When using LAGs, a method for selecting what traffic will go down what physical port must be selected. Each session created between one device and another can only use one single port inside the LAG. It is a common misconception to think of traffic being sprayed across multiple ports—going back to the car on a highway example, you can’t slice up your car and drive in four lanes at the same time. And even if you could, the pieces couldn’t get to your destination any faster—and who would insure you?
Every LAG, static or dynamic, uses a load distribution method to determine which traffic session maps to which specific port. A LAG is capable of examining the header of frames and packets for three types of information:
Q Source information (src)
Q Destination information (dst)
Q Source and destination information (src-dst)
The load distribution logic can extract the following information:
Q IP Address (ip)
Q MAC Address (mac)
Q TCP/UDP Port (port)
You can choose what to look at based on what will best distribute your specific traf-fic load. The typical default method is to use source and destination MAC addresses, or src-dst-mac. This method is used when you simply do not know if there would be a benefit to source-based or destination-based distribution. Packets from host A to host B, host A to host C, and host C to host B could all use different ports in the LAG.
Here are some example use cases and load distribution methods:
Q Traffic headed for a collection of web servers might benefit from using source IP address, or src-ip. With this method, packets from different IP addresses will poten-tially use different ports in the LAG, but packets from the same IP address will use the same port in the LAG.
42 CHAPTER 4 Advanced Layer 2
Q Streaming file transfers to an office building of PC workstations might benefit from destination MAC address, or dst-mac. With this method, packets to the same desti-nation MAC are forwarded over the same port in the LAG, and packets to a different destination are sent on a different port in the LAG.
Q Application servers that must pass traffic among each other might benefit from source and destination port, or src-dst-port. With this method, packets sent between hosts using different port numbers could be forwarded on different ports in the LAG, while packets sent between hosts on the same port number would be for-warded over the same port in the LAG.
PITFALL
Prior to vSphere 5.5, the VMware vSphere Distributed Switch only supports src-dst-ip load distribution. Make sure to tell your network administrator to use this method and set the load distribution algorithm on the VMware virtual switch to “Route based on IP hash.”
Summary
Now that you’ve been down the road of advanced Layer 2 topics, you might be thinking that most of what we’re trying to accomplish here involves the removal of logical loops.
But keep in mind that a vast amount of effort has been put into creative ways to trick the network into thinking that one path exists when, in actuality, there might be mul-tiple physical paths along the topology. Fortunately, most of what you’ll deal with on the vSphere side of the network is completely unable to be looped—this is covered in much greater detail in Part 2.
Of much greater importance for your future career as a high-performance networking ninja will revolve around the understanding, consumption, and configuration of VLANs.
Most of your vSphere environment will depend on the correct numbering and presenta-tion of VLANs so that the guest virtual machines (VMs) can properly communicate with one another. Additionally, there are many performance benefits to be realized by using VLANs to isolate various types of traffic. Finally, link aggregation increases reliability and capacity for critical high volume loads such as vMotion and IP storage. Have no fear: We go very deep into these exact topics later on, with real, working examples of the physical and logical configuration.
Reference 43
Reference
1. IEEE Standards Association. 2013. 802.1AX-2008 - IEEE Standard for Local and Metropolitan Area Networks—Link Aggregation. Available at: http://
standards.ieee.org/findstds/standard/802.1AX-2008.html.
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Chapter 5
Layer 3
Key Concepts
Q Router
Q IP Address
Q Subnet
Q Default Gateway
Q DHCP
Q DNS
Q ARP
Q Ping
Introduction
With Layers 1 and 2 behind us, we can move on to Layer 3, the Network layer. There comes a time when every packet needs to leave its local segment and go off into the world to seek its fortune, while we look on with a single tear in our eye and wish it well. In this chapter, we describe the process of routing data from one device to another, between net-works. We start with an overview of the Network layer functions, then move on to logical addressing at the Network layer, talk through routing mechanics, and close with describ-ing a few of the tools supportdescrib-ing the functions at this layer.
46 CHAPTER 5 Layer 3