Bonding

Principles of bonding mechanisms

Bonding is a mechanism that groups interfaces in layer 2 (on the Ethernet layer), while the whole group gets a single IP address. This mechanism can work in several common ways, including the following:

Redundant mode

Usually two interfaces: one for carrying the traffic, and one for backup. This is usually called Active-Backup mode.

Transmit and/or Receive load balancing

Receive and/or transmit load is distributed independently of the LAN switches to which they are connected.

802.3ad mode

Servers, and the switches they are connected to, must support the 802.3 standard. Load distribution is according to this standard.

The first two methods are topologies that are switchless, which means that the switch does not have to support any specific standard. In the last method, the switches in the topology must support the 802.3ad standard, or in some cases the Cisco Etherchannel implementation.

Redundant mode

In redundant mode, one of the adapters is active and one is passive. When the active adapter fails, the passive takes its place.

Transmit and receive load balancing

In transmit mode, the server adapter usually works in the following way:

• In transmit load balancing, the traffic is usually sent in a round-robin manner, while the bond sends out packets with a single MAC address. The load is balanced on the way out from the server, while all traffic comes in on a single interface.
• In receive load balancing, the question is how to tell the receiver to send the packets back to the interface that originated them. The problem occurs because multiple MAC addresses are going out with a single IP address. Therefore, when the receiver starts to send data, it sends an ARP request, and then sends all packets back to the interface included the receiver’s address in their ARP responses. The solution is to send the receiver multiple ARP responses with all the MAC addresses of the bond. The receiver concludes that the IP address is coming from different MACs, resultin in receive load balancing. Active Load Balancing (ALB) is a receive load balancing technique.

802.3ad topologies

This standard does not mandate any particular distribution algorithm. However, no distribution algorithm shall cause the following:

• Misorder of frames that are part of any given conversation
• Duplication of frames

The standard suggests, but does not mandate, that the algorithm can assign one or more conversations to the same port. However, it must not allocate some of the frames of a given conversation to one port and the remainder to different ports. The information used to assign conversations to ports can include any combination of the following:

• Source MAC address
• Destination MAC address
• Source IP address
• Destination IP address
• The reception port
• The type of destination address (individual or group MAC address)
• Ethernet Length/Type value (i.e., protocol identification)
• Higher layer protocol information (e.g., addressing and protocol identification information from the LLC sub layer or above)

The policy that determines how the bond distributes the frame across the ports is referred to as hash algorithm or hash policy. The hash policy specifies which parameters are used to decide how frames are distributed. For example, when a server exchanges information with several hosts on the same subnet, configuring a source/destination MAC hash usually gives a reasonable load distribution. On the other hand, load balancing is used over a router, a layer-3 hash does not help, because the server sees only one IP address (that of the router), and therefore all traffic is sent over the same interface. In this case, layer-4 hash must be used.