CHAPTER
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Cisco IOS XR Troubleshooting Guide for the Cisco CRS-1 Router
OL-21483-02
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Troubleshooting Load Balancing
This chapter explains the troubleshooting procedures for load balancing on the Cisco CRS-1 Router.
This chapter has the following sections:
• About Load Balancing with Cisco Express Forwarding
• Troubleshooting Layer 3 or Layer 4 Load Balancing
• Troubleshooting Layer 2 Load Balancing
• Configuration Examples for Troubleshooting Load Balancing
About Load Balancing with Cisco Express Forwarding
This document explains how Cisco IOS XR implements load balancing for Layer 2 (data link layer),
Layer 3 (network layer) and Layer 4 (transport layer) flows across multiple parallel links when using
Cisco Express Forwarding (CEF). This procedure explains load balancing on the Cisco CRS-1 Router
only.
Load Balancing Function
Load balancing is a Cisco IOS XR software feature that improves the utilization of parallel links by
distributing traffic flows among them. A traffic flow consists of packets that have common Layer 3 and
Layer 4 characteristics, such as the same source and destination IP addresses. The concept of a flow also
extends to Layer 2 bundles and to virtual links such as MPLS traffic-engineering tunnels.
Cisco IOS XR uses a hash algorithm to identify individual flows and to distribute them across multiple
parallel links. Each flow uses a specific link, but there are many flows in a production environment.
Balancing traffic flows is similar to the earlier method of per-destination balancing, which grouped all
packets by destination. Grouping packets by flow provides a better degree of granularity, and ultimately
balancing, because the hash algorithm considers up to seven packet characterizes instead of just the
destination. Cisco IOS XRsoftware does not use the per-packet load balancing option, which rotates
individual packets around the available links.
Source Information for Load Balancing
Per-flow load balancing in Cisco IOS XR has two options: 3-tuple (the default) and 7-tuple. The 3-tuple
option uses Layer 3 packet information to identify flows, and the 7-tuple option uses Layer 3 and Layer 4
information. A unique flow consists of packets match all three of the 3-tuple values or all seven of the
7-tuple values.
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