Only the IP address of the next-hop IPv6 is specified in the next-hop static IPv6 route. The exit interface is derived from the next hop. For example, in Figure 1, three next-hop static routes are configured on Router 0.
Before forwarding any packet, the router must resolve the route to determine the exit interface to forward the packet. This is called a router resolvability process.
It will vary depending on the type of forwarding mechanism being used by the router. CEF (Cisco Express Forwarding) is the default behaviour on most platforms running IOS 12.0 or later
Next-Hop IPv6
If CEF is not enabled in router1 and the packet is destined for the IP address 2001:AD10:110B:0001::/64 network, on Router1, the process should look like this:
Router 0 looks for a match in the routing table and finds that it has to forward the packets to the next-hop IPv6 address 2001:AD10:110B:0003::2. Each route that references only a next-hop IPv6 address and does not know an exit interface must resolve using another route in the routing table with an exit interface.
Router 0 must now search for an exit interface to reach 2001:AD10:110B:0003::2, So the router searches a second time for a match.
In this case, the IPv6 address matches the route for the directly connected network 2001:AD10:110B:0003::/64, with the exit interface Fast Ethernet 0/0. So, the router looks up Fast Ethernet 0/0 for this network and then forwards the packet to the exit interface.
So, the router takes two routing table lookup processes to forward a packet, and the router will do a routing lookup for each packet at any time. When the router has to perform multiple lookups in the routing table before forwarding a packet, it performs a recursive lookup process.
A recursive static IPv6 route is valid only when the particular following hop resolves directly or indirectly to a valid exit interface. We can verify the IPv6 static route using the figure’s command syntax.
FAQs
What is a static IPv6 Route with a next-hop?
A next-hop IPv6 static route specifies only the next router’s IPv6 address, deriving the exit interface via recursive lookup in the routing table. It’s valid if the next-hop resolves to a connected interface.
How does recursive lookup work in IPv6 routing?
The router matches the packet to a next-hop route, then searches again for the next-hop’s route to find the exit interface, like FastEthernet0/0. Multiple lookups occur per packet without CEF.
What role does CEF play in IPv6 static routes?
CEF precomputes the forwarding table to resolve next-hops once, avoiding recursive lookups and reducing CPU load. It’s a default on modern IOS, optimizing performance.
How to verify a static IPv6 route?
Use show ipv6 route to check for ‘S’ entries, next-hops, and interfaces. The route is valid only if the next-hop resolves; otherwise, it won’t install in the table.
When is a recursive static IPv6 route valid?
It’s valid when the next-hop resolves directly or indirectly to an exit interface via another route. Without resolution, the route is invalid and packets may drop.
