Alcatel-Lucent 7450 Manual page 602

LDP Session Capabilities
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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 SR OS implementation
will enable the above FEC types by default and will thus send the 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 will be exchanged between
the two peers for the lifetime of the LDP session unless a Capability message is sent subsequently
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 above FEC types to update the enabled or
disabled state after the LDP session initialization phase. An LSR informs its peer that it supports
the Dynamic Capability by including the Dynamic Capability Announcement TLV in the LDP
Initialization message. If both LSRs advertise this capability, the user is allowed to 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 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 will only take effect at the next time the LDP session is restarted.
The user can enable or disable a specific FEC type for a given LDP session to a peer by using the
following CLI commands:
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or enable this FEC type respectively. The LSR can send this TLV in the LDP Initialization
message and subsequently in a 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 a LDP Capability message to update the state of this FEC
type.
P2MP FEC—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. Unlike the IPv4 SAC and IPv6 SAC capabilities, the
P2MP capability does not distinguish between IPv4 and IPv6 P2MP FEC. The LSR can
send this TLV in the LDP Initialization message and, subsequently, in a LDP Capability
message to update the state of this FEC type.
configure>router>ldp>session-parameters>peer>fec-type-capability p2mp
configure>router>ldp>session-parameters>peer>fec-type-capability prefix-ipv4
configure>router>ldp>session-parameters>peer>fec-type-capability prefix-ipv6
7450 ESS MPLS Guide