MPLS Configuration Tutorial – Step by Step

If you are looking for an MPLS Tutorial or step by step mpls configuration examples, this basic MPLS l3vpn configuration example will guide you from configuring the first router to  a 3 router MPLS core with 2 external sites.

If are you looking for an explanation of MPLS then I would advise you read the “What is MPLS” post first before attempting this lab.

MPLS Configuration Video

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.

I will be using GNS3 and configuring the routers as we go so you can follow along.

I have also started to get into Network Automation – if this is more your thing check out my pages on Ansible for Network Engineers

Cisco MPLS Tutorial Topology

Step 1 – IP addressing of MPLS Core and OSPF

First bring 3 routers into your topology R1, R2, R3 position them as below. We are going to address the routers and configure ospf to ensure loopback to loopback connectivity between R1 and R3

R1
hostname R1
int lo0 
ip add 1.1.1.1 255.255.255.255
ip ospf 1 area 0 

int f0/0
ip add 10.0.0.1 255.255.255.0
no shut
ip ospf 1 area 0

R2
hostname R2
int lo0
ip add 2.2.2.2 255.255.255.255
ip ospf 1 are 0 

int f0/0
ip add 10.0.0.2 255.255.255.0
no shut
ip ospf 1 area 0

int f0/1 
ip add 10.0.1.2 255.255.255.0 
no shut 
ip ospf 1 area 0 


R3
hostname R3
int lo0 
ip add 3.3.3.3 255.255.255.255
ip ospf 1 are 0 

int f0/0 
ip add 10.0.1.3 255.255.255.0 
no shut 
ip ospf 1 area 0 

You should now have full ip connectivity between R1, R2, R3 to verify this we need to see if we can ping between the loopbacks of R1 and R3

R1#ping 3.3.3.3 source lo0

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
Packet sent with a source address of 1.1.1.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 40/52/64 ms
R1#

You could show the routing table here, but the fact that you can ping between the loopbacks is verification enough and it is safe to move on.

Step 2 – Configure LDP on all the interfaces in the MPLS Core

In order to run MPLS you need to enable it, there are two ways to do this.

• At each interface enter the mpls ip command
• Under the ospf process use the mpls ldp autoconfig command

For this tutorial we will be using the second option, so go int the ospf process and enter mpls ldp autoconfig – this will enable mpls label distribution protocol on every interface running ospf under that specific process.

R1
router ospf 1
mpls ldp autoconfig

R2
router ospf 1
mpls ldp autoconfig

R3
router ospf 1
mpls ldp autoconfig

You should see log messages coming up showing the LDP neighbors are up.

R2#
*Mar  1 00:31:53.643: %SYS-5-CONFIG_I: Configured from console 
*Mar  1 00:31:54.423: %LDP-5-NBRCHG: LDP Neighbor 1.1.1.1:0 (1) is UP
R2#
*Mar  1 00:36:09.951: %LDP-5-NBRCHG: LDP Neighbor 3.3.3.3:0 (2) is UP

To verify the mpls interfaces the command is very simple – sh mpls interface
This is done on R2 and you can see that both interfaces are running mpls and using LDP

R2#sh mpls interface
Interface              IP            Tunnel   Operational
FastEthernet0/0        Yes (ldp)     No       Yes
FastEthernet0/1        Yes (ldp)     No       Yes

You can also verify the LDP neighbors with the sh mpls ldp neighbors command.

R2#sh mpls ldp neigh
    Peer LDP Ident: 1.1.1.1:0; Local LDP Ident 2.2.2.2:0
        TCP connection: 1.1.1.1.646 - 2.2.2.2.37909
        State: Oper; Msgs sent/rcvd: 16/17; Downstream
        Up time: 00:07:46
        LDP discovery sources:
          FastEthernet0/0, Src IP addr: 10.0.0.1
        Addresses bound to peer LDP Ident:
          10.0.0.1        1.1.1.1
    Peer LDP Ident: 3.3.3.3:0; Local LDP Ident 2.2.2.2:0
        TCP connection: 3.3.3.3.22155 - 2.2.2.2.646
        State: Oper; Msgs sent/rcvd: 12/11; Downstream
        Up time: 00:03:30
        LDP discovery sources:
          FastEthernet0/1, Src IP addr: 10.0.1.3
        Addresses bound to peer LDP Ident:
          10.0.1.3        3.3.3.3

One more verification to confirm LDP is running ok is to do a trace between R1 and R3 and verify if you get MPLS Labels show up in the trace.

