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Difficulty: HardRouting Table Components

An administrator reviews the following IPv4 routing table snippet on a Cisco router:

text
Gateway of last resort is 172.20.10.1 to network 0.0.0.0

B 10.100.0.0/16 [200/0] via 172.20.10.2, 04:12:30
O IA 10.100.16.0/20 [110/35] via 192.168.20.2, 01:05:18, GigabitEthernet0/1
D 10.100.20.0/22 [90/156160] via 192.168.30.2, 00:18:42, GigabitEthernet0/2
C 192.168.20.0/24 is directly connected, GigabitEthernet0/1
L 192.168.20.1/32 is directly connected, GigabitEthernet0/1

A packet arrives at the router addressed to 10.100.21.5010.100.21.50. Which TWO statements correctly describe how the routing table components are evaluated to forward this packet? (Select TWO.)

  1. The router selects the route for 10.100.20.0/22 because it provides the longest prefix match (/22) for the destination IP address.Answer
  2. The routing entry for 10.100.20.0/22 indicates an administrative distance of 90 and a metric of 156160.Answer
  3. C
    The router selects the route for 10.100.16.0/20 because the OSPF cost metric of 35 is lower than the EIGRP metric of 156160.
  4. D
    The router selects the BGP route for 10.100.0.0/16 because BGP routes take precedence over EIGRP and OSPF regardless of prefix length.

Answer

The router selects the route for 10.100.20.0/22 based on the longest prefix match rule, and the routing entry defines an administrative distance of 90 with a metric of 156160.
When evaluating incoming packets for forwarding, Cisco routers determine all matching routes and choose the route with the longest subnet mask (most specific prefix). The address 10.100.21.50 matches three entries, but 10.100.20.0/22 has the longest prefix length (/22). Furthermore, Cisco routing tables present protocol metadata in the bracketed structure [Administrative Distance / Metric], which identifies an administrative distance of 90 and a metric value of 156160 for the EIGRP route.

Step-by-Step Solution

1
Determine which routing table entries match the destination IP address 10.100.21.50.
The destination address matches 10.100.0.0/16, 10.100.16.0/20, and 10.100.20.0/22.
10.100.21.50 falls within the host ranges of all three subnets.
2
Apply the Longest Prefix Match (LPM) forwarding rule.
Select 10.100.20.0/22 because /22 has the longest subnet mask length.
Cisco routers always prefer the route with the most specific prefix length first before evaluating administrative distance or metric.
3
Parse the bracketed components [90/156160] for the selected route.
Administrative Distance is 90 and Metric is 156160.
Cisco output displays bracketed metrics formatted as [Administrative Distance / Metric].

Key Concept

Routing Table Component Interpretation and Longest Prefix Match Selection
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Topics

Routing Table Components

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