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Facts or key information about MPLS: – StalinBabu.com – My Personal Blog
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Facts or key information about MPLS:

  • In a MPLS network, the forwarding decision is made based on labels.MPLS labels are inserted between layer 2 and layer 3
  • The “P” router is the router inside the service provider network in the MPLS path
  • LDP router ID is the configured highest IP address of the loopback interface. If no loopback interface is configured, the IP address on the interface is selected as LDP router ID
  • MPLS transport address is LDP router IP, But this can be configured using the command – mplsldp router-id gig2/0 force globally or mplsldp discovery transport-address interface under interface config
  • Frame mode MPLS – Let’s say a local label to a route is 305 and the router has 10 interfaces, the router will advertise 305 for through all interfaces.
  • When same prefix are known to the router by two different routing protocols, the longest prefix matched route wins. If the prefix length is same, the route with the lowest administrative distance is used by the router
  • Using MPLS L3 VPN we can keep routing information private per customer. So two customers sharing the same MPLS network will get a logical private connection using the shared resources.

Lab Scenario:

Service provider network should provide MPLS transport to Customer A and Customer B. Both customers are using the same private IP addressing in their internal network. Each connection to the provider edge router they are using different IGP protocol.


Step – 1: Configure IP address on interfaces for router R1 to R5 and configure OSPF routing


Step 1: Verification

This display shows that the traceroute from R1 to R2 is IP routed and reaching the ingress interface of R2


Step – 2: Turn on MPLS in all routers R1 to R6

The following configuration in the routers will turn on MPLS


mpls label range: This command is used to limit the label range to be used in that router only for lab purposes. This command may not be needed in real world as we may need more 100 labels

mplsip: this is configured in the global config and also in the interface that is participating in mpls

MPLS Verification:



Indicates that this is a MPLS label switched and also indicates the label used in each of the hop.


The following information can be noted in the above output

  • The peer LDP ID is and it is R3 loopback IP address
  • The local LDP ID is and it is the R1 loopback IP address
  • A TCP connection is established from Router 3 to Router 1 on port 646. This is because router 3 has the highest router ID


The following information can be noted in the above output

  • The imp-null mean that route is directly connected to the router. R3 is connected directly to R1 which has the loopback IP address of This means the MPLS label is removed if the traffic is destined for network by router 3
  • The router applies a label for a prefix in both direction (both interfaces in this example). Example – for prefix – to route via R1 it will use label 101 and to route via router 4 it will use label 404. This is called liberal label retention that will help failover to next active in case of a failure in a redundant network. However this does not apply to this topology as path is possible only in one direction and there is no redundant path. The forwarding decision is made based on forwarding table


The following information can be noted in the above output

  • Pop-label is performed when the route in the router that is directly connected
  • Bytes label switched column indicates the number of bytes label switched for that destination
  • Next hop IP is determined via IGP


The following information can be noted in the above output

  • The local LDP ID is
  • The local transport ID is – this can be changed using the command – mplsldp router-id gig2/0 forceglobally or mplsldp discovery transport-addressinterface under interface config
  • Router 3 has two interfaces and those two interfaces participate in discovery process
  • Hello interval and hold time

Step-4: Establish PE to PE iBGP

The following are the information about mBGP that needs to be noted:

  • It is an of Border Gateway Protocol (BGP)
  • It is defined in RFC 4760
  • BGP supports only IPV4 unicast address but mBGP supports IPV4 , IPv6, unicast and multicast
  • Address families are used to support different address types
  • mBGP is widely used in MPLS L3 VPN to exchange VPN labels learned for the routes from the customer sites over the MPLS network


Step – 5 : Assign customer facing interfaces in PE routers to VRF


Route Distinguisher – This is used to separate a customer in an MPLS customer to setup the VPN route. It helps to maintain uniqueness when same IP addressing scheme is use between customers.

Route Target – Extended community tag to import and export to BGP and these tags are used to distinguish customer traffic. In the near side it is exported into BGP and in the far end it is imported from BGP and uniqueness is maintained.




Step – 5 :Configure IP address and routing in customer edge routers


Step – 5 :Configure routing on customer facing interfaces in PE routers







BGP Routes did not need redistribution and so it so it is already showing up in this router


Step – 6 :Redistributing IGP routes into BGP




  • 209 and 210 are VPN labels. These area routes advertised by R2 . When R1 sending a packet to network, it will add the next hop label on top of VPN label and send to R3. R3 will remove the top label and put the next hop label and send to R4. This process happens until the packet reaches R2. The intermediate P routers will keep the VPN label of 209 intact. R2 will remove the VPN label and forward it to R10.
  • Network and are networks from R7 that is connected to R1



VPN label = 209

Next HOP label = 304


Ping Test:






Useful Verification Commands

showipbgp vpnv4 rd labels
showbgp vpnv4 unicast rd labels
showbgp vpnv4 unicast vrf 101:CUSTA labels
showmpls forwarding-table vrf 101:CUSTA
showipcefvrf 101:CUSTA
showbgp vpnv4 unicast vrf 101:CUSTA
showbgp vpnv4 unicast vrf 101:CUSTA