An edge routing gateway processes an incoming packet addressed to 10.50.12.99. The gateway… — CompTIA Network+

An edge routing gateway processes an incoming packet addressed to $10.50.12.99$ . The gateway's active routing table contains four operational routes that encompass this destination address:

- Route 1: $10.50.12.96/29$ learned via EIGRP (Administrative Distance: $90$ , Metric: $25600$ , Next-Hop Interface: eth1)
- Route 2: $10.50.12.96/28$ configured as a Static Route (Administrative Distance: $1$ , Metric: $0$ , Next-Hop Interface: eth2)
- Route 3: $10.50.12.0/24$ learned via OSPF (Administrative Distance: $110$ , Metric: $4$ , Next-Hop Interface: eth3)
- Route 4: $10.50.0.0/16$ learned via eBGP (Administrative Distance: $20$ , Metric: $10$ , Next-Hop Interface: eth4)

Which next-hop interface will the router select to forward this packet, and what is the primary routing rule governing this decision?

A
The router forwards the packet out eth2 because the static route has the lowest administrative distance (AD 1), overriding dynamic routing protocol entries.
The router forwards the packet out eth1 because 10.50.12.96/29 provides the longest prefix match (/29), which takes precedence over administrative distance and metric during route selection.Answer
C
The router forwards the packet out eth3 because OSPF presents the lowest metric value (4) among the matching internal routing protocols.
D
The router forwards the packet out eth4 because eBGP is an exterior gateway protocol with a lower administrative distance (AD 20) than internal dynamic protocols.

Answer

The router selects eth1 as the next-hop interface because 10.50.12.96/29 offers the longest prefix match (/29), which overrides administrative distance and metric considerations.

The correct response identifies eth1 as the next-hop interface because the router evaluates path selection using the Longest Prefix Match (LPM) algorithm first. The destination IP address 10.50.12.99 falls within the subnet range of 10.50.12.96/29 (10.50.12.96 to 10.50.12.103). Because /29 is the longest and most specific subnet mask among all matching routing table entries (/29 vs /28, /24, and /16), the router selects this route immediately without comparing administrative distance or metric values.

Step-by-Step Solution

Determine subnet ranges for all routes matching destination IP 10.50.12.99

10.50.12.96/29 covers 10.50.12.96 to 10.50.12.103. 10.50.12.96/28 covers 10.50.12.96 to 10.50.12.111. 10.50.12.0/24 covers 10.50.12.0 to 10.50.12.255. 10.50.0.0/16 covers 10.50.0.0 to 10.50.255.255. All four routes are valid matches.

The router must identify all matching prefix entries in the forwarding table before applying selection rules.

Compare prefix length (subnet mask specificity) across all valid candidate routes

Prefix lengths are /29 (255.255.255.248), /28 (255.255.255.240), /24 (255.255.255.0), and /16 (255.255.0.0).

Longest Prefix Match (LPM) is the absolute first criteria evaluated by an IP routing table lookup.

Apply the Longest Prefix Match (LPM) rule to select the forwarding route

The route with the longest prefix mask (/29) associated with next-hop interface eth1 is selected.

Administrative Distance and Metric are only evaluated when breaking ties between identical prefix lengths.

Key Concept

Longest Prefix Match (LPM) Precedence

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic