Implementing BGP on Cisco IOS XR Software

Information About Implementing BGP on Cisco IOS XR Software

The BGP best path algorithm implementation is in three parts:

•Part 1—Compares two paths to determine which is better.

•Part 2—Iterates over all paths and determines which order to compare the paths to select the overall best path.

•Part 3—Determines whether the old and new best paths differ enough so that the new best path should be used.

Note The order of comparison determined by Part 2 is important because the comparison operation is not transitive; that is, if three paths, A, B, and C exist, such that when A and B are compared, A is better, and when B and C are compared, B is better, it is not necessarily the case that when A and C are compared, A is better. This nontransitivity arises because the multi exit discriminator (MED) is compared only among paths from the same neighboring autonomous system (AS) and not among all paths.

Comparing Pairs of Paths

The following steps are completed to compare two paths and determine the better path:

1.If either path is invalid (for example, it has the maximum possible MED value, or it has an unreachable nexthop), then the other path is chosen (provided that the path is valid).

2.If the paths have unequal weights, the path with the highest weight is chosen. Note: the weight is entirely local to the router, and can be set with the weight command or using a routing policy.

3.If the paths have unequal local preferences, the path with the higher local preference is chosen. Note: If a local preference attribute was received with the path or was set by a routing policy, then that value is used in this comparison. Otherwise, the default local preference value of 100 is used. The default value can be changed using the bgp default local-preferencecommand.

4.If one of the paths is a redistributed path, which results from a redistribute or network command, then it is chosen. Otherwise, if one of the paths is a locally generated aggregate, which results from an aggregate-addresscommand, it is chosen.

Note Steps 1 through 4 implement the “Degree of Preference” calculation from Section 9.1.1 of draft-ietf-idr-bgp4-24.txt.

5.If the paths have unequal AS path lengths, the path with the shorter AS path is chosen. This step is skipped if bgp bestpath as-path ignore command is configured. Note: when calculating the length of the AS path, confederation segments are ignored, and AS sets count as 1. (See Section 9.1.2.2a of draft-ietf-idr-bgp4-24.txt.)

6.If the paths have different origins, the path with the lower origin is selected. Interior Gateway Protocol (IGP) is considered lower than EGP, which is considered lower than INCOMPLETE. (See Section 9.1.2.2b of draft-ietf-idr-bgp4-24.txt.)

7.If appropriate, the MED of the paths is compared. If they are unequal, the path with the lower MED is chosen.

A number of configuration options exist that affect whether or not this step is performed. In general, the MED is compared if both paths were received from neighbors in the same AS; otherwise the MED comparison is skipped. However, this behavior is modified by certain configuration options, and there are also some corner cases to consider. (See Section 9.1.2.2c of draft-ietf-idr-bgp4-24.txt.)

Cisco IOS XR Routing Configuration Guide

RC-19

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Cisco Systems IOS XR manual Comparing Pairs of Paths, RC-19
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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.
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