Mohamed Anwar asked the following question on my post "4 Types of Port Channels and When They're Used".

"I need a clarification, where if a member link fails, what will happen to the traffic already sent over that link ? Is there any mechanism to notify the upper layer about the loss and ask it to resend ? How this link failure will be handled for data traffic and control traffic ?"

— Mohamed Anwar

I think his questions are really important because he hits on two really key aspects of a failure event: what happens in the data plane and what happens in the control plane.

A network designer needs to bear both of these aspects in mind as part of their design. Overlooking either aspect will almost always open the network up to additional risk.

I think it's well understood that port channels add resiliency in the data plane (I cover some of that in the previous article). What may not be well understood is that port channels also contribute to a stable control plane! I'll talk about that below. I'll also address Mohamed's question about what happens to traffic on the failed link.

I'm not sure why I've taken such an interest in mDNS, service discovery, and the Bonjour protocol, but I have. It probably has something to do with my not being able to use AirPlay at home for such a long time because, like any true network geek, I put my wireless devices on a separate VLAN from my home media devices. I mean, duh. So now I keep an eye out for different methods of enabling mDNS in the network in anticipation of my own experience in my home network becoming one of my customer's experience in their enterprise network.

As I've written about in the past (here), Apple's AirPlay technology relies on Bonjour which is Apple's implementation of "zero config" networking. One of the things that Bonjour enables is the automatic discovery of services on the network. For example, an Apple TV might advertise itself as being able to receive AirPlay streams. An iPad that is looking for AirPlay receivers would use Bonjour to discover the Apple TV and present it to the user as an AirPlay destination. Both the Apple TV and iPad do all this without any user intervention or configuration (hence the "zero config" part).

That's fine and dandy but what my earlier article focused on was how Bonjour broke down in a network where what I'll call the "server" and the "client" are not in the same Layer 2 domain/VLAN. This is because the service discovery aspect of Bonjour relies on link-local scope multicast. These packets will not cross Layer 3 boundaries in the network.

As a follow-up to my previous article on Port Channels titled "4 Types of Port Channels and When They're Used" I wanted to talk a bit about the long-standing rule that says you should always create your Etherchannel (EC) bundles with a number of links that works out to a power of two (ie, 2,4 or 8 links). That rule is less applicable today than it used to be.

The other day I was catching up on recorded content from Cisco Live! and I saw mention of yet another implementation of port channels (this time called Enhanced Virtual Port Channels). I thought it would make a good blog entry to describe the differences of each, where they are used, and what platforms each is supported on.

The shared services area of the network is meant to provide common services — such as DNS, DHCP, and Internet access — to multiple logical networks/VRFs/customers. Cisco publishes a validated design for shared services that describes the use of multiple virtual firewalls and routers to provide connectivity between the shared services module and the VRFs in the network. I'm going to describe a method of collapsing the shared services firewalls and virtual routers into a single instance running on a single box using some of the features found in Juniper's Junos platform.

This post is going to provide a very basic introduction to configuring VRFs on Cisco IOS and Juniper's Junos. There's so many configuration combinations and options for virtual routing that it would be impossible to go through everything in great detail. At the end of the post I'll provide links to documentation where you can get detail if you want it.

All network engineers should be familiar with the method for virtualizing the network at Layer 2: the VLAN. VLANs are used to virtualize the bridging table of Layer 2 switches and create virtual switching topologies that overlay the physical network. Traffic traveling in one topology (ie VLAN) cannot bleed through into another topology. In this way, traffic from one group of users or devices can be kept isolated from other users or devices.

VLANs work great in a Layer 2 switched network, but what happens when you need to maintain this traffic separation across a Layer 3 boundary such as a router or firewall?