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352-001 · Question #481

You work for a service provider and must design a simple solution to provide Internet connectivity to an MPLS Layer 3 VPN customer whose IPv4 address block is 172.32.100.0/24. Which two options…

The correct answer is A. Implement a route toward 172.32.100.0/24 in the global RIB of the PE. C. Implement a default route in the VRF with a next hop in the global RIB of the PE. Providing Internet access to an MPLS L3 VPN customer requires both leaking a default route from the global table into the VRF and advertising the customer prefix in the global RIB to enable bidirectional reachability.

Designing Network Services

Question

You work for a service provider and must design a simple solution to provide Internet connectivity to an MPLS Layer 3 VPN customer whose IPv4 address block is 172.32.100.0/24. Which two options, when combined, allow this customer to have access to the Internet? (Choose 2)

Options

  • AImplement a route toward 172.32.100.0/24 in the global RIB of the PE.
  • BImplement a VRF source NAT for hosts on the 172.32.100.0/24 subnet.
  • CImplement a default route in the VRF with a next hop in the global RIB of the PE.
  • DImplement policy-based routing between the PE and CE.
  • EImplement destination NAT between the VRF and the global RIB of the PE.

How the community answered

(38 responses)
  • A
    68% (26)
  • B
    5% (2)
  • D
    11% (4)
  • E
    16% (6)

Why each option

Providing Internet access to an MPLS L3 VPN customer requires both leaking a default route from the global table into the VRF and advertising the customer prefix in the global RIB to enable bidirectional reachability.

AImplement a route toward 172.32.100.0/24 in the global RIB of the PE.Correct

A route to 172.32.100.0/24 in the global RIB makes the customer subnet reachable from the Internet-facing side of the PE, allowing return traffic from the Internet to be forwarded back into the VRF. Without this route in the global table, the PE has no way to deliver inbound Internet traffic back to the correct VRF.

BImplement a VRF source NAT for hosts on the 172.32.100.0/24 subnet.

NAT is not required when route leaking between VRF and global RIB is implemented; the customer retains its original addressing and bidirectional reachability is handled by the leaked routes.

CImplement a default route in the VRF with a next hop in the global RIB of the PE.Correct

A default route in the VRF with a next hop pointing into the global RIB allows customer traffic originating in the VRF to exit toward the Internet via the PE's global routing table. This route leaking between the VRF and the global RIB is the standard Cisco mechanism for providing Internet access from an L3 VPN without NAT.

DImplement policy-based routing between the PE and CE.

Policy-based routing adds unnecessary complexity and does not resolve the fundamental routing separation between the VRF and the global RIB that prevents Internet access.

EImplement destination NAT between the VRF and the global RIB of the PE.

Destination NAT is used for inbound connections to servers behind a translated address, not for providing general outbound Internet connectivity from a VRF to the global table.

Concept tested: MPLS L3 VPN Internet access via VRF route leaking

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_l3_vpns/configuration/xe-16/mp-l3-vpns-xe-16-book/mp-l3-vpn-internet-access.html

Topics

#MPLS L3 VPN#VRF Internet access#global RIB#default route leaking

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