CCIE DC Written – 1.1.a Link Aggregation – Load Balancing

Port Channels

The Cisco NX-OS software load balances traffic across all operational interfaces in a port channel by hashing the addresses in the frame to a numerical value that selects one of the links in the channel. Port channels provide load balancing by default. Port-channel load balancing uses MAC addresses, IP addresses, or Layer 4 port numbers to select the link. Port-channel load balancing uses either source or destination addresses or ports, or both source and destination addresses or ports.

You can configure the load-balancing mode to apply to all port channels that are configured on the entire device or on specified modules. The per-module configuration takes precedence over the load-balancing configuration for the entire device. You can configure one load-balancing mode for the entire device, a different mode for specified modules, and another mode for the other specified modules. You cannot configure the load-balancing method per port channel.

The default load-balancing mode for Layer 3 interfaces is the source and destination IP address (src-dst-ip), and the default load-balancing mode for non-IP traffic is the source and destination MAC address (src-dst-mac). Use the port-channel load-balance  command to set the load-balancing method among the interfaces in the channel-group bundle.

NOTE: F1-series modules do not support load balancing of non-IP traffic based on a MAC address. If ports on an F1-series module are used in a port channel and non-IP traffic is sent over the port channel, Layer 2 traffic might get out of order.

Here are the additional load balancing methods you can choose from:

  •  Destination MAC address
  • Source MAC address
  • Source and destination MAC address
  • Destination IP address
  • Source IP address
  • Source and destination IP address
  • Source TCP/UDP port number
  • Destination TCP/UDP port number
  • Source and destination TCP/UDP port number

Symmetric Hashing and Random Load Balancing

Here are a couple of other related topics you should be aware of:

Symmetric Hashing – To effectively monitor traffic on a port channel, it is essential that each interface connected to a port channel receives both forward and reverse traffic flows.

Normally, there is no guarantee that the forward and reverse traffic flows will use the same physical interface. However, when you enable symmetric hashing on the port channel, bidirectional traffic is forced to use the same physical interface and each physical interface in the port channel is effectively mapped to a set of flows. When symmetric hashing is enabled, the parameters used for hashing, such as the source and destination IP address, are normalized before they are entered into the hashing algorithm. This process ensures that when the parameters are reversed (the source on the forward traffic becomes the destination on the reverse traffic), the hash output is the same. Therefore, the same interface is chosen.

Random Load Balancing – Random load balancing on port channels is a software solution that enables better port-link bandwidth utilization for GPRS Tunneling Protocol (GTP) over IP-UDP packets. The existing M1, M2, F1, F2 and F2e line card hardware does not have the capability to perform random load balancing and hence, this software solution helps in load balancing and optimizing the port channels bandwidth. Random load balancing is supported only on F3 series line cards. Random load balancing is applicable on all types of traffic and is effective on egress ports of Layer 3 traffic. The Cisco NX-OS software does random load balancing of all traffic across all interfaces in a port channel by using polynomial scheme.

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