An ISP has an MPLS VPN-based network with 12 PE routers. How many peerings are required between the 12 routers if the engineer has not configured route reflectors?

The correct answer is B. 66. In a standard iBGP full-mesh topology (no route reflectors or confederations), every BGP speaker must establish a direct peering session with every other BGP speaker to ensure complete route propagation. The formula for the number of peering sessions in a full mesh is n(n-1)/2, w

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(45 responses)

A
2% (1)
B
84% (38)
C
4% (2)
D
9% (4)

Explanation

In a standard iBGP full-mesh topology (no route reflectors or confederations), every BGP speaker must establish a direct peering session with every other BGP speaker to ensure complete route propagation. The formula for the number of peering sessions in a full mesh is n(n-1)/2, where n is the number of routers. With 12 PE routers: 12 × (12 - 1) / 2 = 12 × 11 / 2 = 132 / 2 = 66 peering sessions. This full-mesh requirement is why route reflectors are commonly deployed in large MPLS VPN networks - to reduce the O(n²) peering complexity to a hub-and-spoke model.

Topics

#MPLS VPN#iBGP#Full Mesh#PE Routers

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