Refer to the exhibit. A new TAC engineer comes to you for advice. The engineer wants to
configure RIPv2- OSPF two-way redistribution while avoiding routing loops. Which two
additions to the router B1 configuration should theengineer make? (Choose two.)

A.
access-list 40 deny 172.16.1.0 0.0.0.255
access-list 40 permit any
router rip
redistribute ospf 100 metric 5
distribute-list 40 out ospf 100

B.
ip prefix-list rip_routes permit 172.16.1.16/25 ge 26 le 28
route-map redis-ospf deny 10
match ip address prefix-list rip_routes
router rip
redistribute ospf 10 route-map redis-ospf subnets

C.
ip prefix-list rip-to-ospf permit 10.1.1.8/25 ge 26le 28
route-map redis-rip deny 20
match ip address prefix-list rip-to-ospf
router ospf 100
redistribute rip route-map redis-rip subnets

D.
access-list 15 deny 10.1.1.0 0.0.0.63
access-list 15 permit any
route-map redis-rip deny 10
match ip address 15
route-map redis-rip permit 20
router ospf 100
redistribute rip route-map redis-rip subnets

Explanation:
B1 is not the only router that redistributes between RIP & OSPF. The “small” router below
B1 can be configured for this task too so B1 can try to redistribute networks advertised by
that “small” router again. Therefore it is necessary to filter out networks that have been
advertised by the “small” router. For example, we need to prevent network 172.16.1.0/24
from advertised back into RIPv2 or network 10.1.1.0/26 from advertised back into OSPF.
Notice that all networks in OSPF domain (including 10.1.1.8/30, 10.1.1.12/30, 10.1.1.48/28,
10.1.1.32/28) can be summarized as 10.1.1.0/26 and all networks in RIP domain (including
172.16.1.24/30, 172.16.1.20/30, 172.16.1.32/28, 172.16.1.48/28) can be summarized as
172.16.1.0/24 -> answers A & D are correct.
In answer B, the command “ip prefix-list rip_routespermit 172.16.1.16/25 ge 26 le 28″
means:
+ First check the first 25 bits of the address -> this will allow addresses from 172.16.1.0 to
172.16.1.127
+ If those match then check the subnet mask, which in this case can be GREATER THAN
or EQUAL to 26 bits & LESS THAN or EQUAL to 28 bits-> meaning that /26, /27, /28
subnet masks would match.
For example, networks 172.16.1.0/26; 172.16.1.16/28would match (but notice networks
172.16.1.0/25; 172.16.1.128/26 wouldn’t).
In the “ip prefix-list rip_routes permit 172.16.1.16/25 ge 26 le 28″, the prefix-list “rip_routes”
only covers networks 172.16.1.32/28 & 172.16.1.48/28 but can’t cover networks
172.16.1.24/30 & 172.16.1.20/30. Also, the OSPF process in the “redistribute” command
should be 100, not 10 -> B is not correct.
Same problem as answer B, the prefix-list in answerC can’t cover networks 10.1.1.8/30 &
10.1.1.12/30 -> C is not correct.