As I've written about previously (The Importance of BGP NEXT_HOP in L3VPNs), the BGP NEXT_HOP attribute is key to ensuring end to end connectivity in an MPLS L3VPN. In the other article, I examine the different forwarding behavior of the network based on which of the egress PE's IP addresses is used as the NEXT_HOP. In this article I'll look at the subnet mask that's associated with the NEXT_HOP and the differences in forwarding behavior when the mask is configured to different values.

There is a lot of (mis-)information on the web stating that the PE's loopback address — which, as I explain in the previous article, should always be used as the NEXT_HOP — must have a /32 mask. This is not exactly true. I think this is an example of some information that has been passed around incorrectly, and without proper context, and is now taken as a rule. I'll explain more about this further on in the article.

In an MPLS network with L3VPNs, it's very easy for the NEXT_HOP attribute of a VPN route to look absolutely correct but be very wrong at the same time. In a vanilla IP network, the NEXT_HOP can point to any IP address that gets the packets moving in the right direction towards the ultimate destination. In an MPLS network, the NEXT_HOP must get the packets moving in the right direction but it must also point to the exact right address in order for traffic to successfully reach the destination.

I like MPLS. And I don't necessarily mean as a solution to solve a problem, but as something to configure in the lab. It's fun to build things that do something when you're done. Setting up OSPF or EIGRP and being able to traceroute across routers is meh. But configuring MPLS with all the associated technologies — an IGP, LDP, MP-BGP, — and then getting all of them working in unison... when you get the traceroute working, it's rewarding.

Here's something to keep an eye out for when you're troubleshooting MPLS: An LFIB entry (that is, the Label Forwarding Information Base) that states "No Label" versus one that states "Pop Label". These mean very different things and can be the difference between a working Label Switched Path (LSP) and a non-working LSP.

Think about this for a minute: An MPLS network with a two Provider Edge (PE) routers and some Provider (P) routers. The P routers have no VRFs configured on them and therefore have no routes whatsoever for any of the customer networks. A customer then does a traceroute from one of their sites, across the MPLS cloud, and into one of their other sites. The traceroute output shows the P routers as hops along the path.

How is it possible for the P routers to reply to the traceroute if they don't have routes back to the customer network?

Normally for these FFF articles I've taken to writing about new protocols as a way of introducing others to it and also edumacating myself about it. For this post I get all nostalgic and look at good old Enhanced Interior Gateway Routing Protocol (EIGRP).

Let's take a look at EIGRP and the state a route can get into where EIGRP tells you "FD is Infinity".

Here's the scenario: An enterprise network with an MPLS core and two branch locations connected to their own Provider Edge (PE) router. In addition to the MPLS link, the PEs are also connected via a DMVPN tunnel. The PEs are peering via iBGP (of course) and are also OSPF neighbors on the DMVPN. Both Customer Edge (CE) routers at the branch are OSPF neighbors with their local PE.

Task: Use the high speed MPLS network as the primary path between the CE routers and only use the DMVPN network if the MPLS network becomes unavailable.

Question: Is the solution as simple as adjusting the Admin Distance (AD) so that the iBGP routes are more preferred?

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.

This is a quick calculator I came up that I could use in the CCIE lab to translate between various IPv4 header QoS markings. As long as I could remember how to draw out the calculator, all I had to do was some basic math and I could translate between markings quite easily.

When I started studying in earnest for my CCIE, I started a log of how I was spending my time studying, which books and papers I'd read, videos I'd watched, and so on. I thought it would be a neat exercise to look back afterwards at what it took to achieve this goal. I'm also somewhat self-deprecating and tend to minimize my accomplishments, so having this data is a way for me to remember that this wasn't a small accomplishment at all.