R1#trace 3.3.3.3

Type escape sequence to abort.
Tracing the route to 3.3.3.3

  1 10.0.0.2 [MPLS: Label 17 Exp 0] 84 msec 72 msec 44 msec
  2 10.0.1.3 68 msec 60 msec *

As you can see the trace to R2 used an MPLS Label in the path, as this is a very small MPLS core only one label was used as R3 was the final hop.

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.

The next step is to configure MP-BGP between R1 and R3

This is when you start to see the layer 3 vpn configuration come to life

Step 3 – MPLS BGP Configuration between R1 and R3

We need to establish a Multi Protocol BGP session between R1 and R3 this is done by configuring the vpnv4  address family as below

R1#
router bgp 1
 neighbor 3.3.3.3 remote-as 1
 neighbor 3.3.3.3 update-source Loopback0
 no auto-summary
 !
 address-family vpnv4
  neighbor 3.3.3.3 activate

R3#
router bgp 1
 neighbor 1.1.1.1 remote-as 1
 neighbor 1.1.1.1 update-source Loopback0
 no auto-summary
 !
 address-family vpnv4
  neighbor 1.1.1.1 activate

*Mar  1 00:45:01.047: %BGP-5-ADJCHANGE: neighbor 1.1.1.1 Up

You should see log messages showing the BGP sessions coming up.

To verify the BGP session between R1 and R3 issue the command sh bgp vpnv4 unicast all summary

R1#sh bgp vpnv4 unicast all summary
BGP router identifier 1.1.1.1, local AS number 1
BGP table version is 1, main routing table version 1

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
3.3.3.3         4     1     218     218        1    0    0 03:17:48        0

You can see here that we do have a bgp vpnv4 peering to R3 – looking at the PfxRcd you can see it says 0 this is because we have not got any routes in BGP. We are now going to add two more routers to the topology. These will be the customer sites connected to R1 and R3. We will then create a VRF on each router and put the interfaces connected to each site router into that VRF.

Step 4 – Add two more routers, create VRFs

We will add two more routers into the topology so it now looks like the final topology

Router 4 will peer OSPF using process number 2 to a VRF configured on R1. It will use the local site addressing of 192.168.1.0/24.

R4 
int lo0
ip add 4.4.4.4 255.255.255.255 
ip ospf 2 area 2 
int f0/0
ip add 192.168.1.4 255.255.255.0 
ip ospf 2 area 2
no shut 

R1 
int f0/1 
no shut 
ip add 192.168.1.1 255.255.255.0 

Now at this point we have R4 peering to R1 but in the global routing table of R1 which is not what we want.

We are now going to start using VRF’s

What is a VRF  in networking?

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.

As an example if R1 was a PE Provider Edge router of an ISP and it had two customers that were both addressed locally with the 192.168.1.0/24 address space it could accommodate both their routing tables in different VRFs – it distinguishes between the two of them using a Route Distinguisher

So back to the topology – we now need to create a VRF on R1

For this mpls tutorial I will be using VRF RED

R1 
ip vrf RED 
rd 4:4
route-target both 4:4

The RD and route-target do not need to be the same – and for a full explanation please read this post on Route Distinguishers
Route Distinguisher vs Route Target before proceeding.

