To traverse an inter-AS domain, recursive FECs are required (see the ‟Inter-AS Non-segmented MLDP” section in the 7450 ESS, 7750 SR, 7950 XRS, and VSR MPLS Guide for more information).
The operator must enable dynamic signaling on interfaces where Inter-AS joins are expected to be received using the existing configuration (p2mp-ldp-tree-join [ipv4] [ipv6]). When enabled, the following describes the required processing of a PIM join, as shown in Figure: In-band signaling with non-segmented inter-AS MLDP trees in GRT .
When the leaf receives a PIM join for group (S1, G1) and, through configuration, knows dynamic signaling is required, the leaf fails to resolve the source S1 via IGP and attempts to resolve route via BGP. The leaf learns that source is reachable via Next-Hop ASBR3 and the route was advertised by PE1 (Root-1) (from VRF Import Ext Community). PE2 (leaf) sources a Recursive mLDP FEC with a root node of ASBR3, and an opaque value containing the MLDP in-band signaling information identifying the (S1, G1) group and the Root-1 (the root of the inter-AS non-segmented MLDP tree), as shown below:
LEAF FEC {Root = ASBR3, Opaque Value = {Root: ROOT-1, Opaque Value (S1, G1)}}—The FEC is forwarded using IGP to ASBR3.
When the Recursive MLDP FEC arrives at ASBR3, it notes that it is the identified root node in the local AS, and that the opaque value is a Recursive Opaque Value. Because ASBR3 fails to resolve the Recursive FEC’s root (Root-1) in IGP, ASBR3 attempts to resolves the root via BGP. Similarly to processing on LEAF, this yields a Next-Hop of ASBR1. ASBR3 creates a new mLDP FEC element with a root node of ASBR1, and the Opaque value as per received opaque value, as shown below:
ASBR3 FEC {Root = ASBR1, Opaque Value = {Root: Root-1, Opaque Value (S1, G1)}}—ASBR 3 forwards the FEC using IGP or EBGP.
When the MLDP FEC arrives at ASBR1, it notes that it is the identified root node, and that the opaque value is a Recursive Opaque value. Because ASBR1 can resolve the Recursive FEC’s root (Root-1) via IGP, no further recursive processing is required. ASBR 1 forwards mLDP FEC containing in-band signaling using IGP toward ROOT-1.