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MPLS Guide
5.4.4 LDP Session Capabilities
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.
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.
3HE 18686 AAAB TQZZA
• 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.
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