bgpd: Use IPv6 LL address as nexthop if global was set to ::/LL

This happens between Bird and FRR. Maybe others as well, dunno.

Bird sends ::(fe80::1588) and we have a nexthop as :: which is inaccessible:

```
BGP routing table entry for fdff:b87d:f5b0::/48
Paths: (1 available, no best path)
  Not advertised to any peer
  4242421588 4242422547 4242422601 4242423605
    :: (inaccessible) from fe80::1588 (172.20.16.140)
    (fe80::1588) (used)
      Origin IGP, invalid, external
      Last update: Mon May 25 14:27:02 2020
```

bgpd[9554]: fe80::1588 went from OpenConfirm to Established
bgpd[9554]: fe80::1588 [FSM] Timer (routeadv timer expire)
bgpd[9554]: fe80::1588 rcvd UPDATE w/ attr: , origin i, mp_nexthop ::(fe80::1588)
bgpd[9554]: fe80::1588 rcvd UPDATE wlen 0 attrlen 120 alen 0
bgpd[9554]: fe80::1588 rcvd fda9:26a9:1c47:2d42::/64 IPv6 unicast
bgpd[9554]: Allocated bnc ::/128(VRF default) peer 0x0
bgpd[9554]: bgp_update(0.0.0.0): NH unresolved
bgpd[9554]: fe80::1588 rcvd fda9:26a9:1c47:d42::/64 IPv6 unicast

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>

diff --git a/bgpd/bgp_nht.c b/bgpd/bgp_nht.c
index fced2fbcab9771fb0804be4e01050f8f21a5ffd3..2a4de5694434a6c89baa2138ad0d4a7ecbc6c293 100644 (file)
--- a/bgpd/bgp_nht.c
+++ b/bgpd/bgp_nht.c
@@ -558,7 +558,18 @@ static int make_prefix(int afi, struct bgp_path_info *pi, struct prefix *p)
                        p->u.prefix6 = p_orig->u.prefix6;
                        p->prefixlen = p_orig->prefixlen;
                } else {
-                       p->u.prefix6 = pi->attr->mp_nexthop_global;
+                       /* If we receive MP_REACH nexthop with ::(LL)
+                        * or LL(LL), use LL address as nexthop cache.
+                        */
+                       if (pi->attr->mp_nexthop_len
+                                   == BGP_ATTR_NHLEN_IPV6_GLOBAL_AND_LL
+                           && (IN6_IS_ADDR_UNSPECIFIED(
+                                       &pi->attr->mp_nexthop_global)
+                               || IN6_IS_ADDR_LINKLOCAL(
+                                       &pi->attr->mp_nexthop_global)))
+                               p->u.prefix6 = pi->attr->mp_nexthop_local;
+                       else
+                               p->u.prefix6 = pi->attr->mp_nexthop_global;
                        p->prefixlen = IPV6_MAX_BITLEN;
                }
                break;