For inter-AS networks where the leaf node does not have the root in the RTM or where the leaf node has the root in the RTM using BGP, and the leaf’s local AS intermediate nodes do not have the root in their RTM because they are not BGP-enabled, RFC 6512 defines a recursive opaque value and procedure for LDP to build an LSP through multiple ASs.
For mLDP to be able to signal through a multiple-AS network where the intermediate nodes do not have a routing path to the root, a recursive opaque value is needed. The LDP FEC root resolves the local ASBR, and the recursive opaque value contains the point-to-multipoint FEC element, encoded as specified in RFC 6513, with a type field, a length field, and a value field of its own.
RFC 6826 section 3 defines the Transit IPv4 opaque for point-to-multipoint LDP FEC, where the leaf in the local AS wants to establish an LSP to the root for point-to-multipoint LSP. Figure: mLDP FEC for Single AS with Transit IPv4 Opaque shows FEC in this network configuration.
Figure: mLDP FEC for Inter-AS with Recursive Opaque Value shows an inter-AS FEC with recursive opaque based on RFC 6512.
As shown in Figure: mLDP FEC for Inter-AS with Recursive Opaque Value, the root ‟10.0.0.21” is an ASBR and the opaque value contains the original mLDP FEC. Therefore, in the leaf AS where the actual root ‟10.0.0.14” is not known, the LDP FEC can be routed using the local root of ASBR. When the FEC arrives at an ASBR that is colocated in the same AS as the actual root, an LDP FEC with transit IPv4 opaque is generated.
Figure: Non-VPN mLDP with Recursive Opaque for Inter-AS shows an end-to-end example of a non-VPN mLDP with recursive opaque for inter-AS.
As shown in Figure: Non-VPN mLDP with Recursive Opaque for Inter-AS, the leaf is in AS3s where the AS3 intermediate nodes do not have ROOT-1 in their RTM. The leaf has S1 installed in the RTM via BGP. All ASBRs are acting as next-hop-self in the BGP domain. The leaf resolving S1 via BGP generates an mLDP FEC with recursive opaque, represented as:
Leaf FEC: <Root=ASBR-3, opaque-value=<Root=Root-1, <opaque-value = S1,G1>>>
This FEC is routed through the AS3 Core to ASBR-3.
Note: AS3 intermediate nodes are not BGP-capable because they do not have ROOT-1 in their RTM.
At ASBR-3, the FEC is changed to:
ASBR-3 FEC: <Root=ASBR-1, opaque-value=<Root=Root-1, <opaque-value = S1,G1>>>
This FEC is routed from ASBR-3 to ASBR-1. ASBR-1 is colocated in the same AS as ROOT-1. Therefore, ASBR-1 does not need a FEC with a recursive opaque value.
ASBR-1 FEC: <Root=Root-1, <opaque-value =S1,G1>>
This process allows all multicast services to work over inter-AS networks.