The 7705 SAR uses a 128-bit LSR ID as defined in draft-pdutta-mpls-ldp-v2 to establish an LDP IPv6 session with a peer LSR. This is so that a routable system IPv6 address can be used by default to bring up the LDP task on the router and establish link LDP and T-LDP sessions to other LSRs. More importantly, using a 128-bit LSR ID allows for the establishment of control plane-independent LDP IPv4 and IPv6 sessions between two LSRs over the same interface or different set of interfaces because each session uses a unique LSR ID (32-bit for IPv4 and 128-bit for IPv6).
The 7705 SAR LDP implementation does not interoperate with a system using a 32-bit LSR ID (as defined in draft-ietf-mpls-ldp-ipv6) to establish an IPv6 LDP session. The latter specifies that an LSR can send both IPv4 and IPv6 Hello messages over an interface, allowing the system to establish either an IPv4 or an IPv6 LDP session with LSRs on the same subnet. It does not allow for separate LDP IPv4 and LDP IPv6 LDP sessions between two routers.
The 7705 SAR LDP implementation interoperates with systems using a 32-bit LSR ID (as defined in draft-ietf-mpls-ldp-ipv6) to establish an IPv4 LDP session and to resolve both IPv4 and IPv6 prefix FECs.
The 7705 SAR otherwise complies with all other aspects of draft-ietf-mpls-ldp-ipv6, including the support of the dual-stack capability TLV in the Hello message. The latter is used by an LSR to inform its peer that it is capable of establishing either an LDP IPv4 or LDP IPv6 session and to convey the IP family preference for the LDP Hello adjacency and thus for the resulting LDP session. This is required because the implementation described in draft-ietf-mpls-ldp-ipv6 allows for a single session between LSRs, and both LSRs must agree if the session should be brought up using IPv4 or IPv6 when both IPv4 and IPv6 Hellos are exchanged between the two LSRs. The 7705 SAR implementation has a separate session for each IP family between two LSRs and, as such, this TLV is used to specify the family preference and to indicate that the system supports resolving IPv6 FECs over an IPv4 LDP session.