An engineer configures an IPv6 static route on a router with a point-to-point serial interface: 'ipv6 route 2001:db8:200::/64 Serial0/0/1'. No next-hop address is specified. Why is this command valid?
- APoint-to-point interfaces have only one possible neighboring device, so IOS can forward traffic out that interface without needing a next-hop address
- BIPv6 automatically substitutes the interface's own link-local address as the next hop in every case
- CThe command is invalid and will be rejected because IPv6 static routes always require a link-local next hop
- DIPv6 static routes never require a next-hop address under any circumstances
Show answer & explanationAnswer & explanation
Correct answer: A. Point-to-point interfaces have only one possible neighboring device, so IOS can forward traffic out that interface without needing a next-hop address
On point-to-point links (such as serial interfaces), there is only one possible remote device, so specifying just the exit interface is sufficient for the router to forward traffic correctly, no next-hop address is required. This differs from multi-access links (like Ethernet), where a next-hop address is typically necessary to resolve Layer 2 addressing.
Why the other options are wrong
- B. IOS does not automatically substitute a next hop; it simply forwards out the specified interface since there's only one endpoint.
- C. The command is syntactically valid on point-to-point interfaces and will be accepted by IOS.
- D. IPv6 static routes on multi-access interfaces generally do require a next-hop address for proper forwarding.
IPv6 Exit-Interface Static Route
On point-to-point interfaces, an IPv6 static route can be configured with only the exit interface (no next-hop address) because there is only one reachable neighbor on that link.
- Valid syntax: ipv6 route <prefix>/<len> <interface>
- Works reliably on point-to-point links like serial or point-to-point GRE tunnels
- On multi-access (Ethernet) links, using only an exit interface can cause resolution issues; a next hop or link-local address is preferred
Memory trick: One neighbor, one interface, no address needed.