SIMULATION
OSPF is configured on routers Amani and Lynaic. Amani’s S0/0 interface and Lynaic’s S0/1
interface are in Area 0. Lynaic’s Loopback0 interface is in Area 2.
Your task is to configure the following:
• Portland’s S0/0 interface in Area 1
• Amani’s S0/1 interface in Area 1
• Use the appropriate mask such that ONLY Portland’s S0/0 and Amnani’s S0/1 could
be in Area 1.
Area 1 should not receive any external or inter-area routes (except the default route).
Answer: See the explanation
Explanation:
First, we configure Portland’s S0/0 interface so that it belongs to Area 1. So, we have to find
out which sub-network the IP address 192.168.4.5/30 (the IP of interface S0/0 of Portland)
belongs to.
This address belongs to a subnetwork which has:
Increment: 4 (/30 = 255.255.255.252 or 1111 1111.1111 1111.1111 1111.1111 1100)
Network address: 192.168.4.4 (because 4 = 4 * 1 and 4 < 5)
Broadcast address: 192.168.4.7 (because 7 = 4 + 4 – 1) (It is not necessary to find out the
broadcast address but we should know it)
The question requires that only Portland’s S0/0 and Amani’s S0/1 could be in Area 1,
therefore we must use a wildcard of 0.0.0.3 (this wildcard is equivalent with a subnet mask of
/30) so that there are only 2 IP addresses can participate in area 1 (they are 192.168.4.5 &
192.168.4.6).
The full command we use here is network 192.168.4.4 0.0.0.3 area 1
The question also requires that “Area 1 should not receive any external or inter-area routes
(except the default route)”. Recall that if we don’t want the router to receive external routes,
we have to stop LSA Type 5. And if we don’t want to receive inter-area routes, we have to
stop LSA Type 3 and Type 4. Therefore we have to configure area 1 as a totally stubby are
A)
For your information, here is the definition of a totally stubby area:“Totally stubb area – This area does not accept summary LSAs from other areas (types 3 or
4)
or external summary LSAs (Type 5). Types 3,4 and 5 LSAs are replaced by the Area Border
Router(ABR) with a default router. Totally stubby areas protect internal routers by minimizing
the routing table and summarizing everything outside the area with a default route.” (CCNP
BSCI Official Exam Certification Guide, Fourth Edition)
In conclusion, we have to configure area 1 as a totally stubby areA. We do that by
configuring
Portland as stub and configuring Amani (ABR router) as a stub + “no-summary”suffix.
+ Configure Portland router as a stub:
Portland#configure terminal
Portland(config)#router ospf 1
Allow network 192.168.4.4/30 to join Area 1, notice that you have to convert subnet mask
into wildcard mask:
Portland(config-router)#network 192.168.4.4 0.0.0.3 area 1
Configure Portland as a stub:
Portland(config-router)#area 1 stub
Portland(config-router)#end
Portland#copy running-config startup-config
+ Configure Amani router as a “totally stub”:
Amani#configure terminal
Amani(config)#router ospf 1
Amani(config-router)#network 192.168.4.4 0.0.0.3 area 1
Make area 1 become a totally stubby area, notice that we can only use this command on
ABR router:
Amani(config-router)#area 1 stub no-summary
Amani(config-router)#end
Amani#copy running-config startup-config