MPLS LDP (Label Distribution Protocol)

LDP is a protocol that automatically generates and exchanges labels between routers. Each router will locally generate labels for its prefixes and will then advertise the label values to its neighbors.

It’s a standard, based on Cisco’s proprietary TDP (Tag Distribution Protocol). It’s pretty much the same story as 802.1Q/ISL or PaGP/LACP. Cisco created a protocol and a standard was created later. Nowadays almost everyone uses LDP instead of TDP.

Like many other protocols, LDP first establishes a neighbor adjacency before it exchanges label information. It works a bit different than most protocols though…

First we send UDP multicast hello packets to discover other neighbors. Once two routers decide to become neighbors, they build the neighbor adjacency using a TCP connection. This connection is then used for the exchange of label information. Normally a loopback interface is used for the neighbor adjacency. Here’s an example:

MPLS LDP Multicast Unicast


The two routers above will send multicast hello packets on their FastEthernet interfaces. Within this hello packet, they will advertise a transport IP address. This IP address is then used to establish the TCP connection between the two routers. Here’s what the hello packet looks like in wireshark:

MPLS LDP Hello Packet

In the capture above you can see a couple of interesting things:

  • The hello packets are sent to multicast address using source/destination UDP port 646.
  • Each router has a unique ID called the LSR (Label Switch Router) ID. This is similar to how most protocols select an ID, by default it will select the highest IP address on a loopback interface. If you don’t have any loopback interfaces then we will use the highest IP address on a physical interface.
  • At the bottom you find the transport address. This is what we use to build the actual TCP connection.  Like the LSR ID, the router selected the IP address on the loopback interface as the transport address.
Make sure that the IP address that LDP has selected for the transport address is advertised in your routing protocol. Otherwise your routers will be able to hear each others hello packets but they can’t form a neighbor adjacency since the transport address(es) are unreachable.

This is different compared to how routing protocols like OSPF or EIGRP form neighbor adjacencies. For example, when you run OSPF then your routers will form neighbor adjacencies on all interfaces that run OSPF:

OSPF two neighbor adjacencies

LDP will only form a single neighbor adjacency, no matter how many interfaces you have in between your routers:

LDP single neighbor adjacency

LDP is a bit similar to BGP when you use the loopback interfaces for the neighbor adjacency. When we use BGP we have to use the update-source command to select the source, LDP does it automatically.

So once our LDP routers have become neighbors, how do we exchange label information? To explain this, let’s do a quick review of how normal routing uses the RIB and FIB. If you have no idea what these two are then I recommend you to read my CEF tutorial first before you continue.

control plane rib forwarding plane fib

With normal routing, we use routing protocols like EIGRP, OSPF or BGP to learn prefixes from other routers. These are all stored in the RIB (Routing Information Base), this is your routing table.

The information in the RIB is used to build the FIB (Forwarding Information Base) which is what we use for actual forwarding of IP packet. These tables are all used for IP packets but for MPLS we use something else:

Fib Lfib Cisco

When we use LDP on Cisco IOS, we locally generate a label for each prefix that we can find in the RIB, except for BGP prefixes. This information is then added to the LIB (Label Information Base).

The information in the LIB is used to build the LFIB (Label Forwarding Information Base). When the router has to forward a packet with a MPLS label on it, it will use the LFIB for forwarding decisions.

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Forum Replies

  1. Hi Rene,

    Thanks for your reply.
    As you mentioned, information inside the RIB is use to build the LIB.
    So i would imagine to have a LABEL for all the available routes.

    e.g. I have 3 best routes to the same destination with rip, eigrp, ospf in the RIB.  via  - rip  via  - eigrp  via  - ospf

    q1) Can i check for FIB, does it contain the same amount of available routes as in the RIB ?

    q2) if the above is yes, i would believe FLIB and FIB would have the same amount of labels ?

    q3) it is

    ... Continue reading in our forum

  2. Hi Alan,

    If OSPF, EIGRP and RIP have learned the route then only one will be installed in the routing table. EIGRP will be selected since it has the lowest administrative distance.

    By default, LDP will generate a label for each route in the routing table. This is something that you can change by using access-lists if you want.

    The LFIB has pretty much the same information as the FIB, however some entries in the FIB (like directly connected interfaces) won’t show up in the LFIB.

    Theoretically, switching based on labels is faster than routing where

    ... Continue reading in our forum

  3. Hi Stuart,

    Did you see this tutorial?

    I think that one will be helpful to understand this better. Within the MPLS “core”, we only advertise the loopback interfaces of the PE routers in our IGP, OSPF in most of my examples. The P router will only have these networks in its routing table. The LIB is based on the information of the RIB (Routing Table), so you only find labels for those loopbacks there, these are the transport labels needed to get from one PE to another PE.

    Only the PE routers have to know

    ... Continue reading in our forum

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