So now we have configured the VRF on R1 we need to move the interface F0/1 into that VRF

R1 
int f0/1
ip vrf forwarding RED

Now notice what happens when you do that – the IP address is removed

R1(config-if)#ip vrf fo
R1(config-if)#ip vrf forwarding RED
% Interface FastEthernet0/1 IP address 192.168.1.1 removed due to enabling VRF RED

You just need to re-apply it

R1
int f0/1
ip address 192.168.1.1 255.255.255.0

Now if we view the config on R1 int f0/1 you can see the VRF configured.

R1


R1#sh run int f0/1
Building configuration...

Current configuration : 119 bytes
!
interface FastEthernet0/1
 ip vrf forwarding RED
 ip address 192.168.1.1 255.255.255.0
 duplex auto
 speed auto
end

R1#

Now we can start to look int VRF’s and how they operate – you need to understand now that there are 2 routing tables within R1

• The Global Routing Table
• The Routing Table for VRF RED

If you issue the command sh ip route this shows the routes in the global table and you will notice that you do not see 192.168.1.0/24

R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
 D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
 N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
 E1 - OSPF external type 1, E2 - OSPF external type 2
 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
 ia - IS-IS inter area, * - candidate default, U - per-user static route
 o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is directly connected, Loopback0
 2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/11] via 10.0.0.2, 01:03:48, FastEthernet0/0
 3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/21] via 10.0.0.2, 01:02:29, FastEthernet0/0
 10.0.0.0/24 is subnetted, 2 subnets
C 10.0.0.0 is directly connected, FastEthernet0/0
O 10.0.1.0 [110/20] via 10.0.0.2, 01:02:39, FastEthernet0/0
R1#

If you now issue the command sh ip route vrf red – this will show the routes in the routing table for VRF RED

R1#sh ip route vrf red
% IP routing table red does not exist

NOTE: The VRF name is case sensitive!

R1#sh ip route vrf RED

Routing Table: RED
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
 D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
 N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
 E1 - OSPF external type 1, E2 - OSPF external type 2
 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
 ia - IS-IS inter area, * - candidate default, U - per-user static route
 o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

C 192.168.1.0/24 is directly connected, FastEthernet0/1
R1#

We just need to enable OSPF on this interface and get the loopback address for R4 in the VRF RED routing table before proceeding.

R1
 int f0/1
 ip ospf 2 area 2

You should see a log message showing the OSPF neighbor come up

R1(config-if)#
 *Mar 1 01:12:54.323: %OSPF-5-ADJCHG: Process 2, Nbr 4.4.4.4 
on FastEthernet0/1 from LOADING to FULL, Loading Done

If we now check the routes in the VRF RED routing table you should see 4.4.4.4 in there as well.

R1#sh ip route vrf RED

Routing Table: RED
 Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
 D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
 N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
 E1 - OSPF external type 1, E2 - OSPF external type 2
 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
 ia - IS-IS inter area, * - candidate default, U - per-user static route
 o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

4.0.0.0/32 is subnetted, 1 subnets
 O 4.4.4.4 [110/11] via 192.168.1.4, 00:00:22, FastEthernet0/1
 C 192.168.1.0/24 is directly connected, FastEthernet0/1
 R1#

We now need to repeat this process for R3 & R6

Router 6 will peer OSPF using process number 2 to a VRF configured on R3. It will use the local site addressing of 192.168.2.0/24.

R6
int lo0
ip add 6.6.6.6 255.255.255.255 
ip ospf 2 area 2 
int f0/0
ip add 192.168.2.6 255.255.255.0 
ip ospf 2 area 2
no shut 

R3
int f0/1 
no shut 
ip add 192.168.2.3 255.255.255.0 

We also need to configure a VRF onto R3 as well.

R3
ip vrf RED
rd 4:4
route-target both 4:4

So now we have configured the VRF on R3 we need to move the interface F0/1 into that VRF

R3
int f0/1
ip vrf forwarding RED

Now notice what happens when you do that – the IP address is removed

R3(config-if)#ip vrf forwarding RED
% Interface FastEthernet0/1 IP address 192.168.2.1 removed due to enabling VRF RED

You just need to re-apply it

R3
int f0/1
ip address 192.168.2.1 255.255.255.0

Now if we view the config on R3 int f0/1 you can see the VRF configured.

