Implementing Routing Policy on Cisco IOS XR Software

Information About Implementing Routing Policy

Attached Policy Modification

Policies that are in use do, on occasion, need to be modified. Traditionally, configuration changes are done by completely removing the relevant configuration and then re-entering it. However, this allows for a window of time in which no policy is attached and the default action takes place. RPL provides a mechanism for an atomic change so that if a policy is redeclared, or edited using the emacs editor, the new configuration is applied immediately, which allows for policies that are in use to be changed without having a window of time in which no policy is applied at the given attach point.

Verification of Attribute Comparisons and Actions

The policy repository knows which attributes, actions, and comparisons are valid at each attach point. When a policy is attached, these actions and comparisons are verified against the capabilities of that particular attach point. Take, for example, the following policy definition:

route-policy bad set med 100

set level level-1-2 set cost 200

end-policy

This policy attempts to perform actions to set the BGP attribute med, IS-IS attribute level, and OSPF attribute cost. The system allows you to define such a policy, but it does not allow you to attach such a policy. If you had defined the policy bad and then attempted to attach it as an inbound BGP policy using the BGP configuration statement neighbor 1.2.3.4 address-family ipv4 unicast route-policy bad in the system would reject this configuration attempt. This rejection results from the verification process checking the policy and realizing that while BGP could set the MED, it has no way of setting the level or cost as the level and cost are attributes of IS-IS and OSPF, respectively. Instead of silently omitting the actions that cannot be done, the system generates an error to the user. Likewise, a valid policy in use at an attach point cannot be modified in such a way as to introduce an attempt to modify a nonexistent attribute or to compare against a nonexistent attribute. The verifiers test for nonexistent attributes and reject such a configuration attempt.

Policy Statements

Four types of policy statements exist: remark, disposition (drop and pass), action (set), and if (comparator).

Remark

A remark is text attached to policy configuration but otherwise ignored by the policy language parser. Remarks are useful for documenting parts of a policy. The syntax for a remark is text that has each line prepended with a pound sign (#):

#This is a simple one-line remark.

#This

#is a remark

#comprising multiple

#lines.

In general, remarks are used between complete statements or elements of a set. Remarks are not supported in the middle of statements or within an inline set definition.

Cisco IOS XR Routing Configuration Guide

RC-219

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Cisco Systems IOS XR manual Policy Statements, Remark, Attached Policy Modification, RC-219

IOS XR specifications

Cisco Systems IOS XR is an advanced operating system designed specifically for high-performance routers and service provider networks. It serves as the backbone for many of Cisco's high-end routing platforms, enabling service providers to manage their networks with increased efficiency, flexibility, and scalability.

One of the main features of IOS XR is its modular architecture. This allows for the independent operation of various components within the OS, facilitating the deployment of new features and updates without affecting the overall stability of the system. This modularity ensures that service providers can implement rapid changes and enhancements while maintaining service continuity.

Another characteristic of IOS XR is its support for 64-bit architecture, which provides enhanced performance and the ability to manage larger amounts of data. This is particularly beneficial for service providers that deal with high traffic volumes and require robust data processing capabilities. The utilization of 64-bit technology also enables the operating system to utilize memory more efficiently, allowing for greater scalability.

IOS XR incorporates advanced technologies such as Distributed System Architecture (DSA) and Multiple Routing Instances (Merging Routes). DSA allows for the distribution of routing processes across multiple hardware resources, maximizing performance and redundancy. Multiple Routing Instances enable operators to create separate logical routing tables for different services, improving isolation and efficiency in managing network traffic.

The operating system also focuses heavily on security, featuring extensive encryption methods and access controls to safeguard network resources. IOS XR supports various authentication protocols, ensuring secure access to routers and switches. In addition, the OS includes comprehensive logging and monitoring capabilities, allowing network administrators to track activities and respond quickly to potential threats.

Another critical aspect of IOS XR is its adherence to the principles of service-oriented architecture (SOA). This approach permits the development of applications and services that can operate independently, fostering innovation and enabling service providers to tailor their offerings based on customer demands.

Ultimately, Cisco IOS XR is a powerful, reliable operating system that meets the complex needs of modern telecommunications networks. With its focus on modularity, performance, security, and scalability, it enables service providers to deliver high-quality, resilient services to their customers while efficiently managing network resources. As the industry continues to evolve, IOS XR remains a vital tool for those aiming to stay competitive in the ever-changing landscape of networking.