A core border router receives an IP packet destined for host address 10.20.105.42. The rou… — CompTIA Network+

A core border router receives an IP packet destined for host address $10.20.105.42$ . The router's active routing table contains four operational candidate entries that match this target network space:

1. Static route to $10.20.96.0/19$ via next-hop $172.16.1.1$ (Administrative Distance: $1$ )
2. OSPF route to $10.20.104.0/21$ via next-hop $172.16.2.1$ (Administrative Distance: $110$ , Metric: $20$ )
3. Internal EIGRP route to $10.20.104.0/22$ via next-hop $172.16.3.1$ (Administrative Distance: $90$ , Metric: $15$ )
4. External BGP route to $10.20.0.0/16$ via next-hop $172.16.4.1$ (Administrative Distance: $20$ , Metric: $100$ )

Which next-hop IP address will the router select to forward this packet?

A
172.16.1.1, because static routes possess the lowest administrative distance among all matching table entries
172.16.3.1, because the /22 subnet mask provides the longest matching prefix for the destination addressAnswer
C
172.16.4.1, because BGP exterior gateway routes take precedence over interior routing protocols when crossing autonomous system boundaries
D
172.16.2.1, because OSPF link-state metrics are preferred over distance-vector summary calculations

Answer

172.16.3.1, because the /22 subnet mask provides the longest matching prefix for the destination address.

The fundamental decision rule for IPv4/IPv6 packet forwarding is Longest Prefix Match (LPM). When a router consults its forwarding table, it selects the route with the most specific (longest) subnet mask that covers the target IP address. Here, host address

10.20.105.42

falls within all four candidate ranges, but

/22

(22 matching network bits) is more specific than

/21

/19

, or

/16

. Consequently, the router routes to next-hop

172.16.3.1

without evaluating Administrative Distance or protocol metrics.

Step-by-Step Solution

Determine which routing entries encompass the destination IP address 10.20.105.42

All four entries match: 10.20.0.0/16 (10.20.0.0 - 10.20.255.255), 10.20.96.0/19 (10.20.96.0 - 10.20.127.255), 10.20.104.0/21 (10.20.104.0 - 10.20.111.255), and 10.20.104.0/22 (10.20.104.0 - 10.20.107.255).

Before comparing routing metrics, all matching candidate prefixes must be identified.

Apply the Longest Prefix Match (LPM) rule across all valid matches

The prefix length of /22 (22 matching network bits) is the most specific subnet mask compared to /21, /19, and /16.

Routers always evaluate prefix specificity first; Administrative Distance (AD) and metric are only compared when two identical prefix lengths exist from different sources.

Select the next-hop associated with the longest prefix entry

The entry 10.20.104.0/22 points to next-hop 172.16.3.1.

Because /22 has the highest number of matching bits, the router immediately forwards the packet to 172.16.3.1.

Key Concept

Longest Prefix Match (LPM) Routing Precedence

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