R3
R3#sh run int f0/1
Building configuration...

Current configuration : 119 bytes
!
interface FastEthernet0/1
ip vrf forwarding RED
ip address 192.168.2.1 255.255.255.0
duplex auto
speed auto
end

Finally we just need to enable OSPF on that interface and verify the routes are in the RED routing table.

R3 
int f0/1
ip ospf 2 area 2

Check the routes in vrf RED

R3
R3#sh ip route vrf RED

Routing Table: RED
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

Gateway of last resort is not set

     6.0.0.0/32 is subnetted, 1 subnets
O       6.6.6.6 [110/11] via 192.168.2.6, 00:02:44, FastEthernet0/1
C    192.168.2.0/24 is directly connected, FastEthernet0/1
R3#

Ok so we have come a long way now let’s review the current situation. We now have this setup

R1,R2,R3 form the MPLS Core and are running OSPF with all loopbacks running a /32 address and all have full connectivity. R1 and R3 are peering with MP-BGP. LDP is enabled on all the internal interfaces. The external interfaces of the MPLS core have been placed into a VRF called RED and then a site router has been joined to that VRF on each side of the MPLS core – (These represent a small office)

The final step to get full connectivity across the MPLS core is to redistribute the routes in OSPF on R1 and R3 into MP-BGP and MP-BGP into OSPF, this is what we are going to do now.

We need to redistribute the OSPF routes from R4 into BGP in the VRF on R1, the OSPF routes from R6 into MP-BGP in the VRF on R3 and then the routes in MP-BGP in R1 and R3 back out to OSPF

Before we start lets do some verifications

Check the routes on R4

R4#sh ip route

4.0.0.0/32 is subnetted, 1 subnets
C 4.4.4.4 is directly connected, Loopback0
C 192.168.1.0/24 is directly connected, FastEthernet0/0

As expected we have the local interface and the loopback address.

When we are done we want to see 6.6.6.6 in there so we can ping across the MPLS

Check the routes on R1

R1#sh ip route 

1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is directly connected, Loopback0
 2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/11] via 10.0.0.2, 00:01:04, FastEthernet0/0
 3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/21] via 10.0.0.2, 00:00:54, FastEthernet0/0
 10.0.0.0/24 is subnetted, 2 subnets
C 10.0.0.0 is directly connected, FastEthernet0/0
O 10.0.1.0 [110/20] via 10.0.0.2, 00:00:54, FastEthernet0/0

Remember we have a VRF configured on this router so this command will show routes in the global routing table (the MPLS Core) and it will not show the 192.168.1.0/24 route as that is in VRF RED – to see that we run the following command

R1#sh ip route vrf RED

Routing Table: RED

4.0.0.0/32 is subnetted, 1 subnets
O 4.4.4.4 [110/11] via 192.168.1.4, 00:02:32, FastEthernet0/1
C 192.168.1.0/24 is directly connected, FastEthernet0/1

Here you can see Routing Table: RED is shown and the routes to R4 are now visible with 4.4.4.4 being in OSPF.

So we need to do the following;

• Redistribute OSPF into MP-BGP on R1
• Redistribute MP-BGP into OSPF on R1
• Redistribute OSPF into MP-BGP on R3
• Redistribute MP-BGP into OSPF on R3

Redistribute OSPF into MP-BGP on R1

R1
router bgp 1
address-family ipv4 vrf RED 
redistribute ospf 2

Redistribute OSPF into MP-BGP on R3

R3
router bgp 1
address-family ipv4 vrf RED 
redistribute ospf 2

This has enabled redistribution of the OSPF routes into BGP. We can check the routes from R4 and R6 are now showing in the BGP table for their VRF with this command

sh ip bgp vpnv4 vrf RED

R1#sh ip bgp vpnv4 vrf RED
BGP table version is 9, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, 
 r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

 Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 4:4 (default for vrf RED)
*> 4.4.4.4/32 192.168.1.4 11 32768 ?
*>i6.6.6.6/32 3.3.3.3 11 100 0 ?
*> 192.168.1.0 0.0.0.0 0 32768 ?
*>i192.168.2.0 3.3.3.3 0 100 0 ?

