Multiprotocol BGP

Identify AFI, SAFI and multiprotocol reachability

IPv4 unicast, IPv6 unicast and VPN routes use distinct address-family semantics. AFI identifies the address family and SAFI identifies the subsequent type. MP_REACH_NLRI carries reachable prefixes and the appropriate next hop; MP_UNREACH_NLRI withdraws them. Capability negotiation tells peers which families they can exchange. The four-byte router ID remains required even on an IPv6-only router.

IPv6 transport and IPv6 routes

For the first stage, the physical link is 2001:db8:0:12::/64; the IPv4 endpoint suffixes in the generic diagram apply only when added in the second stage.

hostname R1
ipv6 unicast-routing
interface Loopback0
 ipv6 address 2001:db8::1/128
exit
interface GigabitEthernet1
 ipv6 address 2001:db8:0:12::1/64
 ipv6 address FE80::1 link-local
 no shutdown
exit
router bgp 65001
 bgp router-id 10.255.0.1
 no bgp default ipv4-unicast
 neighbor 2001:db8:0:12::2 remote-as 65002
 address-family ipv6 unicast
  neighbor 2001:db8:0:12::2 activate
  network 2001:db8::1/128
 exit-address-family
end

hostname R2
ipv6 unicast-routing
interface Loopback0
 ipv6 address 2001:db8::2/128
exit
interface GigabitEthernet1
 ipv6 address 2001:db8:0:12::2/64
 ipv6 address FE80::2 link-local
 no shutdown
exit
router bgp 65002
 bgp router-id 10.255.0.2
 no bgp default ipv4-unicast
 neighbor 2001:db8:0:12::1 remote-as 65001
 address-family ipv6 unicast
  neighbor 2001:db8:0:12::1 activate
  network 2001:db8::2/128
 exit-address-family
end

R1# show bgp ipv6 unicast summary
BGP router identifier 10.255.0.1, local AS number 65001
Neighbor                 V     AS       State/PfxRcd
2001:db8:0:12::2          4     65002    1

R1# show ipv6 route 2001:db8::2/128
B   2001:DB8::2/128 [20/0]
     via FE80::2, GigabitEthernet1

One established TCP session carries IPv6 reachability; no IPv4 NLRI is activated. A numeric prefix count in the family summary means the session is established.

Move only the transport to IPv4

On both routers add IPv4 addressing to the same link, remove the old IPv6 neighbor, then activate the IPv4-addressed neighbor inside IPv6 unicast. The IPv6 interface addresses remain for data-plane reachability.

R1

interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
exit
router bgp 65001
 no neighbor 2001:db8:0:12::2
 neighbor 192.0.2.2 remote-as 65002
 address-family ipv6 unicast
  neighbor 192.0.2.2 activate
 exit-address-family

R2

interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
exit
router bgp 65002
 no neighbor 2001:db8:0:12::1
 neighbor 192.0.2.1 remote-as 65001
 address-family ipv6 unicast
  neighbor 192.0.2.1 activate
 exit-address-family

Some implementations initially advertise an IPv4-mapped IPv6 next hop for this combination. Merely receiving the prefix does not establish a usable IPv6 forwarding path.

Set a reachable IPv6 next hop

Apply an outbound route map on each router so the IPv6 advertisements use its real IPv6 link address. This command family supports the shown cross-family transport; other images need their documented equivalent.

R1

route-map V6-NEXT-HOP permit 10
 set ipv6 next-hop 2001:db8:0:12::1
exit

router bgp 65001
 address-family ipv6 unicast
  neighbor 192.0.2.2 route-map V6-NEXT-HOP out
 exit-address-family

R2

route-map V6-NEXT-HOP permit 10
 set ipv6 next-hop 2001:db8:0:12::2
exit

router bgp 65002
 address-family ipv6 unicast
  neighbor 192.0.2.1 route-map V6-NEXT-HOP out
 exit-address-family

R1# show bgp ipv6 unicast 2001:db8::2/128
BGP routing table entry for 2001:db8::2/128
  65002
    2001:db8:0:12::2 from 192.0.2.2
      Origin IGP, valid, external, best

Verify and restore

R1# ping ipv6 2001:db8::2 source 2001:db8::1
Success rate is 100 percent (5/5)

The peer address is IPv4, while the route and usable next hop are IPv6. MP-BGP supplies reachability, not a data-plane encapsulation service by itself. Restore the initial full configuration to return to IPv6 transport.