Examkin

Question

Difficulty: MediumIPv4 Static Routing

A network engineer is configuring an IPv4 static route on router HQ to reach the destination subnet 172.16.40.0/24172.16.40.0/24. Router HQ connects to the next-hop router over an Ethernet broadcast multi-access segment via interface GigabitEthernet0/1. The engineer enters the command `ip route 172.16.40.0 255.255.255.0 GigabitEthernet0/1` without specifying the next-hop IP address. Which operational issue will occur on router HQ as a result of configuring only the exit interface on a multi-access link?

  1. The router treats the destination network as directly connected and sends an ARP request for every destination IP address in the 172.16.40.0/24 subnet, causing ARP table bloat and performance degradation.Answer
  2. B
    The router assigns an administrative distance of 255 to the route, rendering it unusable and removing it from the IPv4 routing table.
  3. C
    The route is rejected by Cisco IOS syntax validation because an administrative distance value is required whenever an interface parameter is used.
  4. D
    The router drops packets matching the route because recursive lookup logic is automatically disabled on Layer 3 GigabitEthernet interfaces.

Answer

The router treats the destination network as directly connected and sends an ARP request for every destination IP address in the 172.16.40.0/24 subnet, causing ARP table bloat and performance degradation.
Specifying only an exit interface on a broadcast multi-access link (like Ethernet) causes the router to treat the remote prefix as directly connected. When forwarding traffic, the router issues an ARP request for every individual destination IP within that prefix. This relies heavily on proxy ARP from the next-hop router, causing unnecessary ARP broadcast traffic and consuming significant Memory/CPU resources due to ARP cache growth.

Step-by-Step Solution

1
Analyze static route exit interface behavior on broadcast networks.
When a static route specifies only a multi-access exit interface (such as Ethernet), Cisco IOS assumes the destination network resides directly on that interface segment.
Directly connected interface routes prompt the router to attempt Layer 2 MAC address resolution for every individual target destination IP address.
2
Evaluate layer 2 address resolution mechanics (ARP).
Router HQ generates an ARP request broadcast for each unique IP address in the 172.16.40.0/24172.16.40.0/24 network.
Without a explicit next-hop IP specified in the route, the router relies on proxy ARP from the neighboring router, creating large ARP tables and high broadcast overhead.

Key Concept

Directly Attached Static Routes on Multi-Access Ethernet Links
Estimated Time:1m 30s
Rate this question

Topics

IPv4 Static Routing

More questions from this topic