Mpls Configuration Commands

[Tonja Witcraft]

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The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

To implement the MPLS feature, you must have a router from the range of Cisco 2600 or higher. To select the required Cisco IOS with MPLS feature, use the Software Research tool. Also check for the additional RAM and Flash memory required to run the MPLS feature in the routers. WIC-1T, WIC-2T, and serial interfaces can be used.

When used with MPLS, the VPN feature allows several sites to interconnect transparently through a service provider network. One Service Provider network can support several different IP VPNs. Each of these appears to its users as a private network, separate from all other networks. Within a VPN, each site can send IP packets to any other site in the same VPN.

Each VPN is associated with one or more Virtual Routing and Forwarding (VRF) instances. A VRF consists of an IP routing table, a derived Cisco Express Forwarding (CEF) table, and a set of interfaces that use this forwarding table. The router maintains a separate Routing Information Base (RIB) and CEF table for each VRF. Therefore, the information is not sent outside the VPN and allows the same subnet to be used in several VPNs and does not cause duplicate IP address problems. The router that uses Multiprotocol BGP (MP-BGP) distributes the VPN routing information with the MP-BGP extended communities.

2. Configure an IGP on the service provider core, either Open Shortest Path First (OSPF) or Intermediate System-to-Intermediate System (IS-IS) protocols are the recommended options, and advertise the Loopback0 from each P and PE routers.

Set up the import and export properties for the MP-BGP extended communities. These are used to filter the import and export process with the command route-target both as shown in the next output:

There are several ways to configure BGP, for example, you can configure PE routers as BGP neighbors or use a Route Reflector (RR) or Confederation methods. A Route Reflector is used in the next example, which is more scalable than the use of direct neighbors between PE routers:

In this next sample, the show ip route vrf commands show the same prefix 10.0.6.0/24 in both the outputs. This is because the remote PE has the same network for two Cisco clients, CE_B2 and CE_A3, which is allowed in a typical MPLS VPN solution.

When you run a traceroute between two sites, in this example two sites of Client_A (CE-A1 to CE-A3), it is possible to see the label stack used by the MPLS network (if it is configured to do so by mpls ip propagate-ttl ).

From what meager knowledge of Junos I have, the first command will configure the router to accept MPLS/labeled packets. By default, the interface can only accept IP packets so this command is needed to activate MPLS on the forwarding plane.

From the perspective or the Routing/Forwarding Engine, MPLS is also a protocol, needing its own LIB/LFIB ("show route table mpls.0") where all the labels including the default labels such as Router Alert and explicit nulls will be stored.

Thus, as far as I understand, the redundant configuration is needed to keep the configuration in line with the Junos philosophy of separate forwarding and control planes. First command for the forwarding plane, second to activate the control plane.

My reasoning (completely subjective) was that IPv4 is on by default on Junos so it does not need to be explicitly turned on. I would extend that reasoning (again completely subjective and I cannot back this up via independent sources) for IPv6.

Regarding ISO, Junos to my knowledge does not support CLNS as data traffic but only the IS-IS implementation. For that, the configuration would be congruent to the MPLS configuration in that IS-IS has to be turned on by "set protocols is-is" to enable the only ISO protocol supported.

If you are looking for an MPLS Tutorial or step by step mpls configuration examples, this basic MPLS VPN configuration example will guide you from configuring the first router to a 3 router MPLS core with 2 external sites.

The entire tutorial is covered in this video above so if you like to just watch the video is there, if you want to follow along I suggest you open this page twice or print it out so you can make notes.

Building the simple MPLS topology below this will consist of a 3 router MPLS core and two remote sites in the same VRF running OSPF as the PE-CE routing protocol. This will be quite a long post as I will be taking you through every single verification along the way to ensure you understand how each section works.

So to review we have now configured IP addresses on the MPLS core, enabled OSPF and full IP connectivity between all routers and finally enabled mpls on all the interfaces in the core and have established ldp neighbors between all routers.

Virtual routing and forwarding (VRF) is a technology included in IP (Internet Protocol) that allows multiple instances of a routing table to co-exist in a router and work together but not interfere with each other.. This increases functionality by allowing network paths to be segmented without using multiple devices.

In the next MPLS Tutorial I will add a second customer site into the mix and also go through some MPLS Troubleshooting where I will go through turning off different features and trying to break the MPLS and show you the logical steps to troubleshoot it.

Multiprotocol Label Switching (MPLS) is a way of routing traffic within a telecommunications network that directs data from one node to the next based path labels rather than long network addresses, It also allows the sharing of address space for clients as it is labels that are being routed not prefixes.

No, MPLS is a method to route networks across a service provider network, routing protocols like OSPF and BGP are used to make MPLS work. MPLS operates using BGP and typically uses OSPF to exchange routes with the customer.

You spoke of additional information such as

In the next MPLS Tutorial I will add a second customer site into the mix and also go through some MPLS Troubleshooting where I will go through turning off different features and trying to break the MPLS and show you the logical steps to troubleshoot it. Im interested in the troubleshooting, where can I find this?

Thanks very much, I am just re-learning my way around MPLS so this was a great refresh. I got caught out on GNS3 playing games with me and dropping the LDP config in OSFP so it all looked good except in the core. I am just going to expand out now to BGP but thanks for taking the time to document this so well.

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Generally if a corporate company takes MPLS from any service provider, I heard that mostly we use BGP as routing protocol for mpls which is called MP -BGP. We need to work with service provider for completing configuration part and establishing neighbor relationships with ISP routers.

I think , ISP will give us some AS numbers to be configured on our routers. we need to configure that AS number and IP addresses on our routers in BGP processes. Later my doubt is do we have to configure MPLS commands on our router interfaces ? or will ISP do that part assuming their routers as Edge routers.

You are not going to run MPLS. The MPLS is used and seen only on ISP routers. Your routers will not use MPLS. Also, no MPLS will be run between your and ISP's edge routers. The MPLS is purely the stuff of your ISP and you do not need to care about it at all.

Why are we allocated AS number in the private range. Is it not a problem for routing private AS numbers over internet. Generally if we take leasedlines also, ISP will give a public ip to configure on our routers . (point to point) Then what is the exception here coming to BGP. Why are we getting private AS numbers ?

You said that some automated process will take care of Label assignment for customer routes . Where does it happen ? on ISP router ? If so in what form ? is it VRF ? can you give me more info on this ?

It is because you do not need any unique AS. You are not a standalone AS, rather, you are simply a part (a customer) of your ISP. You will either use your ISP's AS number, or you will be assigned a private AS. If you are assigned a private AS number, your ISP will take care of removing that private AS when - and if - it advertises your company networks to other providers and out to public internet. If you are purchasing only an MPLS VPN service then your company routes won't probably be advertised to public internet, in which case having a unique AS number would be completely useless.

Yes, the ISP will create a VRF on his PE routers where your company is connected. Each network learned from your company and stored in the VRF will be assigned a unique label value and advertised to other PE routers via BGP. This unique enumeration of entries in different VRFs is performed by the IOS running on the PE router.

Multiprotocol label switching (MPLS) is a telecom routing technique that uses labels to direct data between nodes. It is supported by the Linux networking stack, and many articles and tutorials have been written about how to configure it with ip route. However, you can also handle MPLS at a lower level with tc.

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