A packet is received with a destination IP address of 10.2.16.10. What would the next hop IP address be for this packet?
D. None; the packet will be dropped.
Correct Answer: B
The packet will be routed to the next hop IP address of 192.168.4.2, since this routing table entry is the most specific match for the remote network. Packets are routed according to the most specific, or “longest,” match in the routing table.
The packet in the scenario has a destination IP address of 10.2.16.10, which matches two entries in the routing table.
10.0.0.0 /8: this matches based on the /8 mask, where only the first byte has to match. The destination IP address of 10.2.16.10 has a first byte matching 10. If this were the only matching route table entry, it would be selected.
10.2.16.0 /24: The first 24 bits of this entry match the first 24 bits of the destination IP address of 10.2.16.10.
Therefore, the 10.2.16.0 /24 entry is selected for routing this packet because it most specifically matches the destination IP address, or has the longest number of matching bits.
The next hops of 192.168.1.10 and 192.168.10.254 will not be used, as these routes are not the most specific matches for the destination IP address of the packet.
It is interesting to note that packets that are destined for the 10.2.32.0 network will be load balanced across both serial 0/0 and serial 0/1 because the cost (2172425) is the same for both paths.
The packet will not be dropped because there is at least one routing table entry that matches the destination IP address of the packet.
To ensure that no packets are dropped, even if there is no matching route in the routing table, a default route could be configured as follows (next hop picked at random for illustration):
Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.1
This configuration would instruct the router to send any packets that do match the existing routes to 192.168.1.1. For example, a packet destined for 184.108.40.206/24 would not match any routes in the table, and would thus be forwarded to 192.168.1.1.
If you understand how routing tables and routing advertisements work, it is relatively simple to describe the contents of a router’s routing table without seeing the table directly. To do so, you would view the router’s configuration and the configuration of its neighbors using show run, along with a diagram of its network connections. For example, examine the diagram of the two routers shown below along with their respective configurations:
It will contain S*0.0.0.0/0 [1/0] via 220.127.116.11 because of the static default route indicated in line 4 of its configuration output.
It will contain R 192.168.110.128/26 [120/1] via 18.104.22.168 00:00:22, Serial 0/0 because Router 2 has a network 192.168.110.128 statement indicating that it will advertise this network to its neighbors.
It will contain the two routes C 22.214.171.124/30 is directly connected, S0/0 and C 192.168.54.64/26 is directly connected, Fa0/0 because all directly connected routes are automatically placed in the table.
Interpret the components of routing table
Cisco > Support > IP > IP Routing > Design > Design TechNotes > Route Selection in Cisco Routers > Document ID: 8651