A network engineer configures the following static route on R1: 'ip route 172.20.5.0 255.255.255.0 GigabitEthernet0/1'. R1's GigabitEthernet0/1 connects to a multi-access Ethernet segment shared with several other devices. What is a potential drawback of using an exit interface instead of a next-hop IP address in this configuration?
- AR1 may generate excessive proxy ARP requests for every destination in the 172.20.5.0/24 network
- BThe route will not appear in the routing table because exit-interface static routes are invalid on Ethernet links
- CThe route will only work if the destination network uses the same subnet mask as GigabitEthernet0/1
- DThe administrative distance of the route automatically increases to 254
Show answer & explanationAnswer & explanation
Correct answer: A. R1 may generate excessive proxy ARP requests for every destination in the 172.20.5.0/24 network
Specifying only an exit interface on a multi-access (broadcast) segment causes the router to treat every packet as if it were directly connected, generating an ARP request (via proxy ARP) for each individual destination IP in that network, which is inefficient and increases ARP table load. This is a well-documented drawback compared to specifying a next-hop IP address, which avoids this behavior. On point-to-point links this isn't an issue since there's only one possible next hop.
Why the other options are wrong
- B. Exit-interface static routes are valid syntax but behave differently on multi-access vs point-to-point links.
- C. Subnet masks of the destination network and outgoing interface are unrelated requirements.
- D. The AD remains the default static value of 1 (or as configured), it does not automatically change based on interface-only syntax.
Exit-Interface Static Routes on Multi-Access Links
Configuring a static route with only an exit interface on a broadcast multi-access network causes the router to ARP for every destination host, unlike specifying a next-hop IP.
- Best practice: use next-hop IP on multi-access networks
- Exit-interface-only is fine on point-to-point links
- Causes proxy ARP overhead and potential performance issues
Memory trick: On a busy street, ask for directions (IP), don't guess every door (interface)