Technical Tip: Explanation on IPv6 link-local address as next-hop address in an IPv6 network

[tino_p]

Description	This article explains the use of IPv6 link-local address (of BGP neighbor for example) in an IPv6 network.
Scope	IPv6, FortiGate.
Solution	This topology is used for demonstration: The remote device redistributes the connected network (loopback network 2402:800:6a00:24::100/64) to the local FortiGate, assuming that BGP was enabled on both IPv4 and IPv6 at its neighbor 2402:800:6a00:27::303 and 10.211.244.81. The local FortiGate also has both neighbors 2402:800:6a00:27::301 and 10.211.244.82 activated BGP on IPv4 and IPv6. After establishing BGP with the  neighbor, FortiGate has the following outputs: The above results showed that FortiGate learned the network 2402:800:6a00:24::100/64 with the next-hop 2402:800:6a00:27::301 from both neighbors, but neighbor 10.211.244.82 only advertised the IPv6 global address, while the neighbor 2402:800:6a00:27::301 advertised both global and link-local IPv6 address.     Based on RFC2545, it is expected behavior for IPv6 routing in BGP: https://datatracker.ietf.org/doc/html/rfc2545 (Part 3 - Constructing the Next Hop field). If it is preferred to use only the IPv6 global address, there are several workarounds: To use the different weight values under BGP neighbor settings, to prefer the route that was learnt from IPv4 neighbor 10.211.244.82: As a result, next-hop shows the global address instead of the link-local address in the routing table, because the route with a higher weight was chosen: To Use prefix list, route-map, and apply route-map-in6 in the neighbor setting. In the route map, the IPv6 global address of the neighbor is defined as ip6-nexthop-local address in local FortiGate:      As a result, next-hop shows the global address instead of the link-local address in the routing table:    Another workaround is to set ip6-nexthop-local to '::', which means unspecified. It then will not use a link-local address as nexthop while installing the route into the routing table. Currently, the feature is under development and will be updated here when available.