When an LDP LSR initializes the LDP session to the peer LSR and the session comes up, IP addresses and FECs are distributed. Local IPv4 and IPv6 interface addresses are exchanged using the Address and Address Withdraw messages. FECs are exchanged using label mapping messages.
By default, IPv6 address distribution is determined by whether the dual-stack capability TLV, which is defined in draft-ietf-mpls-ldp-ipv6, is present in the Hello message from the peer. This requirement is designed to address interoperability issues found with existing third-party LDP IPv4 implementations.
IP address and FEC distribution behavior is described below.
If the peer LSR sent the dual-stack capability TLV in the Hello message, then local IPv6 addresses are sent to the peer. The user can configure an address export policy to restrict which local IPv6 interface addresses are sent to the peer.
If the peer explicitly stated enabling of LDP IPv6 FEC type by including the IPv6 SAC TLV in the initialization message with the D-bit set to 0, then IPv6 FECs are also sent to the peer.
If the peer sent the dual-stack capability TLV in the Hello message, but explicitly stated disabling of LDP IPv6 FEC type by including the IPv6 SAC TLV in the initialization message with the D-bit set to 1, then IPv6 local addresses instead of IPv6 FECs are sent to the peer. The user can configure an address export policy to further restrict which local IPv6 interface addresses to send to the peer.
If the peer did not send the dual-stack capability TLV in the Hello message, then no IPv6 addresses or IPv6 FECs are sent to that peer, regardless of the presence or not of the IPv6 SAC TLV in the initialization message. This case is added to prevent interoperability issues with some third-party LDP IPv4 implementations. The user can override the distribution defined by the initial Hello message by explicitly configuring an address export policy and a FEC export policy to select IPv6 addresses and FECs to send to the peer.
The above behavior applies to LDP IPv4 and IPv6 addresses and FECs. The procedure is summarized in the flowchart diagrams in Figure: LDP IPv6 Address and FEC Distribution Procedure and Figure: LDP IPv6 Address and FEC Distribution Procedure.