Here we can see that 4.4.4.4 is now in the BGP table in VRF RED on R1 with a next hop of 192.168.1.4 (R4) and also 6.6.6.6 is in there as well with a next hop of 3.3.3.3 (which is the loopback of R3 – showing that it is going over the MPLS and R1 is not in the picture)

The same should be true on R3

R3#sh ip bgp vpnv4 vrf RED
BGP table version is 9, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 4:4 (default for vrf RED)
*>i4.4.4.4/32 1.1.1.1 11 100 0 ?
*> 6.6.6.6/32 192.168.2.6 11 32768 ?
*>i192.168.1.0 1.1.1.1 0 100 0 ?
*> 192.168.2.0 0.0.0.0 0 32768 ?

Which it is!  6.6.6.6 is now in the BGP table in VRF RED on R3 with a next hop of 192.168.2.6 (R6) and also 4.4.4 is in there as well with a next hop of 1.1.1.1 (which is the loopback of R1 – showing that it is going over the MPLS and R1 is not in the picture)
The final step is to get the routes that have come across the MPLS back into OSPF and then we can get end to end connectivity

R1

router ospf 2 
redistribute bgp 1 subnets 

R3 

router ospf 2 
redistribute bgp 1 subnets

If all has worked we should be now able to ping 6.6.6.6 from R4

Before we do let’s see what the routing table looks like on R4 

R4#sh ip route 


4.0.0.0/32 is subnetted, 1 subnets
C 4.4.4.4 is directly connected, Loopback0
 6.0.0.0/32 is subnetted, 1 subnets
O IA 6.6.6.6 [110/21] via 192.168.1.1, 00:01:31, FastEthernet0/0
C 192.168.1.0/24 is directly connected, FastEthernet0/0
O E2 192.168.2.0/24 [110/1] via 192.168.1.1, 00:01:31, FastEthernet0/0

Great we have 6.6.6.6 in there

Also check the routing table on R6 

R6#sh ip route

4.0.0.0/32 is subnetted, 1 subnets
O IA 4.4.4.4 [110/21] via 192.168.2.1, 00:01:22, FastEthernet0/0
6.0.0.0/32 is subnetted, 1 subnets
C 6.6.6.6 is directly connected, Loopback0
O IA 192.168.1.0/24 [110/11] via 192.168.2.1,00:01:22,FastEthernet0/0
C 192.168.2.0/24 is directly connected, FastEthernet0/0

Brilliant we have 4.4.4.4 in there so we should be able to ping across the MPLS

R4#ping 6.6.6.6

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 6.6.6.6, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max= 40/48/52ms

Which we can – to prove this is going over the MPLS and be label switched and not routed, lets do a trace

R4#trace 6.6.6.6

Type escape sequence to abort.
Tracing the route to 6.6.6.6

1 192.168.1.1 20 msec 8 msec 8 msec
 2 10.0.0.2 [MPLS: Labels 17/20 Exp 0] 36 msec 40 msec 36 msec
 3 192.168.2.1 [MPLS: Label 20 Exp 0] 16 msec 40 msec 16 msec
 4 192.168.2.6 44 msec 40 msec 56 msec
R4#

More MPLS Configuration Resources

Want to learn more about MPLS? Check out these other great posts on MPLS configuration and troubleshooting.

• MPLS LDP Troubleshooting – This postscovers some basic MPLS LDP Troubleshooting commands and how to use them.
• Route Distinguisher vs Route Target – This post covers the difference between the RD & RT values – and no they are no the same thing!

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.

If you have enjoyed this tutorial, please take a minute to enter your name and email below to be kept up to date on new articles and upcoming training courses.

Roger