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Difficulty: MediumRouting Table Components

A Cisco router receives an IPv4 packet destined for host 10.200.15.42. The router inspects its routing table and finds the following entry routes:

text
O 10.200.15.0/24 [110/20] via 172.16.1.1, GigabitEthernet0/0/1
S 10.200.0.0/16 [1/0] via 172.16.2.1, GigabitEthernet0/0/2
D 10.200.15.32/28 [90/307200] via 172.16.3.1, GigabitEthernet0/0/3
B 10.200.15.40/30 [200/0] via 172.16.4.1, GigabitEthernet0/0/4

Which next-hop IP address and outgoing interface will the router select to forward the packet?

  1. 172.16.4.1 via GigabitEthernet0/0/4 because the 10.200.15.40/30 route has the longest prefix match.Answer
  2. B
    172.16.2.1 via GigabitEthernet0/0/2 because the static route has the lowest administrative distance.
  3. C
    172.16.3.1 via GigabitEthernet0/0/3 because EIGRP has a lower administrative distance than BGP.
  4. D
    172.16.1.1 via GigabitEthernet0/0/1 because the /24 route encompasses the entire subnet.

Answer

The router forwards the packet to next-hop 172.16.4.1 via interface GigabitEthernet0/0/4 because the 10.200.15.40/30 prefix offers the longest prefix match.
When a router receives an IP packet, it compares the destination IP address (10.200.15.42) against all entries in its routing table. The matching entries cover the following ranges:
- 10.200.0.0/16 (/16)
- 10.200.15.0/24 (/24)
- 10.200.15.32/28 (/28: 10.200.15.32 to 10.200.15.47)
- 10.200.15.40/30 (/30: 10.200.15.40 to 10.200.15.43)

Because 10.200.15.42 falls within the /30 range, and /30 is the most specific prefix (longest prefix match), the router selects the entry pointing to 172.16.4.1 via GigabitEthernet0/0/4. Administrative Distance is not evaluated across different prefix lengths.

Step-by-Step Solution

1
Identify all candidate routes in the routing table that encompass the destination IPv4 address 10.200.15.42.
Candidate routes are 10.200.0.0/16, 10.200.15.0/24, 10.200.15.32/28 (range 10.200.15.32–10.200.15.47), and 10.200.15.40/30 (range 10.200.15.40–10.200.15.43). All four entries match the destination address.
Before comparing routing metrics or administrative distance, the router identifies every route that includes the target IP address within its range.
2
Apply the Longest Prefix Match (LPM) rule to choose among matching entries.
Prefix lengths are /16, /24, /28, and /30. The /30 route has the longest mask length (30 set subnet bits).
Cisco IOS packet forwarding decision logic prioritizes prefix specificity above all other metrics, including Administrative Distance and routing protocol metrics.
3
Determine the next-hop IP and outgoing interface from the winning route entry.
The entry 10.200.15.40/30 points to next-hop 172.16.4.1 via GigabitEthernet0/0/4.
The router uses the next-hop parameters specified by the chosen LPM entry to perform Layer 2 frame encapsulation and egress transmission.

Key Concept

Longest Prefix Match (LPM) Rule
Estimated Time:1m 15s
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Topics

Routing Table Components

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