Do MPLS LSRs require a Layer 3 IP routing protocol?

Question

Recently I've been trying to get my head around MPLS, it's a part of the project we're doing. During one of our meetings one of the participant ask a question:

Since LSRs doesn't check IP routing table when it's forwarding packets. If we're sure that every packet is going through MPLS not the normal IP networks, is it necessary to have IP protocols running on LSRs?

I've been trying to get the answer to this question, but can't seems to find a clear enough answer. The closest one I can find is here. MPLS can reroute, but can it route?. That question says MPLS requires a layer 3 protocol to work properly.

But what if the whole network from end-to-end is an MPLS network, will the IP protocol still be necessary? My guess is it's still necessary but my understanding to MPLS is not clear enough so that I can point out a situation that this won't work.

Answer 1

In theory you can run MPLS without an IGP, any sane network admin would not though. MPLS labels can be assigned through:

LDP
RSVP-TE

Generally a router runs IGP to populate the Routing Information Base (RIB). This is the control plane, routes are then installed into the Forwarding Information Base (FIB). This is the data plane forwarding, depending on platform, there may also be distributed forwarding on linecards so the linecard will have a local database.

When running LDP, labels are advertised to adjacent routers based on the RIB. The labels are then stored in the Label Information Base (LIB). This is the control plane and equivalent to the RIB but contains labels. The LIB is then one of the components used to build the Label Forwarding Information Base (LFIB) which is the equivalent of the FIB.

Running LDP, without an IGP, the labels would not be assigned because there are no routes in the RIB. This could be solved by entering static routes but it's obviously not a scalable solution.

When using RSVP-TE, labels are not assigned through the RIB. Tunnels are manually defined, connectivity is still needed between the headend and tailend, which is usually done through IGP but could be done through static routing as well.

There are special cases where routing is not needed with RSVP-TE because the neighbor is statically defined such as in the configuration below:

interface FastEthernet1/0
ip address 20.0.23.2 255.255.255.0
mpls traffic-eng tunnels
mpls traffic-eng passive-interface nbr-te-id 3.3.3.3 nbr-if-addr 20.0.23.3

Example above came from RSVP-TE without IGP

So the configuration is possible but definitely not recommended. To be able to reach the router remotely for management you would want to have an IGP running, also possible to solve through static routes of course. In the end it's about scalability and how prone to error each solution is.

Answer 2

CovaraC: MPLS is a switching (S) protocol it's not MPLR (R) Routing, it doesn't know about subnets in 2 3 routers away, it only knows about the connected interface. It also doesn't know which path is the best one to X subnets. MPLS just put a lable for any outgoing packet to any route, it doesn't care if this packet is going out to the best route or not, routing protocols will decide which path is the best. After routing protocols (OSPF, EIGRP, ISIS, ...) decide which interface and next hop is the best, MPLS will insert one lable for that next hop. Why ? so that next router doesn't need to open the packet and see what is inside for layer 3 ip once mpls knows all lables and next hopes, this is the main purpose of MPLS , reducing CPU and Memory usage. Second is that, when a lable is inserted in low layer before layer 3 , you can put any info and subnets even ipv6 in layer 3, routers with MPLS don't care what is there. they know lables now.