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Difficulty: MediumRouter Forwarding Decision Logic

Refer to the following routing table extract from a Cisco router:

text
Gateway of last resort is 10.10.10.1 to network 0.0.0.0

D 192.168.16.0/20 [90/307200] via 10.1.1.2, GigabitEthernet0/0
O 192.168.24.0/22 [110/20] via 10.2.2.2, GigabitEthernet0/1
S 192.168.24.0/24 [1/0] via 10.3.3.2, GigabitEthernet0/2
S* 0.0.0.0/0 [1/0] via 10.10.10.1, GigabitEthernet0/3

Which TWO statements correctly describe how the router makes forwarding decisions based on this routing table?

  1. A packet with destination IP 192.168.24.45 is routed via GigabitEthernet0/2 because 192.168.24.0/24 represents the longest prefix match.Answer
  2. Longest prefix match is evaluated first during packet forwarding lookup, independent of administrative distance values across different prefix lengths.Answer
  3. C
    A packet with destination IP 192.168.24.45 is routed via GigabitEthernet0/0 because EIGRP has a lower administrative distance than OSPF.
  4. D
    Traffic for destination IP 192.168.24.45 is load-balanced across GigabitEthernet0/1 and GigabitEthernet0/2 because their subnet boundaries overlap.

Answer

The router routes traffic destined for 192.168.24.45 out interface GigabitEthernet0/2 using the longest prefix match (/24), and longest prefix match logic is evaluated prior to administrative distance during the forwarding lookup.
When a router receives a packet, it compares the packet's destination IP address against all network entries in its routing table. The rule of Longest Prefix Match (LPM) specifies that the route with the most specific (longest) subnet mask is chosen for forwarding, regardless of protocol or administrative distance. Furthermore, administrative distance is utilized when populating the routing table between equal prefix matches, not during the packet packet forwarding lookup process.

Step-by-Step Solution

1
Evaluate all candidate matching routes for destination address 192.168.24.45.
The destination falls within 192.168.16.0/20 (range 192.168.16.0–192.168.31.255), 192.168.24.0/22 (range 192.168.24.0–192.168.27.255), 192.168.24.0/24 (range 192.168.24.0–192.168.24.255), and 0.0.0.0/0.
Routing decision logic compares the destination binary pattern against all active routes in the RIB.
2
Determine the winning route based on the Longest Prefix Match (LPM) rule.
192.168.24.0/24 has 24 matching bits, which is the maximum matching prefix length among all candidate entries.
The router always selects the route entry with the highest number of matching leading bits.
3
Analyze the relationship between Administrative Distance (AD) and Longest Prefix Match.
Administrative distance is used during routing table selection when installing paths from different routing sources with exact matching prefixes. For packet forwarding lookups, LPM is evaluated first.
Administrative distance does not override a longer subnet mask during packet forwarding decisions.

Key Concept

Router Forwarding Decision Logic (Longest Prefix Match precedence over Administrative Distance)
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Topics

Router Forwarding Decision Logic

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