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Difficulty: HardRouting Table Components

An enterprise Cisco IOS-XE router displays the following IPv4 routing table entries:

text
Gateway of last resort is not set

172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks
O 172.16.10.0/24 [110/65] via 10.0.0.1, 01:15:22, GigabitEthernet0/1
D 172.16.10.0/25 [90/307200] via 10.0.0.2, 00:45:10, GigabitEthernet0/2
S 172.16.0.0/16 [1/0] via 10.0.0.3
C 172.16.10.128/25 is directly connected, GigabitEthernet0/3

Which two statements correctly describe how the router processes traffic based on these routing table components?

  1. A packet destined to 172.16.10.50 is forwarded out interface GigabitEthernet0/2 because the /25 EIGRP route represents the longest prefix match.Answer
  2. A packet destined to 172.16.10.200 is forwarded out interface GigabitEthernet0/3 because the connected subnet 172.16.10.128/25 is the longest prefix match.Answer
  3. C
    A packet destined to 172.16.10.50 is forwarded via next-hop 10.0.0.3 because the static route has the lowest Administrative Distance.
  4. D
    A packet destined to 172.16.10.200 is forwarded via next-hop 10.0.0.1 because the OSPF route has a lower metric than the connected interface.

Answer

Packets destined to 172.16.10.50 are egressed via GigabitEthernet0/2 due to the /25 EIGRP route being the longest prefix match, and packets destined to 172.16.10.200 are egressed via GigabitEthernet0/3 because the connected /25 route provides the longest prefix match for that address range.
Router forwarding decision logic evaluates routes using the Longest Prefix Match (LPM) rule. For 172.16.10.50, the EIGRP subnet 172.16.10.0/25 (/25 mask length) is longer than the OSPF /24 and static /16 routes. For 172.16.10.200, the directly connected subnet 172.16.10.128/25 (/25 mask length) is longer than the OSPF /24 and static /16 routes. Thus, both statements reflecting LPM selections out GigabitEthernet0/2 and GigabitEthernet0/3 are correct.

Step-by-Step Solution

1
Analyze matching prefix routes for destination address 172.16.10.50.
Destination 172.16.10.50 matches 172.16.0.0/16 (static), 172.16.10.0/24 (OSPF), and 172.16.10.0/25 (EIGRP range 172.16.10.0 - 172.16.10.127).
Routing decisions prioritize prefix mask length over Administrative Distance or metric.
2
Determine the forwarding interface for 172.16.10.50.
The /25 route (172.16.10.0/25) has the longest mask length. Traffic is routed via 10.0.0.2 out GigabitEthernet0/2.
Longest Prefix Match (LPM) rule dictates selecting the longest subnet mask among all matching routes.
3
Analyze matching prefix routes for destination address 172.16.10.200.
Destination 172.16.10.200 matches 172.16.0.0/16 (static), 172.16.10.0/24 (OSPF), and 172.16.10.128/25 (Connected range 172.16.10.128 - 172.16.10.255). It does NOT match 172.16.10.0/25.
Address 172.16.10.200 falls strictly into the upper half of the /24 network block.
4
Determine the forwarding interface for 172.16.10.200.
The connected route 172.16.10.128/25 has the longest prefix match (/25 vs /24 vs /16). Traffic egresses directly out GigabitEthernet0/3.
LPM resolution selects /25 over /24 and /16.

Key Concept

Longest Prefix Match (LPM) Rule in Cisco IPv4 Forwarding
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Topics

Routing Table Components

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