This post is still part of my learning journey towards my CCIE Routing and Switching…I will be going through a basic configurations of MPLS and dig deeper into how MPLS labels were switched between routers…MPLS is using LDP or Label Distribution Protocol which used a label to switched packets across Label Edge Routers and  Label Switch Routers…..

So what are the components of a MPLS?

CE: Customer Edge Router
PE: Provider Edge Router also known as LER or Label Edge Router
P: Label Switch Routers (LSR) or Transit Router..

Moreover, there are three actions that happens as labels are switched between the PE to P routers:

Label Push: This means that labels are being added to a prefix
Label Swap: This means that labels were swap or replace with another labels as it passes from PE to 
                     P
Label Pop:  This means that the label are being remove. This happens before the PE router on the 
                    destinations side.


I will be using the diagram below to show how MPLS works:

  CE1 Configurations:

  CE2 Configurations:

  PE1 Configurations

  ISP or Provider Router Configuration

  PE2 Configurations:

So here’s the BGP Status of each of the routers…

CE1#sh ip bgp summary
!
Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
172.16.12.2     4        65512      81      78        3    0    0 01:06:58        1
CE1#

PE1#show ip bgp summary
!
Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
4.4.4.4         4        65512     129     124        9    0    0 01:48:25        1
172.16.12.1     4         1000      78      82        9    0    0 01:10:36        1
PE1#

So for ISP, I never set up BGP on this router but rather, it’s running an IGP between PE1 and PE2..

ISP#show ip bgp summary
% BGP not active

PE2#show ip bgp summary
!
Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
2.2.2.2             4          65512     126     130        9    0    0 01:49:34        1
172.16.45.5     4         5000        80      82           9    0    0 01:11:32        1
PE2#

CE2#sh ip bgp summary
!

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
172.16.45.4     4        65512      73      71        3    0    0 01:00:19        1

 Now, let me show how to enable Label Distribution Protocol on the PE1, ISP or P and PE2 router…

   So the moment that I have enabled the Label Distribution Protocol using the command “mpls ip”  
   under the interface between PE1 to P/ISP to PE2.., the LDP neighbors were formed…

  As we can see from my configs, I have advertised 1.1.1.1/24 in CE1 and 5.5.5.5/24 in CE2…This is 
  something we can check on the PE1, P and PE2…

  Ping from CE1 to CE2 loopback and vice-versa is working fine…

  Now, let’s check how the traffic from R1 reached R5..

   From CE1 BGP database, in order to reach 5.5.5.5, it has to go through PE1..

  Now, let’s check PE1 …As observed, in order to reach 5.5.5.5, it has to go through with PE2…

   But since I have enabled LDP, let’s check the MPLS forwarding table instead on PE1 ( This is 
   analogous to BGP database table) with the command “show mpls forwarding-table”

   There are three types of packet switching in which by default Cisco routers are using IP CEF. This 
   is used by LDP to switched packets. The other two types include process switching and route
   caching or fast forwarding.

   Now, let me check the specific CEF table towards the destination loopback of R5 using the 
   command “show ip cef”

    PE1#show ip cef 5.5.5.5
     5.5.5.0/24
      nexthop 172.16.23.3 Serial2/1 label 17
    PE1#

So what does the output of “show ip cef 5.5.5.5” on PE1 indicates…

But what does P or ISP router or Transit router is telling us? So it shows below that there is no route towards 5.5.5.5..This is true because, I never set up IBGP between ISP and PE1 and PE2…

ISP#show ip cef 5.5.5.5
0.0.0.0/0
  no route

IMPORTANT: Take note that labels are only local to the Router…When it pass the label to the next router, it will be change (LABEL SWAP) by the router and stores it in its LDP table…The Label might be the same number though as the labels by default always starts with 16 and increments by 1…

Now, let me check the P or ISP router Forwarding Table ( LDP Table):

ISP#show mpls forwarding-table
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop
Label      Label      or Tunnel Id     Switched      interface
16         No Label   2.2.2.2/32       9236          Se2/1      point2point
17         No Label   4.4.4.4/32       7276          Se2/2      point2point

 So here is what happens:

1. When ISP received a label tag with 17, then it has to forward that to 4.4.4.4 (PE2)..
2. Similarly, when it received a label with tag of 16, it has to forward to 2.2.2.2 (PE1)..

As expected, there should no CEF table for 5.5.5.5 from the P router..

 ISP#show ip cef 5.5.5.5
        0.0.0.0/0
        no route

Now, let’s take a look on the LDP forwarding table of PE2..

PE2#show mpls forwarding-table
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop
Label      Label      or Tunnel Id     Switched      interface
16         16         2.2.2.2/32       0             Se2/2      point2point
17         No Label   3.3.3.3/32       0             Se2/2      point2point
18         Pop Label  172.16.23.0/24   0             Se2/2      point2point
 
As observed, the Outgoing Label Tag as Pop Label for the prefix 172.16.23.0/24…

By this time, let’s check the CEF table on PE2..

PE2#show ip cef 5.5.5.5
5.5.5.0/24
  nexthop 172.16.45.5 Serial2/0
PE2#
 
>>> So the above results is showing that the label were already removed. As we have seen on the previous configurations of the LDP, it’s only enabled on Serial 2/2 of the PE2 but not on the interface facing CE2…


On the other hand, the IP CEF results towards 1.1.1.1 shows that it’s label has a value of 16 and the next hop is the P or ISP router..

PE2#show ip cef 1.1.1.1
1.1.1.0/24
  nexthop 172.16.34.3 Serial2/2 label 16


Finally, we can check from PE2 that 5.5.5.5 is advertise via BGP…

PE2#show ip route bgp
!
Gateway of last resort is not set

      1.0.0.0/24 is subnetted, 1 subnets
B        1.1.1.0 [200/0] via 2.2.2.2, 01:47:46
      5.0.0.0/24 is subnetted, 1 subnets
B        5.5.5.0 [20/0] via 172.16.45.5, 01:47:49


In summary, here are the important terms/configurations to set up MPLS:

A. MPLS device components:

 –  CE Router
 –  PE Router or Label Edge Router (LER)
 –  P or ISP router also known as Label Switch Router (LSR)  or Transit router…

B.   Label switching happens with three important steps:

   –  Label Push
   –  Label Swap
   –  Label Pop

C.  LDP  or Label Distribution Protocol is the industry standard to enable MPLS…Cisco’s 
     proprietary  protocol which performs the same functionality is called TDP or Tag Distribution 
     Protocol..

    Now, in order to enable LDP neighbors, we can used the following two commands:

   1. On all participating interface to established LDP neighbors, we can configured 
    
        mpls ip

   2. Alternative options is to enable LDP is to apply ” mpls ldp autoconfig “ under any IGP routing 
       protocol such as OSPF..

D. To verify the LDP Table or MPLS Forwarding table, we can used the command:

        show mpls forwarding-table

E. To verify specific label on a destination address, we can used the command below:

        show ip cef “x.x.x.x”



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