Load Balancing; Source-Based Hashing - Cisco Nexus 1000V Deployment Manual

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Links within the PortChannel that are connected to the same physical switch are bundled in the same subgroup

automatically through the use of the Cisco Discovery Protocol packets received from the upstream switch.

Alternatively, interfaces can be manually assigned to a specific subgroup using interface-level configuration.

When vPC-HM is used, each vEth interface on the VEM is mapped to one of the two subgroups using a round-

robin mechanism. All traffic from the vEth interface uses the assigned subgroup unless the assigned subgroup is

unavailable, in which case the vEth interface will fail over to the remaining subgroup. When the originally assigned

subgroup becomes available again, traffic will shift back to its original location. Traffic from each vEth interface is

then hashed within its assigned subgroup based on the configured hashing algorithm.

When multiple uplinks are attached to the same subgroup, the upstream switch needs to be configured in a

PortChannel, bundling those links together. The PortChannel needs to be configured with the option mode on.

Figure 37.

Virtual PortChannel Host Mode

Load Balancing

The Cisco Nexus 1000V provides 17 hashing algorithms to load-balance traffic across physical interfaces in a

PortChannel. These algorithms can be divided into two categories: source-based hashing and flow-based

hashing. The type of load balancing that the Cisco Nexus 1000V uses can be specified in VEM-level detail, so one

VEM can implement flow-based hashing, using the better load sharing offered by that mode, and another VEM not

connected to a clustered upstream switch can use MAC address pinning and thus source-based hashing.

Choose the hashing algorithm used with care because it affects the available configuration options and may

require configuration changes on the access-layer switches. The default hashing algorithm used by the Cisco

Nexus 1000V is source MAC address hashing (a source-based hash).

Source-based hashing algorithms help ensure that a MAC address is transmitted down only a single link in the

PortChannel, regardless of the number of links in a PortChannel.

With source-based hashing, a MAC address can move between interfaces under the following conditions:

The virtual machine moves to a new Microsoft Hyper-V A link fails, causing recalculation of the hashing.

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