Some third-party LDP implementations are compliant with RFC 5036 for LDP IPv4 but are not compliant with RFC 5036 for handling IPv6 address or IPv6 FECs over an LDP IPv4 session.
An LSR based on the 7705 SAR in a LAN with a broadcast interface can peer with any third-party LSR, including those that are incapable of handling IPv6 address or IPv6 FECs over an LDP IPv4 session. When the 7705 SAR uses the IPv4 LDP control plane to advertise IPv6 addresses or IPv6 FECs to that peer, it can cause the IPv4 LDP session to go down.
To address this issue, draft-ietf-mpls-ldp-ipv6 modifies RFC 5036 and requires compliant systems to check for the dual-stack capability TLV in the IPv4 Hello message from the peer. If the peer does not advertise this TLV, the LSR does not send IPv6 addresses and FECs to that peer. The 7705 SAR supports advertising and resolving IPv6 prefix FECs over an LDP IPv4 session using a 32-bit LSR ID in compliance with draft-ietf-mpls-ldp-ipv6.