For VPRN services to function correctly, all Provider Edge (PE) routers must exchange VPN-IPv4 routing information. This is accomplished using Multi-Protocol iBGP (MP-iBGP). In the absence of route reflectors or confederations, the iBGP split-horizon rule mandates that all iBGP speakers within an Autonomous System form a full mesh.

Crucially, only the PE routers participate in the VPRN service control plane; the Provider (P) routers are only involved in MPLS label switching and do not need to process VPN routes. Therefore, the full mesh is only required among the 6 PE routers. The formula for the number of sessions in a full mesh is n (n - 1) / 2, where n is the number of routers.

Applying this formula: 6 (6 - 1) / 2 = 6 5 / 2 = 15.

A. 1: This represents a single session, which is insufficient for any network with more than two PE routers.

B. 5: This is the number of iBGP peers a single PE router would have (n-1), not the total number of unique sessions in the network.

D. 30: This incorrectly calculates the number of sessions as n (n-1), which double-counts each bidirectional BGP session.

E. 45: This incorrectly includes the 4 P routers in the full-mesh calculation (10 9 / 2 = 45). P routers do not participate in the VPRN iBGP control plane.

1. Nokia 7750 SR OS Services Architecture Guide, Release 21.10.R1, Section: VPRN Service Overview. This guide explains the BGP/MPLS IP VPN model. It states, "The PE routers distribute VPN-IPv4 routes to each other by means of the MP-BGP protocol... The P routers in the service provider core are not aware of the VPN routes." This confirms that only PE routers are part of the iBGP mesh for VPRN.

2. Nokia 7750 SR OS MPLS Guide, Release 21.10.R1, Section: BGP. This guide details the iBGP split-horizon rule and the resulting requirement for a full-mesh topology among iBGP speakers when route reflectors are not used. It implicitly supports the n(n-1)/2 calculation for the number of required sessions.

3. RFC 4364: BGP/MPLS IP Virtual Private Networks (VPNs), Section 4. BGP Extended Communities and their use in VPNs. This foundational RFC describes the architecture where "PE-routers use an interior BGP (iBGP) to distribute VPN-IPv4 routes to each other." It also clarifies that P-routers "do not need to run BGP at all." This supports the exclusion of P routers from the iBGP session calculation.