You are writing the requirements for an MPLS L3VPN environment that uses MP-BGP between PE routers. In this environment, route targets and route distinguishers need to be advertised between the PE…

Question

Options

• Amandatory creation of PE-to-PE BGP sessions between the outgoing interface IP addresses
• Badvertisement of standard communities, enabled on the PE-to-PE BGP neighbors
• Ccreation of PE-to-PE BGP sessions between loopback IP addresses
• Dfull mesh of IBGP sessions
• Efull mesh of EBGP sessions and partial mesh of IBGP sessions
• Fadvertisement of extended communities, enabled on the PE-to-PE BGP neighbors

• A
  9% (3)
• B
  24% (8)
• C
  62% (21)
• E
  6% (2)

Explanation

In MPLS L3VPN, PE routers use IBGP (not EBGP) to exchange VPNv4 routes within the same AS, requiring a full mesh of IBGP sessions (D) or route reflectors - option E is wrong because EBGP applies across AS boundaries, not between PEs in the same provider AS. BGP sessions are established between loopback addresses (C) because loopbacks are always up regardless of physical interface state, providing session stability - option A is wrong because using outgoing physical interface IPs makes sessions fragile and is not standard practice. Route targets (RTs) are encoded as BGP extended communities (F), which are 64-bit values capable of carrying the full RT format; option B (standard communities) is wrong because standard communities are only 32 bits and cannot represent RTs.

Memory tip: Think "LEF" - Loopbacks, Extended communities, Full-mesh IBGP - the three structural requirements for PE-to-PE MP-BGP in L3VPN. If you remember that RTs are extended (not standard) communities, and that PE routers share one AS (so IBGP, not EBGP), the wrong answers eliminate themselves.

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