Similarly, when the new option is invoked on a targeted IPv6 peer, the router sends a IPv6 targeted Hello message with source IP address set to the global unicast IPv6 address corresponding to the primary IPv6 address of the specified interface and a destination IP address set to configured IPv6 address of the peer. The LSR-ID field in the LDP identifier in the common LDP PDU header is set the 32-bit address of the specified interface. If the specified interface does not have an IPv4 address configured the adjacency does not come up. Any subsequent adjacency or session level messages is sent with the common LDP PDU header set as above.
When the targeted IPv6 peer contexts are brought up, the following procedures are performed.
LSR A (LSR B) sends a IPv6 Hello message with source IP address set to the primary IPv6 unicast address of the specified local LSR ID interface LoA2(LoB2), for example, A4/128 (B4/128), and a destination IP address set to the peer address B4/128(A4/128).
LSR A (LSR B) sets the LSR-ID in LDP identifier field of the common LDP PDU header to the 32-bit IPv4 address of the specified local LSR-ID interface LoA2(LoB2), for example, A3/32 (B3/32).
If the specified local LSR-ID interface is unnumbered or does not have an IPv4 address configured, the adjacency does not come up and an error is returned.
LSR A (LSR B) sets the transport address TLV in the Hello message to the IPv6 address of the specified local LSR-ID interface LoA2 (LoB2), for example, A4/128 (B4/128).
If the specified local LSR-ID interface is unnumbered or does not have an IPv6 address configured, the adjacency does not come up and an error is returned.
LSR A (LSR B) includes in each IPv6 Hello message the dual-stack TLV with the preference set to IPv6 family.
If the peer is a third-party LDP IPv6 implementation and does not include the dual-stack TLV, then LSR A (LSR B) resolves IPv6 FECs only because IPv6 addresses are not advertised in Address messages as per RFC 7552 [ldp-ipv6-rfc].
If the peer is a third-party LDP IPv6 implementation and includes the dual-stack TLV with transport connection preference set to IPv4, LSR A (LSR B) does not bring up the Hello adjacency and discards the Hello message. If the LDP session was already established, then LSRA(B) sends a fatal Notification message with status code of 'Transport Connection Mismatch' (0x00000032)' and restarts the LDP session [ldp-ipv6-rfc]. In both cases, a new counter for the transport connection mismatches is incremented in the output of show router ldp statistics.
The LSR with highest transport address takes on the active role and initiates the TCP connection for the LDP IPv6 session using the corresponding source and destination IPv6 transport addresses.