Maximum Prefix Outbound Route Filter for BGP

Abstract

This document introduces a Maximum Prefix ORF (Outbound Route Filtering) type for BGP. It aims to provide a mechanism whereby the sender of route information is informed of the maximum number of prefixes that the receiver is willing to accept. This facilitates improved resource management by limiting the number of routes exchanged, avoiding unnecessary or excessive route propagation, and reducing memory and CPU load.¶

1. Introduction

The Outbound Route Filtering (ORF) capability, as defined in [RFC5291], provides a mechanism for a BGP speaker to send to its BGP peer a set of Outbound Route Filters that can be used by its peer to filter outbound routing updates to the speaker.¶

This document defines a new ORF type, called "Maximum Prefix Outbound Route Filter", which enables a BGP receiver to specify the maximum number of prefixes that a sender should transmit. Rather than specifying particular prefixes, the Maximum Prefix ORF allows the receiver to define a threshold for the number of prefixes it is willing to accept, regardless of specific prefix ranges.¶

The Maximum Prefix ORF provides an efficient mechanism for managing resource consumption, particularly in large BGP environments, by enabling a BGP speaker to limit the number of routes it propagates based on the preferences of its peer.¶

2. Efficiency

The Maximum Prefix ORF provides significant operational efficiency by reducing unnecessary BGP UPDATE messages between peers. Without this ORF, all available routes are sent to the receiver, which must then filter and limit the number of prefixes it installs in its BGP table. Although the receiver filters routes upon receipt, it still processes each incoming UPDATE, resulting in increased CPU and memory load, especially in large networks.¶

3. Operation

The Maximum Prefix ORF is designed to work in conjunction with existing maximum prefix filtering mechanisms, such as configuration commands like "neighbor X maximum-prefix N". This configuration allows operators to set a maximum prefix limit for a BGP neighbor.¶

With the addition of the Maximum Prefix ORF, the receiver can proactively communicate this limit to its BGP peer, informing the sender to enforce the same maximum prefix constraint. This ORF type is sent from the receiver to the sender and allows the sender to filter updates before they are sent, aligning with the receiver's specified limit.¶

Upon receiving the Maximum Prefix ORF, the sender MUST ensure that the total number of prefixes in outbound updates to this receiver does not exceed the specified limit. Any prefixes beyond this limit are filtered by the sender, reducing the load on the receiver's BGP processing and eliminating unnecessary UPDATE messages.¶

5. Maximum Prefix ORF-Type

The value of the ORF-Type for the Maximum Prefix ORF-Type is TBD.¶

A Maximum Prefix ORF entry is encoded as defined in the table.¶

    +------------------------------------------------+
    | Address Family Identifier (2 octets)           |
    +------------------------------------------------+
    | Subsequent Address Family Identifier (1 octet) |
    +------------------------------------------------+
    | Prefix Limit (4 octets)                        |
    +------------------------------------------------+

The meaning and use of the fields are as follows:¶

• Address Family Identifier (AFI):

  This field is the same as the one used in [RFC4760].¶

  Subsequent Address Family Identifier (SAFI):

  This field is the same as the one used in [RFC4760].¶

  Prefix Limit:

  This field indicates the maximum prefix limit the receiver wants to receive from its peer. If the received Prefix Limit is zero (0), the tuple SHOULD be ignored.¶

References