.
Step 2
Check if R3 and R5 receive these prefixes.
Step3
Configure area 1 so R3 and R5 do not receive any LSA type 5 but have reachability towards 192.168.1.1 and 192.168.1.17.
Step 4
Check the results.
Lab Solution
Step1
In R1 configure two additional loopback interfaces with IP addresses 192.168.1.1/28 and 192.168.1.17/28. Advertise these IP addresses as OSPF external prefixes. 192.168.1.0/28 should show as E1, 192.168.1.16/28 should show as E2.
R1 Configuration:
!
interface Loopback1
ip address 192.168.1.1 255.255.255.240
!
interface Loopback2
ip address 192.168.1.17 255.255.255.240
!
route-map CONNECTED permit 10
match interface Loopback1
set metric-type type-1
!
route-map CONNECTED permit 20
match interface Loopback2
set metric-type type-2
!
router ospf 1
router-id 1.1.1.1
log-adjacency-changes
redistribute connected subnets route-map CONNECTED
network 10.1.12.1 0.0.0.0 area 0
network 10.1.13.1 0.0.0.0 area 1
network 172.16.101.1 0.0.0.0 area 0
interface Loopback1
ip address 192.168.1.1 255.255.255.240
!
interface Loopback2
ip address 192.168.1.17 255.255.255.240
!
route-map CONNECTED permit 10
match interface Loopback1
set metric-type type-1
!
route-map CONNECTED permit 20
match interface Loopback2
set metric-type type-2
!
router ospf 1
router-id 1.1.1.1
log-adjacency-changes
redistribute connected subnets route-map CONNECTED
network 10.1.12.1 0.0.0.0 area 0
network 10.1.13.1 0.0.0.0 area 1
network 172.16.101.1 0.0.0.0 area 0
!
Step 2
Check if R3 and R5 receive these prefixes.
Pic. 2 - External Prefixes in R5.
Pic. 3 - External Prefixes in R3.
Step 3
Configure area 1 so R3 and R5 do not receive any LSA type 5 but have reachability towards 192.168.1.1 and 192.168.1.17.
Note!
All routers in the area 1 must be configured as 'stub' since this is the flag that must match between neighbors along with: hello/dead intervals, area ID, authentication.
R1 Configuration:
!
router ospf 1
router-id 1.1.1.1
log-adjacency-changes
area 1 stub
redistribute connected subnets route-map CONNECTED
network 10.1.12.1 0.0.0.0 area 0
network 10.1.13.1 0.0.0.0 area 1
network 172.16.101.1 0.0.0.0 area 0
router-id 1.1.1.1
log-adjacency-changes
area 1 stub
redistribute connected subnets route-map CONNECTED
network 10.1.12.1 0.0.0.0 area 0
network 10.1.13.1 0.0.0.0 area 1
network 172.16.101.1 0.0.0.0 area 0
!
R3 Configuration:
!
router ospf 1
router-id 3.3.3.3
log-adjacency-changes
area 1 stub
network 0.0.0.0 255.255.255.255 area 1
router-id 3.3.3.3
log-adjacency-changes
area 1 stub
network 0.0.0.0 255.255.255.255 area 1
!
R5 Configuration:
!
router ospf 1
router-id 5.5.5.5
log-adjacency-changes
area 1 stub
network 0.0.0.0 255.255.255.255 area 1
router-id 5.5.5.5
log-adjacency-changes
area 1 stub
network 0.0.0.0 255.255.255.255 area 1
!
Step 4
Check the results.
Note!
ABR (R1) filters out LSA5 (external routes) while sending updates into area 1. Instead, it installs a default route on R3 (further passed onto R5). This allows R3 and R5 to reach external destinations.
Pic.4 - R3's Routing Table.
Pic. 5 - R5's Routing Table.
Pic. 6 - R5's Reachability to External Prefixes.
