Category: MPLS VPN

If you use multiprotocol BGP (MP-BGP) in your network to support MPLS VPN, IPv6 or IP Multicast over BGP, it's best if you go all the way and configure an explicit ipv4 address family; the resulting BGP configuration is significantly easier to read and understand as the session-specific parameters are clearly separated from the routing-specific parameters and the IPv4 settings are nicely grouped in an explicit section.

To change the format of the BGP configuration, configure the IPv4 address family with the address-family ipv4 unicast router configuration command (the neighbor statements and other configuration settings pertinent to IPv4 configuration are automatically moved into the new address family) or manually activate a BGP neighbor for IPv4 route exchange with the neighbor activate router configuration command.

Cheng sent me an interesting question:

I'm reading your book MPLS and VPN Architectures and I've found the ip vrf forwarding name fallback global command in the “Additional Lookup in the Global Routing Table” section. I can only find this command in Junos, but not in IOS.

… and he was right. When we were writing the book, we described several features that were still in development as it looked like they would be in the production code by the time the book was published. Many of them made it into the public IOS releases (for example, the Carrier's Carrier architecture), but some of them (like this command) simply vanished from the surface.

Occasionally someone would try to persuade me that the layer-2 VPN services are like aspirin (you know, totally harmless plus it could get rid of all your headaches). OK, that might be true if you take the layer-2 VPN offering as a pure transport solution and plug in an extra router (sometimes also called a layer-3 switch by marketing people) between the Service Provider’s Ethernet (or whatever they give you) and your LAN. But there are people who don’t know the details and plug the SP Ethernet straight into their L2 switch … and things might even work for a while … until the whole network collapses.

An IOS device configured for IP+MPLS routing uses three different Maximum Transmission Unit (MTU) values:

• The hardware MTU configured with the mtu interface configuration command
• The IP MTU configured with the ip mtu interface configuration command
• The MPLS MTU configured with the mpls mtu interface configuration command

The hardware MTU specifies the maximum packet length the interface can support … or at least that's the theory behind it. In reality, longer packets can be sent (assuming the hardware interface chipset doesn't complain); therefore you can configure MPLS MTU to be larger than the interface MTU and still have a working network. Oversized packets might not be received correctly if the interface uses fixed-length buffers; platforms with scatter/gather architecture (also called particle buffers) usually survive incoming oversized packets.

The MPLS VPN implementation on Cisco IOS has always allowed you to create VRF static routes that pointed to interfaces belonging to other VRFs. The feature can be used to implement interesting overlapping VPN (or common services VPN) designs, some of which are explained in the MPLS and VPN Architectures books.

However, quite often the ability to create inter-VRF static routes is considered a major security problem, as an operator configuration error could establish undesired inter-VPN connectivity. In these cases, use the no ip route static inter-vrf configuration command to prevent such routes from being installed in the VRF routing table.

When we were writing the MPLS and VPN architectures books, there was a limit on the number of OSPF processes you could configure per PE-router. The limit was based on the fact that IOS supports up to 32 routing information sources. Two of them are static and connected; you also need an IGP and BGP in the MPLS VPN backbone, resulting in 28 OSPF processes that could be configured on a single PE router. This “feature” severely limited OSPF-based MPLS VPN deployments until IOS release 12.3(4)T when the limitation was removed, resulting in the availability of up to 30 routing processes per VRF.

RIP, BGP, and EIGRP never experienced the same limitations as you configure VRF-specific routing instances within address families of a single routing protocol

Every now and then I'm getting questions from my readers regarding the suitability of my MPLS books for the CCIP exam, for example:

I'm pursuing my CCIP and have a hard time finding the right MPLS study guide. I know you have the CCIP edition that was written in 2002, but I think the exam topics have changed. Can you recommend what book or books are best for the CCIP MPLS exam?

Are MPLS VPN Architectures Volume 1 & 2 two completely separate books or is Volume 2 a newer release. I was thinking of going for the CCIP and wanted to know if I should get both books or just the more recent one.

IOS release 12.3(11)T introduced Half-duplex VRF, a great feature for those of us who have to implement hub-and-spoke VPN (the VPN where all traffic has to pass through the central site), but hate the configuration hassle associated with it. Unfortunately, the way this feature is implemented, you can only configure it on virtual access/template interface, making it useless in most access networks. Too bad ...