When the IPv6 context of interfaces I/F1 and I/F2 are brought up, the following procedures are performed.
LSR A (LSR B) sends a IPv6 Hello message with source IP address set to the link-local unicast address of the specified local LSR ID interface, for example, fe80::a1 (fe80::a2), and a destination IP address set to the link-local multicast address ff02:0:0:0:0:0:0:2.
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 LoA1 (LoB1), for example, A1/32 (B1/32).
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 (lsrInterfaceNoValidIp (17) in output of show router ldp interface detail).
LSR A (LSR B) sets the transport address TLV in the Hello message to the IPv6 address of the specified local LSR-ID interface LoA1 (LoB1), for example, A2/128 (B2/128).
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 (interfaceNoValidIp (16) in output of show router ldp interface detail.
LSR A (LSR B) includes in each IPv6 Hello message the dual-stack TLV with the transport connection 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 restart 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.