LDP can advertise all FEC types over an LDP IPv4 or an LDP IPv6 session. The FEC types are: IPv4 prefix FEC, IPv6 prefix FEC, IPv4 P2MP FEC (with MVPN), and PW FEC 128.
LDP also supports signaling the enabling or disabling of the advertisement of the following subset of FEC types during the LDP IPv4 or IPv6 session initialization phase, and when the session is already up.
IPv4 prefix FEC
This is performed using the State Advertisement Control (SAC) capability TLV as specified in draft-ietf-mpls-ldp-ip-pw-capability. The SAC capability TLV includes the IPv4 SAC element having the D-bit (Disable- bit) set or reset to disable or enable this FEC type respectively. The LSR can send this TLV in the LDP Initialization message and subsequently in an LDP capability message.
IPv6 prefix FEC
This is performed using the State Advertisement Control (SAC) capability TLV as specified in draft-ietf-mpls-ldp-ip-pw-capability. The SAC capability TLV includes the IPv6 SAC element having the D-bit (Disable- bit) set or reset to disable or enable this FEC type respectively. The LSR can send this TLV in the LDP Initialization message and subsequently in an LDP capability message to update the state of this FEC type.
P2MP FEC (IPv4 only)
This is performed using the P2MP capability TLV as specified in RFC 6388. The P2MP capability TLV has the S-bit (State-bit) with a value of set or reset to enable or disable this FEC type respectively. The LSR can send this TLV in the LDP initialization message and, subsequently, in an LDP capability message to update the state of this FEC type.
During LDP session initialization, each LSR indicates to its peers which FEC type it supports by including the capability TLV for it in the LDP initialization message. The 7705 SAR enables the IPv4 and IPv6 Prefix FEC types by default and sends their corresponding capability TLVs in the LDP initialization message. If one or both peers advertise the disabling of a capability in the LDP Initialization message, no FECs of the corresponding FEC type are exchanged between the two peers for the lifetime of the LDP session unless a capability message is sent to explicitly enable it. The same behavior applies if no capability TLV for a FEC type is advertised in the LDP initialization message, except for the IPv4 prefix FEC which is assumed to be supported by all implementations by default.
Dynamic Capability, as defined in RFC 5561, allows all FEC types to update the enabled or disabled state after the LDP session initialization phase. An LSR informs its peer that it supports Dynamic Capability by including the Dynamic Capability Announcement TLV in the LDP initialization message. If both LSRs advertise this capability, the user can enable or disable any of the above FEC types while the session is up and the change takes effect immediately. The LSR then sends a SAC capability message with the IPv4 or IPv6 SAC element having the D-bit (Disable-bit) set or reset, or the P2MP capability TLV (IPv4 only) in a capability message with the S-bit (State-bit) set or reset. Each LSR then takes the consequent action of withdrawing or advertising the FECs of that type to the peer LSR. If one or both LSRs did not advertise the Dynamic Capability Announcement TLV in the LDP initialization message, any change to the enabled or disabled FEC types only takes effect the next time the LDP session is restarted.
The user can enable or disable a specific FEC type for a specific LDP session to a peer by using the following CLI commands:
config>router>ldp>session-params>peer>fec-type-capability>prefix-ipv4
config>router>ldp>session-params>peer>fec-type-capability>prefix-ipv6
config>router>ldp>session-params>peer>fec-type-capability>p2mp