Examkin

Question

Difficulty: EasyRouter Forwarding Decision Logic

Refer to the following partial IPv4 routing table output from a Cisco router:

text
Gateway of last resort is 10.0.0.1 to network 0.0.0.0

S* 0.0.0.0/0 [1/0] via 10.0.0.1
O 172.31.0.0/16 [110/20] via 10.1.1.1, GigabitEthernet0/0
D 172.31.50.0/24 [90/307200] via 10.2.2.2, GigabitEthernet0/1
S 172.31.50.96/28 [1/0] via 10.3.3.3, GigabitEthernet0/2

When the router receives an IP packet destined for 172.31.50.100172.31.50.100, which TWO statements correctly describe how the router makes its forwarding decision?

  1. The router selects the route 172.31.50.96/28 because it has the longest matching prefix length (/28) for destination 172.31.50.100.Answer
  2. The packet is forwarded out interface GigabitEthernet0/2 toward next-hop IP address 10.3.3.3.Answer
  3. C
    The router forwards the packet to next-hop 10.2.2.2 because dynamic routing protocols take precedence over static routes regardless of subnet mask.
  4. D
    The router drops the packet because destination address 172.31.50.100 is outside the valid usable host range of the subnet 172.31.50.96/28.

Answer

The router forwards the packet out interface GigabitEthernet0/2 toward next-hop 10.3.3.3 after selecting 172.31.50.96/28 as the longest prefix match.
When an IPv4 packet arrives, a router evaluates all matching routes in its routing table and selects the entry with the longest prefix length (most specific subnet mask). Here, destination 172.31.50.100 matches 0.0.0.0/0 (/0), 172.31.0.0/16 (/16), 172.31.50.0/24 (/24), and 172.31.50.96/28 (/28). The /28 route is the longest match. Therefore, the router forwards the packet out interface GigabitEthernet0/2 to next-hop 10.3.3.3.

Step-by-Step Solution

1
Evaluate all routing table entries that match the destination IP address 172.31.50.100.
The destination matches four entries: 0.0.0.0/0, 172.31.0.0/16, 172.31.50.0/24, and 172.31.50.96/28.
All four network routes encompass the destination IP address 172.31.50.100.
2
Apply the Longest Prefix Match (LPM) rule to select the best route.
172.31.50.96/28 is chosen because /28 is the longest prefix mask (most specific match).
A router always prefers the route with the highest number of matching network bits before considering Administrative Distance or Metric.
3
Identify the next-hop IP and egress interface configured for the winning route.
The entry 172.31.50.96/28 specifies next-hop 10.3.3.3 and egress interface GigabitEthernet0/2.
The router uses the exact forwarding attributes bound to the selected longest matching route entry.

Key Concept

Router Forwarding Decision Logic (Longest Prefix Match)
Rate this question

Topics

Router Forwarding Decision Logic

More questions from this topic