3.5. Inter-Area Considerations

In the following example diagram, it is assumed that the all areas are part of a single SR domain.

The Figure 4 assumes the IPv6 control plane with the MPLS data plane.

       !          !
       !          !
B------C-----F----G-----K
/       |          |     |
S---A/        |          |     |
\        |          |     |
\D------I----------J-----L----Z (2001:DB8::2:1/128, Node-SID 150)
       !          !
Area 1  ! Backbone ! Area 2
       !   area   !

           Figure 4: Inter-Area Topology Example

In Area 2, node Z allocates Node-SID 150 to his local IPv6 prefix 2001:DB8::2:1/128.

Area Border Routers (ABRs) G and J will propagate the prefix and its SIDs into the backbone area by creating a new instance of the prefix according to normal inter-area/level IGP propagation rules.

Nodes C and I will apply the same behavior when leaking prefixes from the backbone area down to area 1. Therefore, node S will see prefix 2001:DB8::2:1/128 with Prefix-SID 150 and advertised by nodes C and I.

Therefore, the result is that a Prefix-SID remains attached to its related IGP prefix through the inter-area process, which is the expected behavior in a single SR domain.

When node S sends traffic to 2001:DB8::2:1/128, it pushes Node-SID(150) as an active segment and forwards it to A.

When a packet arrives at ABR I (or C), the ABR forwards the packet according to the active segment (Node-SID(150)). Forwarding continues across area borders, using the same Node-SID(150) until the packet reaches its destination.