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  • mpls vpn label

    Hi everyone,I have a confusion in understanding the vpn lable.I have configured Inter-AS vpn as given in the below topology.The PE3 router is assigning a lable of 16 to vrf route 192.168.3.1 and transporting this lable to other router PE4 through MP-BGP.My confusion is when the IP packet is transported from PE4 to PE3 the packet uses 2 lables one is PE3 nexthop lable and another is vpn lable,so when it reaches PE3 it has only vpn lable and in the LFIB of PE3 the IN label is 16 and out lable is aggregate.As out is aggregate the vpn label should be removed after removing this, how it knows that it should look into the corresponding vfr CEF to forward the packet to correct outgoing interface as it was not having any identification.Plz anyone can sort out my confusion.Awaiting your response. PE3 ---->ASBR1----->ASBR2----->PE4(topology)

  • #2
    Re: mpls vpn label

    Which option of Inter-AS VPN are you trying to achieve? A, B or C?
    Infrastructure Architect
    CCNA, CCNA Security, MCSE, JNCIS

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    • #3
      Re: mpls vpn label

      Hi, I am trying to achieve optionB of interAS mpls vpn.

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      • #4
        Re: mpls vpn label

        Option B uses base MPLS tunnels within the AS beween PE router and ASBR. Stacked tunnels are used between ASBRs and end to end.

        I think that the ASBRs should remove the top label before sending to the PE.

        Lets say for example:

        PE3(in16) ---->ASBR1(out16/in50)----------->ASBR2(out50/in100)---------------->PE4(out100)

        PE4 will send to ASBR2 with the out label of 100
        ASBR2 receive the in label of 100 and send to ASBR1 with the out label of 50
        ASBR1 will receive the in label of 50 and send to PE3 with the out label of 16.
        PE3 will receive the in label of 16, pop it and deliver to the relevant CE
        Infrastructure Architect
        CCNA, CCNA Security, MCSE, JNCIS

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        • #5
          Re: mpls vpn label

          Hi BigDeesDad, In the same topology which u explained below at the PE3 router the in label is 16 and the out label was aggregate when i gave the command show mpls forwading table.(Eg:in label out label 16 Aggregate 10.1.1.0/24[V]).The output was looking exactly as i mentioned in Eg enclosed in the brackets.So my confusion is if this is the case the packet which is comming with the in label 16 at the PE3 router,since the out label was aggregate it should remove the label and look into the corresponding VRF FIB.How it comes to know that it should look into the FIB of corresponding VRF as their is no anything mentioned in the mpls forwarding tabel as shown above.I hope u got my confusion.If i am wrong kindly implement this scenario and you will find the same forwarding table.Kindly let me know if u got the answer to this.Awaiting your response at the earliest.

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          • #6
            Re: mpls vpn label

            Oh sorry I see what you mean now.

            The packet is received at PE3 with the in-label of 16 as the ARBR1 will have removed the outer label. ON PE3, Label 16 has an out label of aggregate so yes the MPLS label is then removed but a normal IPV4 lookup occurs to find a route to the CE and not a VRF. If the aggregate is from a connected route then the PE might need to ARP to identify the next-hop for the CE.
            Last edited by BigDeesDad; 15th September 2009, 22:05.
            Infrastructure Architect
            CCNA, CCNA Security, MCSE, JNCIS

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            • #7
              Re: mpls vpn label

              Hi BigDeesDad, As u said the normal ipv4 lookup is formed at the PE3 router than it won't find the VPN customer route since it will be available in the particular vrf CEF table and it will not be existing in the global cef table.So how it looks into the corresponding vrf cef table to find the customer routes outgoing interface.I hope now u got the clear picture.Awaiting your response at the earliest.

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              • #8
                Re: mpls vpn label

                Because an MPLS aggregate entry contains an attribute for the VRF entity hence resolving the CEF. This is the only difference between an MPLS entry and an MPLS Aggregate entry.

                MPLS Entry
                In-Label
                Out-Label
                Switching action
                Next-hop

                MPLS Aggregate Entry
                Virtual Routing Entity
                In-Label
                Out-Label
                Switching action
                Next-hop
                Infrastructure Architect
                CCNA, CCNA Security, MCSE, JNCIS

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                • #9
                  Re: mpls vpn label

                  Hi BigDeesDad, I didn't get that properly from ur reply,i am very sorry to say that.I hope it would be better if u explain it with an eg by taking some simple scenario.Also show me the outputs of aggregate entry and mpls entry which u r trying to say in the below reply.Awaiting your response at the earliest.

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                  • #10
                    Re: mpls vpn label

                    Have a look at the following link, it will explain it a lot clearer than I can.

                    http://www.networkworld.com/subnets/...nt.html?page=1

                    I'm afraid I am struggling to understand your confusion. Your CE is advertising route to your PE3. This is showing in the LFIB of PE3 as in-16 and out-aggregate. Now it is an aggregate either because it is a BGP aggregate route, static route or directly connected route or you have configured an aggregate label.

                    Now traffic from PE4 to PE3.
                    PE4 will push label 16. (learned across VRF from PE3)
                    ASBR2 will receive label 16 and push another label (eg50). so now in-16, out-50
                    ASBR1 will receieve in-16/out-50 from ASBR2 and pop outer label (50).
                    PE3 receives label 16, checks LFIB, out-aggregate, so pops and resolves from IPV4. This route is learned from CE so it will be in global routing table

                    Are we talking about the same scenarios here?
                    Infrastructure Architect
                    CCNA, CCNA Security, MCSE, JNCIS

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                    • #11
                      Re: mpls vpn label

                      Can you post the outputs of the following?
                      show ip route vrf <name>
                      show ip bgp vpnv4 all labels
                      show mpls forwarding-table

                      This is the outputs from my test setup. As you can see the in-label is 19 and out-label aggregate. When MPLS traffic destined for 20.1.1.0/24, the in-label is removed, LFIB is queried to find aggregate and no next-hop so a second lookup is performed by normal IPV4 lookup.


                      testPE#show ip ro vrf test

                      Routing Table: test
                      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

                      20.0.0.0/24 is subnetted, 1 subnets
                      C 20.1.1.0 is directly connected, Loopback1
                      192.168.0.0/32 is subnetted, 1 subnets
                      B 192.168.0.1 [20/0] via 172.16.0.3, 00:53:51


                      testPE#show ip bgp vpnv4 all labels
                      Network Next Hop In label/Out label
                      Route Distinguisher: 100:1 (test)
                      192.168.0.1/32 172.16.0.3 nolabel/18
                      20.1.1.0/24 0.0.0.0 19/aggregate(test)


                      testPE#show mpls forwarding-table
                      Local Outgoing Prefix Bytes tag Outgoing Next Hop
                      tag tag or VC or Tunnel Id switched interface
                      18 Pop tag 172.30.0.3/32 0 Fa0/1 172.30.0.3
                      19 Aggregate 20.1.1.0/24[V] 1460
                      Infrastructure Architect
                      CCNA, CCNA Security, MCSE, JNCIS

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