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An Overview of Link Aggregation and LACP

The Network Way - Nir Yechiel's blog

The concept of Link Aggregation (LAG) is well known in the networking industry by now, and people usually consider it as a basic functionality that just works out of the box. With all of the SDN hype that’s going on out there, I sometimes feel that we tend to neglect some of the more “traditional” stuff like this one. As with many networking technologies and protocols, things may not just work out of the box, and it’s important to master the details to be able to design things properly, know what to expect to (i.e., what the normal behavior is) and ultimately being able to troubleshoot in case of a problem.

The basic concept of LAG is that multiple physical links are combined into one logical bundle. This provides two major benefits, depending on the LAG configuration:

  1. Increased capacity – traffic may be balanced across the member links to provide…

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Distributed Virtual Routing – SNAT

Assaf Muller

Where Am I?
Overview and East/West Traffic
* SNAT
Floating IPs

SNAT vs Floating IPs

A quick reminder about two NAT types used in Neutron.

  1. SNAT refers to source NAT, or, changing the source address of packets as they leave the external device of a router. This is used for traffic originating from VMs that have no floating IP attached. A router is allocated a single IP address from the external network which is shared across all VMs connected to all subnets the router is connected to. Sessions are differentiated according to the full tuple of (source IP, destination IP, source port, destination port). This is typically known as ‘PAT’, or port address translation in the networking world.
  2. Floating IPs, sometimes called DNAT (Destination NAT) in Neutronland, implement a much simpler form of NAT, a 1:1 private to public address translation. You can assign a VM a floating IP and…

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Distributed Virtual Routing – Floating IPs

Assaf Muller

Where Am I?
Overview and East/West Traffic
SNAT
* Floating IPs

In The Good Old Days…

Legacy routers provide floating IPs connectivity by performing 1:1 NAT between the VM’s fixed IP and its floating IP inside the router namespace. Additionally, the L3 agent throws out a gratuitous ARP when it configures the floating IP on the router’s external device. This is done to advertise to the external network that the floating IP is reachable via the router’s external device’s MAC address. Floating IPs are configured as /32 prefixes on the router’s external device and so the router answers any ARP requests for these addresses. Legacy routers are of course scheduled only on a select subgroup of nodes known as network nodes.

Things Are About to Get Weird

In the DVR world, however, things are very different. This is going to get very complicated very fast so let’s understand how and…

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