The 7210 SAS implementation 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 allows a routable system IPv6 address to be used by default to bring up the LDP task on the router and establish link LDP and T-LDP sessions to other LSRs, as is the common practice with LDP IPv4 in existing customer deployments. More importantly, this allows for the establishment of control plane-independent LDP IPv4 and LDP IPv6 sessions between two LSRs over the same interface or set of interfaces. The 7210 SAS implementation allows for two separate LDP IPv4 and LDP IPv6 sessions between two LSRs over the same interface or a set of interfaces because each session uses a unique LSR-ID (32-bit for IPv4 and 128-bit for IPv6).
The 7210 SAS LDP implementation does not interoperate with an implementation using a 32-bit LSR-ID, as defined in draft-ietf-mpls-ldp-ipv6, to establish an IPv6 LDP session. The latter specifies an LSR can send both IPv4 and IPv6 Hellos over an interface such that it can 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 7210 SAS LDP implementation should interoperate with an implementation 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 7210 SAS LDP implementation otherwise complies with all other aspects of draft-ietf-mpls-ldpipv6, 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 the resulting LDP session. This is required because the implementation described in draft-ietf-mplsldp-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 7210 SAS implementation has a separate session for each IP family between two LSRs and, as such, this TLV is used to indicate the family preference and also that it supports resolving IPv6 FECs over an IPv4 LDP session.