After an LDP LSR initializes the LDP session to the peer LSR and the session comes up, local IPv4 and IPv6 interface addresses are exchanged using the Address and Address Withdraw messages. Similarly, 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 RFC 7552, is present in the Hello message from the peer. This coupling is introduced to prevent interoperability issues with existing third-party LDP IPv4 implementations.
The following is the detailed behavior for the processing of the Dual-stack capability TLV in conjunction with the IPv6 SAC TLV in the Hello message:
If the peer has sent the dual-stack capability TLV in the Hello message, IPv6 local addresses are sent to the peer. The user can configure a new address export policy to further restrict the local IPv6 interface addresses sent to the peer. If the peer explicitly enabled the LDP IPv6 FEC type by including the IPv6 SAC TLV with the D-bit set to 0 in the initialization message, IPv6 FECs are sent to the peer. The FEC prefix export policies can be used to restrict the LDP IPv6 FEC that can be sent to the peer.
If the peer has sent the dual-stack capability TLV in the Hello message, but explicitly disabled the LDP IPv6 FEC type by including the IPv6 SAC TLV with the D-bit set to 1 in the initialization message, IPv6 FECs are not sent, but IPv6 local addresses are sent to the peer. A CLI is provided to allow the configuration of an address export policy to further restrict the local IPv6 interface addresses that can be sent to the peer. The FEC prefix export policy has no effect because the peer has explicitly requested disabling the IPv6 FEC type advertisement.
If the peer has not sent the dual-stack capability TLV in the Hello message, no IPv6 addresses or IPv6 FECs are sent to that peer, regardless of whether the IPv6 SAC TLV is present in the initialization message. This case is added to prevent interoperability issues with existing third-party LDP IPv4 implementations. The user can override this by explicitly configuring an address export policy and a FEC export policy to select the addresses and FECs to send to the peer.
The preceding behavior applies to LDP IPv4 and IPv6 addresses and FECs. The procedure is summarized in the flowcharts shown in the following figures.
