When a router is unable to find a known route in the routing table, how does it handle the packet

[olly ahmed]

Please help to identify the correct answer with detail explanation.

What is the correct answer?

When a router is unable to find a known route in the routing table, how does it handle the packet?

A. It discards the packet
B. It sends the packet over the route with the best metric
C. It sends the packet to the next hop address
D. It sends the packet to the gateway of last resort

[Georg Pauwen]

Hello,

answer A is the right one. Answer D looks like it could be right, but even the gateway of last resort (the 0.0.0.0 route) needs to be configured and thus appears in the routing table. No route in the routing table means that the packet will be discarded.

[olly ahmed]

Hi Georg,

But if there is a gateway of last resort exists then the traffic will be handed over to the last resort network though there is no default route configured.

[Georg Pauwen]

Hello,

have a look at this routing table. For simplicity's sake I have configured just two interfaces. 

R1#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
+ - replicated route, % - next hop override

Gateway of last resort is not set

172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.0.0/16 is directly connected, FastEthernet0/1
L 172.16.1.1/32 is directly connected, FastEthernet0/1
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.1.0/24 is directly connected, FastEthernet0/0
L 192.168.1.1/32 is directly connected, FastEthernet0/0

Without a default route configured, the gateway of last resort is not set. So packets without a destination are discarded.

Now, in the next routing table, the following command has been added to the router configuration:

ip route 0.0.0.0 0.0.0.0 10.10.10.2

The result is:

R1#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
+ - replicated route, % - next hop override

Gateway of last resort is 10.10.10.2 to network 0.0.0.0

S* 0.0.0.0/0 is directly connected, Serial1/0
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.10.10.0/24 is directly connected, Serial1/0
L 10.10.10.1/32 is directly connected, Serial1/0
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.0.0/16 is directly connected, FastEthernet0/1
L 172.16.1.1/32 is directly connected, FastEthernet0/1
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.1.0/24 is directly connected, FastEthernet0/0
L 192.168.1.1/32 is directly connected, FastEthernet0/0

Now, looking at your original question again, and thinking about it again, the answer might as well indeed be D. The point is that you need to configure the gateway of last resort, it is not in the routing table by default.

Is this an exam question and if so, for which exam ?

[olly ahmed]

Hi Georg,

Thanks for your reply.

I guess its CCNA R&S / CCNP Route exam question. Another thing is that we can also configure "gateway of last resort" without configuring the default route which is by "ip default-network". So if there is any "gateway of last resort" then it should go to the last resort. So answer should be D. But as far as I can remember in all the dumps the answer is A.

Thanks

Olly

[Georg Pauwen]

Olly,

answer A was my first thought, too. With no default route, or gateway of last resort, both of which are 'known routes', packets are discarded.

[muneef81]

ow about this senario ??

Refer to the exhibit. If R1 receives a packet destined to 172.16.1.1, to which IP address does it send the packet ?

A. 192.168.14.4
B. 192.168.12.2
C. 192.168.13.3
D. 192.168.15.5
Correct Answer: A

the 172.16.1.1 is not in the routing table ??

[Harold Ritter]

You are correct in saying that there is no specific entry in the routing table for 172.16.1.1. For that reason the default route (0.0.0.0/0) is used to forward the traffic. The next hop address for the default route in this case is 192.168.14.4, hence the correct answer being A.

Regards,