Layer 2 Load Balancing - Cisco CRS-1 - Carrier Routing System Router Troubleshooting Manual

About Load Balancing with Cisco Express Forwarding
With the 3-tuple option, the hash algorithm uses the following Layer 3 and platform-specific information
to identify unique flows.
•
•
With the 7-tuple option, the hash algorithm uses the following Layer 3, Layer 4, and platform-specific
information to identify unique flows.
•
•
•
For Layer 3 packets that do not contain Layer 4 information, the 7-tuple option substitutes a replacement
value or ignores the field.

Layer 2 Load Balancing

Load balancing at Layer 2 is also done by flow. Cisco IOS XR software provides load balancing for
Layer 2 bundles only. Data is distributed to a link in proportion to the bandwidth of the link in relation
to its bundle. For all IP traffic passing over a bundle interface, load balancing is done by the Forwarding
Information Base (FIB) on the ingress and egress line cards. When a received packet is switched to a
bundle interface, the FIB chooses which member link to use based on the source and destination IP
address of the packets. The Adjacency Information Base (AIB) stores a list of adjacencies that the FIB
uses to determine which member links are available for forwarding.
Layer 2 load balancing uses the 3-tuple algorithm, but uses only the source and destination IP addresses
in the packet. A modulo operation on the hash result is subsequently performed using the number of
entries in the load balancing table, which is driven by the link weighting.
For each member of bundle, the adjacency information includes a weight that balances the load among
bundle members with different bandwidths. The theory behind the weight usage is:
•
•
Cisco IOS XR Troubleshooting Guide for the Cisco CRS-1 Router
13-232
Layer 3 information from the IP header
Source IP address
–
Destination IP address
–
Platform-related information:
Router ID
–
Layer 3 information from the IP header:
Source IP address
–
Destination IP address
–
Protocol
–
Layer 4 information from the TCP or UDP header
Source port
–
Destination port
–
Platform-related information
– Router ID
– Ingress interface handle
The weight reflects the bandwidth ratios of all members. For example, if you have one OC192
member and one OC48 member as part of a bundle, OC192 member is given a relative weight of 4
while OC48 has a weight as 1.
If you have only one member in the bundle, it still gets a relative weight of 2 to initiate hashing.
Chapter 13 Troubleshooting Load Balancing
OL-21483-02