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Difficulty: MediumIPv4 Static Routing

A network engineer is configuring a static route on a Cisco router interface connected to a broadcast multiaccess Ethernet network. If the engineer configures the command using only the egress interface (e.g., `ip route 10.20.30.0 255.255.255.0 GigabitEthernet0/0`) without specifying a next-hop IPv4 address, which network behavior or operational issue will occur?

  1. A
    The router automatically assigns an administrative distance of 255 to the route, preventing it from being installed in the routing table.
  2. The router treats the destination subnet as directly connected and generates an ARP request for every destination IP address, leading to high CPU load and ARP table bloat.Answer
  3. C
    The router drops incoming packets destined for 10.20.30.0/24 because static routes on Ethernet interfaces strictly mandate a recursive lookup using a loopback IP address.
  4. D
    The static route functions identically to specifying a next-hop IPv4 address because Ethernet interfaces automatically resolve the next-hop MAC address using CDP.

Answer

The router treats the destination prefix as a directly connected subnet on the Ethernet segment and attempts to perform an ARP lookup for every host IP address in that destination network.
When an IPv4 static route on a broadcast multiaccess Ethernet interface specifies only the exit interface, Cisco IOS considers the destination prefix directly connected. Consequently, the router attempts to resolve the MAC address for every individual destination IP address in that subnet via ARP requests. This leads to severe ARP table growth, high CPU usage, and dependence on Proxy ARP.

Step-by-Step Solution

1
Analyze the static route syntax used on a multiaccess broadcast interface.
The command specifies an exit interface (`GigabitEthernet0/0`) instead of a next-hop IPv4 address.
On point-to-point links (like HDLC/PPP serial), an exit interface is sufficient because there is only one possible recipient. On multiaccess Ethernet networks, multiple devices share the segment.
2
Determine how the router processes packet forwarding for an exit-interface-only static route on Ethernet.
The router considers the target subnet directly connected to `GigabitEthernet0/0`.
To forward a packet, the router must encapsulate it into an Ethernet frame requiring a destination MAC address. Because no next-hop IP was defined, the router broadcasts an ARP request asking for the MAC address of the final destination IP address.
3
Identify the operational consequences of this configuration.
Excessive ARP traffic, enlarged ARP table size, and elevated CPU utilization occurs.
Every unique destination IPv4 address triggers a separate ARP request. Unless Proxy ARP is enabled on the downstream gateway, traffic will fail completely; if Proxy ARP is enabled, the router's ARP table fills with individual host entries.

Key Concept

Static Route Exit Interface vs. Next-Hop IP on Multiaccess Broadcast Networks
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Topics

IPv4 Static Routing

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