Implementing BGP on Cisco IOS XR Software

Information About Implementing BGP on Cisco IOS XR Software

If the bgp bestpath med always command is configured, then the MED comparison is always performed, regardless of neighbor AS in the paths. Otherwise, MED comparison depends on the AS paths of the two paths being compared, as follows:

a.If a path has no AS path or the AS path starts with an AS_SET, then the path is considered to be internal, and the MED is compared with other internal paths

b.If the AS path starts with an AS_SEQUENCE, then the neighbor AS is the first AS number in the sequence, and the MED is compared with other paths that have the same neighbor AS

c.If the AS path contains only confederation segments or starts with confederation segments followed by an AS_SET, then the MED is not compared with any other path unless the bgp bestpath med confed command is configured. In that case, the path is considered internal and the MED is compared with other internal paths.

d.If the AS path starts with confederation segments followed by an AS_SEQUENCE, then the neighbor AS is the first AS number in the AS_SEQUENCE, and the MED is compared with other paths that have the same neighbor AS.

Note: if no MED attribute was received with the path, then the MED is considered to be 0 unless the bgp bestpath med missing-as-worstcommand is configured. In that case, if no MED attribute was received, the MED is considered to be the highest possible value.

8.If one path is received from an external peer and the other is received from an internal (or confederation) peer, the path from the external peer is chosen. (See Section 9.1.2.2d of draft-ietf-idr-bgp4-24.txt.)

9.If the paths have different IGP metrics to their next hops, the path with the lower IGP metric is chosen. (See Section 9.1.2.2e of draft-ietf-idr-bgp4-24.txt.)

10.If all path parameters in steps 1 through 10 are the same, then the router IDs are compared. If the path was received with an originator attribute, then that is used as the router ID to compare; otherwise, the router ID of the neighbor from which the path was received is used. If the paths have different router IDs, the path with the lower router ID is chosen. Note: where the originator is used as the router ID, it is possible to have two paths with the same router ID. It is also possible to have two BGP sessions with the same peer router, and therefore receive two paths with the same router ID. (See Section 9.1.2.2f of draft-ietf-idr-bgp4-24.txt.)

11.If the paths have different cluster lengths, the path with the shorter cluster length is selected. If a path was not received with a cluster list attribute, it is considered to have a cluster length of 0.

12.Finally, the path received from the neighbor with the lower IP address is chosen. Locally generated paths (for example, redistributed paths) are considered to have a neighbor IP address of 0. (See Section 9.1.2.2g of draft-ietf-idr-bgp4-24.txt.)

Order of Comparisons

The second part of the BGP best path algorithm implementation determines the order in which the paths should be compared. The order of comparison is determined as follows:

1.The paths are partitioned into groups such that within each group the MED can be compared among all paths. The same rules as in the “Comparing Pairs of Paths” section on page RC-19are used to determine whether MED can be compared between any two paths. Normally, this comparison results in one group for each neighbor AS. If the bgp bestpath med always command is configured, then there is just one group containing all the paths.

Cisco IOS XR Routing Configuration Guide

RC-20

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Cisco Systems IOS XR manual Order of Comparisons, RC-20
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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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