Without extended next hop
With a router ID, both sessions come up and each neighbor sends R1 one IPv4 route. But look at the next hops:
- C1's route arrives with next hop 198.51.100.1, C1's loopback. C1 has no IPv4 address on the link, so that was the only IPv4 address it had to offer.
- R2's route arrives with next hop 192.0.2.1, R2's address on the far side, towards C2.
Without the extended next hop capability, an IPv4 route must have an IPv4 next hop, so each router sends the best IPv4 address it has. Neither is reachable from R1, so both routes are inaccessible: not valid, not best, not installed, and not sent on to anyone (PfxSnt 0). C1 never hears about C2's prefix.
Nothing logs an error. Everything looks configured; it just doesn't work. If you ever see IPv4 routes over IPv6 sessions marked inaccessible, check the extended next hop capability first.
vyos@R1:~$ show ip bgp summary
IPv4 Unicast Summary:
BGP router identifier 0.0.0.1, local AS number 64500 VRF default vrf-id 0
BGP table version 0
RIB entries 2, using 320 bytes of memory
Peers 2, using 60 KiB of memory
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd PfxSnt Desc
2001:db8:0:1::2 4 64512 4 3 0 0 0 00:00:02 1 0 C1
2001:db8:0:2::2 4 64501 4 3 0 0 0 00:00:02 1 0 R2
Total number of neighbors 2
vyos@R1:~$ show ip bgp
BGP table version is 0, local router ID is 0.0.0.1, vrf id 0
Default local pref 100, local AS 64500
Status codes: s suppressed, d damped, h history, u unsorted, * valid, > best, = multipath,
i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
198.51.100.0/24 198.51.100.1 0 0 64512 i
203.0.113.0/24 192.0.2.1 0 64501 64502 i
Displayed 2 routes and 2 total paths
vyos@R1:~$ show bgp ipv4 203.0.113.0/24
BGP routing table entry for 203.0.113.0/24, version 0
Paths: (1 available, no best path)
Not advertised to any peer
64501 64502
192.0.2.1 (inaccessible) from 2001:db8:0:2::2 (192.0.2.1)
Origin IGP, invalid, external
Last update: Sat Oct 17 01:25:33 2026
c1# vtysh -c 'show ip bgp'
BGP table version is 1, local router ID is 198.51.100.1, vrf id 0
Default local pref 100, local AS 64512
Status codes: s suppressed, d damped, h history, u unsorted, * valid, > best, = multipath,
i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 198.51.100.0/24 0.0.0.0 0 32768 i
Displayed 1 routes and 1 total paths
c1# ip -4 route
blackhole 198.51.100.0/24 proto 196 metric 20- Lines 10–11: Both sessions up, one route received from each, none sent on (PfxSnt 0).
- Line 24: C1's loopback as the next hop, with no
*or>. - Line 25: R2's far-side address as the next hop.
- Lines 33–34: Inaccessible, so invalid.
- Line 46: C1 has only its own route: it never learns C2's prefix.
Check your understanding
IPv4 routes arrive over an IPv6 session, marked inaccessible, with an IPv4 next hop from somewhere else in the peer's network. What's the likely cause?
- The extended next hop capability isn't negotiated
- The IPv4 address family isn't enabled
- An import route-map is denying them
- The router ID is wrong