4A0-106 practice question 30 of 30

VPN-IPv4 addressing is made unique by defining an address structure consisting of:

Answer options

A: A 32 bit IPv4 address followed by a 64 bit Route Target

B: A 32 bit IPv4 address followed by a 64 bit Route Distinguisher

C: A 64 bit Route Target followed by a 32 bit IPv4 address

D: A 64 bit Route Distinguisher followed by a 32 bit IPv4 address Correct answer

E: A 32 bit Route Distinguisher followed by a 32 bit IPv4 address FA 32 bit Route Target followed by a 32 bit IPv4 address

Correct Answer:

D. A 64 bit Route Distinguisher followed by a 32 bit IPv4 address

Explanation

A VPN-IPv4 address is a 96-bit value used in BGP to uniquely identify customer routes across a service provider's network. This structure is essential for allowing different customers to use overlapping private IPv4 address spaces. It is formed by prepending a 64-bit (8-octet) Route Distinguisher (RD) to the customer's 32-bit (4-octet) IPv4 address. The sole purpose of the RD is to make the customer's non-unique IPv4 prefix globally unique within the BGP domain. The Route Target (RT), in contrast, is a BGP extended community attribute used to control the distribution of routes among different VPNs, not to form the address itself.

Why Incorrect

A. The Route Target (RT) is a BGP extended community attribute for route distribution policy, not a component of the VPN-IPv4 address prefix itself.
B. The order is incorrect; the 64-bit Route Distinguisher is prepended to, not appended to, the 32-bit IPv4 address.
C. The Route Target is not part of the address structure, and the component used for uniqueness is the Route Distinguisher.
E. The Route Distinguisher is 64 bits, not 32 bits. This option is also syntactically malformed.

References

1. IETF RFC 4364, BGP/MPLS IP Virtual Private Networks (VPNs), Section 4: VPN-IPv4 Addresses, Page 6.

"To solve this problem, we define a new address family, which we call 'VPN-IPv4 addresses'. A VPN-IPv4 address is a 12-octet quantity, beginning with an 8-octet Route Distinguisher (RD) and ending with a 4-octet IPv4 address." (Note: 8 octets = 64 bits; 4 octets = 32 bits).

2. Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 3 Services Guide: VPRN, Release 21.10.R1, Document Part Number: 3HE 17275 AAAD TQZZA, Section: VPRN Route Distinguishers.

"A VPRN requires a route distinguisher (RD). The RD is used to make all prefixes in a VPRN unique in the BGP domain. The RD is prepended to the IPv4 prefix to create a unique VPN-IPv4 prefix. The VPN-IPv4 prefix is what is advertised in BGP."

3. Nokia Data Center Fabric with SR Linux Cookbook, Section 3.2.1.2: Route Distinguisher and Route Target.

"The RD is an 8-byte value that is prepended to the customer’s IPv4 prefix to create a unique VPN-IPv4 prefix. This allows for the use of overlapping IP addresses between different tenants."

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