Configuring Modular QoS on Link Bundles on Cisco ASR 9000 Series Routers
Load Balancing
QC-156
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide
OL-23108-02
A link bundle is simply a group of ports that are bundled together and act as a single link. The advantages
of link bundles are as follows:
• Multiple links can span several line cards to form a single interface. Thus, the failure of a single link
does not cause a loss of connectivity.
• Bundled interfaces increase bandwidth availability, because traffic is forwarded over all available
members of the bundle. Therefore, traffic can flow on the available links if one of the links within a
bundle fails. Bandwidth can be added without interrupting packet flow.
All the individual links within a single bundle must be of the same type and the same speed.
Cisco IOS XR software supports the following methods of forming bundles of Ethernet interfaces:
• IEEE 802.3ad—Standard technology that employs a Link Aggregation Control Protocol (LACP) to
ensure that all the member links in a bundle are compatible. Links that are incompatible or have
failed are automatically removed from a bundle.
• EtherChannel —Cisco proprietary technology that allows the user to configure links to join a
bundle, but has no mechanisms to check whether the links in a bundle are compatible.
Load Balancing
Load balancing is supported on all links in the bundle. Load balancing function is a forwarding
mechanism to distribute traffic over multiple links based on layer 3 routing information in the router.
There are two types of load balancing schemes:
• Per-Destination Load Balancing
• Per-Packet Load Balancing
When a traffic stream arrives at the router, per-packet load balancing allows the traffic to be evenly
distributed among multiple equal cost links. Per-packet schemes make routing decision based on
round-robin techniques, regardless of the individual source-destination hosts.
Only Per-Destination Load Balancing is supported.
Per-destination load balancing allows the router to distribute packets over one of the links in the bundle
to achieve load sharing. The scheme is realized through a hash calculating based on the
source-destination address and user sessions.
When the per-destination load balancing is enabled, all packets for a certain source-destination pair will
go through the same link, though there are multiple links available. In other words, per-destination load
balancing can ensure that packets for a certain source-destination pair could arrive in order.
Layer 3 Load Balancing on Link Bundles
By default, load balancing on Layer 2 link bundles is done based on the MAC SA/DA fields in the packet
header. Layer 3 load balancing for link bundles is done on Ethernet Flow Points (EFPs) and is based on
the IPv4 source and destination addresses in the packet.When Layer 3 service-specific load balancing is
configured, all egressing bundles are load balanced based on the IPv4 source and destination addresses.
When packets do not have IPv4 addresses, default load-balancing is used.
Layer 3 load balancing for link bundles is enabled globally, using the following command:
hw-module load-balance bundle l2-service l3-params
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