Tag Archives: IP

AWS ABCs – Network Building Blocks

Given that my technical background is largely in the networking space (exhibit A, exhibit B, exhibit C(CIE)), one of the first things I tried to wrap my head around when being introduced to AWS is how networking works in the AWS cloud.

What I attempted to do was build a mental model by relating cloud networking constructs such as Virtual Private Cloud (VPC), subnets, and routing tables to on-prem, physical networking constructs. This worked pretty well but I did get tripped up at times because some of these constructs don’t map exactly one-for-one.

This post will explain the mental model I used while also calling attention to the elements or behaviors that don’t map exactly between on-prem and AWS.

Continue reading AWS ABCs – Network Building Blocks

Label Switched Multicast – Ethernet Header

I got an interesting email from Ying Lu who had read my posts on LSM:

I am curious about the Ethernet DA and codepoint used for multicast MPLS. Previously, I understand that:
– Ethernet DA is unicast MAC of nexthop of each replication leg.
– codepoint is 0x8847
However, looking at RFC5332, I am not so sure…
“Ethernet is an example of a multipoint-to-multipoint data link. Ethertype 0x8847 is used whenever a unicast ethernet frame carries an MPLS packet.

Ethertype 0x8847 is also used whenever a multicast ethernet frame carries an MPLS packet, EXCEPT for the case where the top label of the MPLS packet has been upstream-assigned.

Ethertype 0x8848, formerly known as the “MPLS multicast codepoint”, is to be used only when an MPLS packet whose top label is upstream assigned is carried in a multicast ethernet frame.

Interesting question. What is the ethernet destination address (DA) and the value of the ethernet type field (codepoint) when the MPLS packet is being sent on an LSM LSP?

Getting back into the lab, I started a ping from CE1 to a group that CE3 had joined. I then ran a sniff on the segment between P and PE3.

Sample LSM Topology
Sample LSM Topology

Examining the capture shows a unicast address in the ethernet DA field and an ethernet type of 0x8847.

LSM Ethernet Dest Address and Type

I started wondering if I could trick the P router into using a multicast ethernet frame so I spun up a fourth PE and attached it to the same segment that P and PE3 are on and had it join the same multicast group.

The P router continued to send unicast ethernet frames with a type of 0x8847 and just started putting two frames on the wire, one for PE3 and one for PE4. It did not, as I had hoped, put a multicast ethernet frame on the wire that would be picked up by both PEs.

So it appears that IOS — and I tested this with a version of IOS 15.4T — sends unicast ethernet frames when sending LSM packets and therefore also uses an ethernet type code of 0x8847.

Label Switched Multicast – Q&A

This post is the last one I’m planning in this series on Label Switched Multicast (LSM). The questions & answers below are meant to expand on topics from the previous posts or address topics that weren’t mentioned in the previous posts at all.

If you’re not familiar with LSM yet then this Q&A likely won’t make much sense to you and I recommend you go back and read through the previous posts.

Please post a comment if one of the answers isn’t clear or you have additional questions! Continue reading Label Switched Multicast – Q&A

Label Switched Multicast – Packet Walk

This post is going to follow a multicast packet as it moves through a sample MPLS network using Label Switched Multicast (LSM). I’ll show how the packet moves through the network by looking at the forwarding tables on different routers and also by doing some packet captures.

This post is part of a series I’m writing on LSM and if you’re not already familiar with LSM, I recommend you go back and read the previous posts.

After reading this post you will be able to precisely describe how LSM forwarding works in the data plane and will be able to do some basic troubleshooting.

Let’s get into the lab!

Continue reading Label Switched Multicast – Packet Walk

Label Switched Multicast – Configuration

In the previous post (Label Switched Multicast – An Introduction) in this series on Label Switched Multicast (LSM) I introduced the concepts behind LSM and draft-rosen, the two most poplar methods for transporting multicast traffic through MPLS Layer 3 VPNs.

In this article I will talk through the configuration of LSM on the PE and P routers and get to the point where two CEs are successfully passing multicast traffic via the MPLS network. All of the configuration examples will be relevant to Cisco IOS.

As was the case in the introduction article in the series, it’s best if you already have a good understanding of multicast and MPLS before reading this article.

At the end of this article you’ll be able to start configuring your own LSM environment using the configuration samples here as a template.

To the CLI! Continue reading Label Switched Multicast – Configuration

Label Switched Multicast – An Introduction

There are two common methods for transporting multicast packets within an MPLS-based Layer 3 VPN:

  1. Generic Routing Encapsulation (GRE) with Protocol Independent Multicast (PIM) (also known as “draft-rosen”)
  2. Label Switched Multicast (LSM)

There’s also a third method which uses Resource Reservation Protocol—Traffic Engineering (RSVP-TE) but I’m not going to get into that one.

In this first post in a series on LSM, I’ll describe how draft-rosen works, how LSM works, and then compare and contrast the two. Subsequent posts will focus solely on LSM.

At the end of this post, you will be able to describe conceptually how the control and data planes work with LSM and what the pros and cons are of LSM as compared to draft-rosen.

I will not be covering any theory on multicast or MPLS and will instead recommend that you be familiar with both topics before reading further.

Here we go!

Continue reading Label Switched Multicast – An Introduction

Choosing a Route: Order of Operations

In Cisco IOS packets are forwarded through the router (or Layer 3 switch) by Cisco Express Forwarding (CEF). A data structure called the CEF table contains a list of known IP prefixes and the outgoing interface that packets should be put on in order to get them onwards to their destination. That’s well and good. But how do the IP prefixes make it into the CEF table? To answer that question you have to work backwards and understand the order of operations that IOS goes through in order for a prefix to make it into the CEF table. Continue reading Choosing a Route: Order of Operations

What the *, traceroute?

If you’ve ever done a traceroute from one IOS box to another, you’ve undoubtedly seen output like this:

R8# traceroute
Tracing the route to
VRF info: (vrf in name/id, vrf out name/id)
  1 4 msec 3 msec 4 msec
  2 4 msec *  0 msec

That “msec * msec” output. Why is the middle packet always lost?? And why only on the last hop?? Continue reading What the *, traceroute?

VRFs and Shared Services Cheating with Junos

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.

Continue reading VRFs and Shared Services Cheating with Junos

An Introduction to Layer 3 Traffic Isolation

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.

Traffic Isolation Using VLANs

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? Continue reading An Introduction to Layer 3 Traffic Isolation