CORRECT TEXT
(OSPF Sim)
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 subnetwork 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 area. 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