Originating Network Layer Reachability Information (NLRI)

Understanding NLRI (Network Layer Reachability Information)

Introduction to NLRI

• NLRI stands for Network Layer Reachability Information.
• In BGP (Border Gateway Protocol), NLRI represents an IP prefix along with its associated prefix length.
• Examples of NLRI include 198.51.52.53/32 for IPv4 and 2001:db8:acad:51::52/64 for IPv6.
• It is common to hear the term "route" used interchangeably with NLRI in the context of BGP, even though BGP technically advertises NLRI and Path Attributes, not routes.

Origination of NLRI

• NLRI can be originated in several ways:
  • The "network" Statement: Using the "network" statement in BGP to specify NLRI. It is important to note that the NLRI specified in the "network" statement must be an exact match.
    • Example: network 198.51.52.0 mask 255.255.255.0
    • Example: network 2001:db8:acad:51::/64
  • Redistribution: NLRI can be introduced into BGP through redistribution from another routing source.
  • Aggregation (Summarization): NLRI aggregation, also known as summarization, can be used to advertise summarized prefixes instead of individual prefixes. This helps reduce the size of the BGP routing table.

NLRI and Mask Options

• The use of the "mask" option is essential when specifying NLRI with the "network" statement, especially for non-classful networks.
• While optional for classful networks, using the "mask" option for clarity is generally recommended.

Conclusion

• Network Layer Reachability Information (NLRI) is a fundamental concept in BGP, representing IP prefixes and their associated prefix lengths.
• NLRI can be originated in BGP through various methods, including the "network" statement, redistribution, and aggregation.
• Understanding how to specify NLRI accurately and efficiently is essential for managing BGP routing effectively.