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Cisco® BGP-4 Command and Configuration Handbook (CCIE Professional Development) by William R. Parkhurst Ph.D.

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2-1. auto-summary

Syntax Description:

This command has no arguments. auto-summary is enabled by default.

Purpose: When auto-summary is enabled, routes injected into BGP via redistribution are summarized on a classful boundary. A 32-bit IP address consists of a network address and a host address. The subnet mask determines the number of bits used for the network address and the number of bits used for the host address. The IP address classes have a natural or standard subnet mask, as shown in Table 2-1.

Table 2-1. IP Address Classes
Class Address Range Natural Mask
A 1.0.0.0 to 126.0.0.0 255.0.0.0 or /8
B 128.1.0.0 to 191.254.0.0 255.255.0.0 or /16
C 192.0.1.0 to 223.255.254.0 255.255.255.0 or /24
Reserved addresses include 128.0.0.0, 191.255.0.0, ...

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