In the ENARSI Official Cert Guide, about IPv6 over IPv4 BGP Session and route map that changes the next hop to a valid IPv6 address, it is written "Now, be very careful with this. It has to be done on the router advertising the route, not on the router receiving the route." with emphasis (page 585).
I understand exactly the need for this route map, but I do not understand why it would only work on the router advertising the route and not on the router receiving the route. Is it supposed to be a recommendation for a best practice ? Or are there technical reasons for this emphasis ?
With some thoughts, you may not want other organizations to have to handle this next-hop manipulation in case of eBGP, though it can be discussed between network admins. In case of iBGP, it seems to be easier to write a re-usable route-map on the router advertising the route to multiple iBGP peers. So it sounds like a best practice in both cases.
I tried a small lab with these configs, it seems to be working as expected (R1 and R2 directly connected on both G0/0).
R1 config
hostname R1
ipv6 unicast-routing
interface Loopback0
ip address 1.1.1.1 255.255.255.255
!
interface GigabitEthernet0/0
ip address 10.0.12.1 255.255.255.252
duplex auto
speed auto
media-type rj45
ipv6 address 2001:CAFE:12::1/64
!
router bgp 65501
bgp log-neighbor-changes
neighbor 2.2.2.2 remote-as 65502
neighbor 2.2.2.2 ebgp-multihop 2
neighbor 2.2.2.2 password CISCO
neighbor 2.2.2.2 update-source Loopback0
!
address-family ipv4
no neighbor 2.2.2.2 activate
exit-address-family
!
address-family ipv6
network 2001:DB8:1::/64
neighbor 2.2.2.2 activate
neighbor 2.2.2.2 soft-reconfiguration inbound
neighbor 2.2.2.2 route-map CHANGE_NH in
exit-address-family
!
ip route 2.2.2.2 255.255.255.255 GigabitEthernet0/0 10.0.12.2
route-map CHANGE_NH permit 10
set ipv6 next-hop 2001:CAFE:12::2
! R2 config
hostname R2
ipv6 unicast-routing
interface Loopback0
ip address 2.2.2.2 255.255.255.255
!
interface Loopback1
no ip address
ipv6 address 2001:DB8:2::1/64
!
interface GigabitEthernet0/0
ip address 10.0.12.2 255.255.255.252
duplex auto
speed auto
media-type rj45
ipv6 address 2001:CAFE:12::2/64
!
router bgp 65502
bgp log-neighbor-changes
neighbor 1.1.1.1 remote-as 65501
neighbor 1.1.1.1 ebgp-multihop 2
neighbor 1.1.1.1 password CISCO
neighbor 1.1.1.1 update-source Loopback0
!
address-family ipv4
no neighbor 1.1.1.1 activate
exit-address-family
!
address-family ipv6
network 2001:DB8:2::/64
neighbor 1.1.1.1 activate
exit-address-family
!
ip route 1.1.1.1 255.255.255.255 GigabitEthernet0/0 10.0.12.1 R2#sh bgp ipv6 unicast neighbors 1.1.1.1 advertised-routes
BGP table version is 2, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 2001:DB8:2::/64 :: 0 32768 i
Total number of prefixes 1
R2#
R1#sh ipv6 route
IPv6 Routing Table - default - 4 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
B - BGP, HA - Home Agent, MR - Mobile Router, R - RIP
H - NHRP, I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea
IS - ISIS summary, D - EIGRP, EX - EIGRP external, NM - NEMO
ND - ND Default, NDp - ND Prefix, DCE - Destination, NDr - Redirect
RL - RPL, O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1
OE2 - OSPF ext 2, ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
la - LISP alt, lr - LISP site-registrations, ld - LISP dyn-eid
lA - LISP away, a - Application
B 2001:DB8:2::/64 [20/0]
via 2001:CAFE:12::2
C 2001:CAFE:12::/64 [0/0]
via GigabitEthernet0/0, directly connected
L 2001:CAFE:12::1/128 [0/0]
via GigabitEthernet0/0, receive
L FF00::/8 [0/0]
via Null0, receive
R1#sh bgp ipv6 unicast
BGP table version is 2, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 2001:DB8:2::/64 2001:CAFE:12::2 0 0 65502 i
R1#
R1#ping 2001:db8:2::1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:DB8:2::1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms
R1#
(On R2, multiple syslogs with debug ipv6 icmp :
*Nov 27 21:09:22.593: ICMPv6: Received echo request, src=2001:CAFE:12::1, Dst=2001:DB8:2::1
*Nov 27 21:09:22.593: ICMPv6: Sent echo reply, src=2001:DB8:2::1, Dst=2001:CAFE:12::1
)
