Cisco GRS User Manual

If the ICMP NAT session timeout is set to a very large duration (240 seconds) it can tie up precious NAT
resources such as Query mappings and NAT Sessions for the whole duration. Also, if the timeout is set to
very low it can result in premature freeing of NAT resources and applications failing to complete gracefully.
The ICMP Query session timeout needs to be a balance between the two extremes. A 60-second timeout is a
balance between the two extremes.
Implementing NAT with TCP
This section explains the various NAT behaviors that are applicable to TCP connection initiation. The detailed
NAT with TCP functionality is defined in RFC 5382.
Address and Port Mapping Behavior
A NAT translates packets for each TCP connection using the mapping. A mapping is dynamically allocated
for connections initiated from the internal side, and potentially reused for certain connections later.
Internally Initiated Connections
A TCP connection is initiated by internal endpoints through a NAT by sending SYN packet. All the external
IP address and port used for translation for that connection are defined in the mapping.
Generally for the client-server applications where an internal client initiates the connection to an external
server, to translate the outbound SYN, the resulting inbound SYN-ACK response mapping is used, the
subsequent outbound ACK, and other packets for the connection.
The 3-way handshake corresponds to method of connection initiation.
Externally Initiated Connections
For the first connection that is initiated by an internal endpoint NAT allocates the mapping. For some situations,
the NAT policy may allow reusing of this mapping for connection initiated from the external side to the
internal endpoint.
Implementing NAT 44 over ISM
These sections provide the information about implementation of NAT.
The following figure illustrates the implementation of NAT 44 over ISM
Cisco IOS XR Carrier Grade NAT Configuration Guide for the Cisco CRS Router, Release 5.2.x
8 OL-32659-01
Implementing Carrier Grade NAT on Cisco IOS XR Software
Implementing NAT with TCP

Cisco GRS Specifications

General

Product Type	Router
Port Interfaces	Multiple 10GE, 40GE, and 100GE ports
Operating System	Cisco IOS XR
Redundancy	Yes
Power Supply	Dual or multiple power supplies

Summary

Preface

Changes to This Document

Lists technical changes made to the document since its first release.

Obtaining Documentation and Submitting a Service Request

Information on obtaining documentation, using Cisco Bug Search Tool, and submitting service requests.

New and Changed Carrier Grade NAT Feature Information

New and Changed Carrier Grade NAT Features

Lists features introduced or changed in Cisco IOS XR Carrier Grade NAT configuration.

Implementing Carrier Grade NAT on Cisco IOS XR Software

Carrier Grade NAT Overview and Benefits

Provides an overview of Carrier Grade NAT implementation and its advantages.

Information About Carrier Grade NAT

Details information about implementing NAT using ICMP and TCP protocols.

Cisco Carrier NAT Applications

Lists applications deployed on the CGSE line card for Carrier Grade NAT.

Policy Functions

Explains various policy functions related to Carrier Grade NAT.

Implementing Carrier Grade NAT on Cisco IOS XR Software

Details tasks to get started with CGN configuration.

Configuration Examples for Implementing the Carrier Grade NAT

Provides practical examples for configuring Carrier Grade NAT.

External Logging

Bulk Port Allocation

Allocates blocks of ports to reduce volume of log data generated by NAT sessions.

Session logging

Includes destination IP and port information in NAT translation records for Netflow or Syslog.

Syslog Logging

Configures Syslog logging for NAT table entries.

Frequently Asked Questions (FAQs)

Answers common questions about external logging features.