Layer 2 Load-Balancing on redundant Architecture

Question

Some PCs with 2 NICs are bound on 2 Switches in order to avoid a Single-Point-of-Failure (the switches are interconnected to each other too). Under which cirumstances is it possible that the PCs can actively use both links to perform load-balancing in order to use the possible bandbith more efficient? Link-Aggregation (802.1ax) does only work when interconnecting to a single device, but then i have a SPoF. How then? Is there a standard or a method to do this? How can I get something like LA on a redundant network architecture?

I think it may be possible, but I'm not sure if you'll find it is a standard. MC-LAG for example, or SMLT, but they are not "standards", here's an article on nortel and avaya switches.. http://forums.networkinfrastructure.info/nortel-ethernet-switching/smlt-on-ms-windows-servers/ — NickW, May 21 '13 at 08:27

score 1 · Accepted Answer · answered Jun 04 '13 at 10:33

You can configure a Mode 4 (LACP or 802.3ad) bond to two separate switches, you'll end up with two separate "bundles", which you can see by the presence of two different "Aggregator ID"s in /proc/net/bonding/bondX in Linux. (I'm not sure about Windows sorry)

If the first switch (one Aggregator ID) goes down, then the second switch (another Aggregator ID) takes over. The whole bond fails over to the second switch.

You can only have one active Aggregator ID at a time. This means if you lose one port of the active Aggregator, then you run on the remaining single port, despite the fact there is a perfectly good Aggregator with two good ports sitting idle.

If you wish to load balance across both switches, then you need switches which share their MAC tables in some way. The vendor-neutral term for this is "Multi Chassis Link Aggregation" or MLAG. Cisco call theirs "Virtual Port-Channel" or VPC.

When MLAG is in use, there is only one Aggregator ID and the bond just gracefully handles port or switch loss and load balances accordingly.

As they say, a picture says a thousand words...

Two Separate Switches:
  +----------+   +----------+
  | Switch A |   | Switch B |
  | Agg ID 1 |   | Agg ID 2 |
  +--+----+--+   +--+----+--+
     |    |         |    |
     |    |         |    |
 +---+----+---------+----+---+
 | eth0 | eth1 | eth2 | eth3 |
 +---------------------------+
 |           bond0           |
 +---------------------------+

Multi-Chassis Link Aggregation:
  +-------------------------+
  |     Aggregator ID 1     |
  +----------+---+----------+
  | Switch A |   | Switch B |
  +--+----+--+   +--+----+--+
     |    |         |    |
     |    |         |    |
 +---+----+---------+----+---+
 | eth0 | eth1 | eth2 | eth3 |
 +---------------------------+
 |           bond0           |
 +---------------------------+

Layer 2 Load-Balancing on redundant Architecture

1 Answers1