Adjacency-level FEC-type capability advertisement is defined in draft-pdutta-mpls- ldp-adj-capability. By default, all FEC types supported by the LSR are advertised in the LDP IPv4 or IPv6 session initialization; see LDP Session Capabilities for more information. If a specific FEC type is enabled at the session level, it can be disabled over a specified LDP interface at the IPv4 or IPv6 adjacency level for all IPv4 or IPv6 peers over that interface. If a specific FEC type is disabled at the session level, then FECs are not advertised and enabling that FEC type at the adjacency level do not have any effect. The LDP adjacency capability can be configured on link Hello adjacency only and does not apply to targeted Hello adjacency.
The LDP adjacency capability TLV is advertised in the Hello message with the D-bit (Disable-bit) set or reset to disable or enable the resolution of this FEC type over the link of the Hello adjacency. It is used to restrict which FECs can be resolved over a specified interface to a peer. This provides the ability to dedicate links and data path resources to specific FEC types. For IPv4 and IPv6 prefix FECs, a subset of ECMP links to an LSR peer may be configured to carry one of the two FEC types. An mLDP P2MP FEC (IPv4 only) can exclude specific links to a downstream LSR from being used to resolve this type of FEC.
Like the LDP session-level FEC-type capability, the adjacency FEC-type capability is negotiated for both directions of the adjacency. If one or both peers advertise the disabling of a capability in the LDP Hello message, no FECs of the corresponding FEC type is resolved by either peer over the link of this adjacency for the lifetime of the LDP Hello adjacency, unless one or both peers sends the LDP adjacency capability TLV subsequently to explicitly enable it.
The user can enable or disable an FEC type for a specific LDP interface to a peer by using the following CLI commands:
config>router>ldp>if-params>if>ipv4>fec-type-capability>p2mp-ipv4
config>router>ldp>if-params>if>ipv4/ipv6>fec-type-capability>prefix-ipv4
config>router>ldp>if-params>if> ipv4/ipv6>fec-type-capability>prefix-ipv6
These commands, when applied to the IPv4 P2MP FEC, deprecate the existing multicast-traffic command under the interface. Unlike the session-level capability, these commands can disable multicast FEC for IPv4.
The encoding of the adjacency capability TLV uses a PRIVATE Vendor TLV. It is used only in a Hello message to negotiate a set of capabilities for a specific LDP IPv4 or IPv6 hello adjacency.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| ADJ_CAPABILITY_TLV | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VENDOR_OUI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| Reserved | |
+-+-+-+-+-+-+-+-+ +
| Adjacency capability elements |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value of the U-bit for the TLV is set to 1 so that a receiver must silently ignore if the TLV is deemed unknown.
The value of the F-bit is 0. After being advertised, this capability cannot be withdrawn; therefore, the S-bit is set to 1 in a Hello message.
Adjacency capability elements are encoded as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|D| CapFlag |
+-+-+-+-+-+-+-+-+
D bit: Controls the capability state.
1 : Disable capability
0 : Enable capability
CapFlag: The adjacency capability
1 : Prefix IPv4 forwarding
2 : Prefix IPv6 forwarding
3 : P2MP IPv4 forwarding
4 : P2MP IPv6 forwarding (not supported on the 7705 SAR)
5 : MP2MP IPv4 forwarding
6 : MP2MP IPv6 forwarding
Each CapFlag appears no more than once in the TLV. If duplicates are found, the D-bit of the first element is used. For forward compatibility, if the CapFlag is unknown, the receiver must silently discard the element and continue processing the rest of the TLV.