This feature allows multicast services to use segmented protocols and span them over multiple autonomous systems (ASs), as done in unicast services. As IP VPN or GRT services span multiple IGP areas or multiple ASs, either due to a network designed to deal with scale or as result of commercial acquisitions, operators may require Inter-AS VPN (unicast) connectivity. For example, an Inter-AS VPN can break the IGP, MPLS and BGP protocols into access segments and core segments, allowing higher scaling of protocols by segmenting them into their own islands. SR OS also allows for similar provision of multicast services and for spanning these services over multiple IGP areas or multiple ASs.
For multicast VPN (MVPN), SR OS previously supported Inter-AS Model A/B/C for Rosen MVPN; however, when MPLS was used, only Model A was supported for Next Generation Multicast VPN (NG-MVPN) and d-mLDP signaling.
For unicast VPRNs, the Inter-AS or Intra-AS Option B and C breaks the IGP, BGP and MPLS protocols at ABR routers (in case of multiple IGP areas) and ASBR routers (in case of multiple ASs). At ABR and ASBR routers, a stitching mechanism of MPLS transport is required to allow transition from one segment to next, as shown in Figure 1 and Figure 2.
In Figure 1, the Service Label (S) is stitched at the ASBR routers.
In Figure 2, the 3107 BGP Label Route (LR) is stitched at ASBR1 and ASBR3. At ASBR1, the LR1 is stitched with LR2, and at ASBR3, the LR2 is stitched with TL2.
Previously, in case of NG-MVPN, segmenting an LDP MPLS tunnel at ASBRs or ABRs was not possible. As such, RFC 6512 and 6513 used a non-segmented mechanism to transport the multicast data over P-tunnels end-to-end through ABR and ASBR routers. The signaling of LDP needed to be present and possible between two ABR routers or two ASBR routers in different ASs.
For unicast VPNs, it was usually preferred to only have EBGP between ASBR routers. The non-segmented behavior of d-mLDP would have broken this by requiring LDP signaling between ASBR routers.
SR OS now has d-mLDP non-segmented intra-AS and inter-AS signaling for NG-MVPN and GRT multicast. The non-segmented solution for d-mLDP is possible for inter-ASs as Option B and C.