Label distribution using LDP and RSVP questions

Answered Question
Sep 1st, 2008
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Hi Guru,

Since RSVP can distribute label, in network scenario where implementing only MPLS TE and VPNl3 and every PE have fullmesh MBGP, I don't need to enable LDP to make it works.


What is the advantage on top of RSVP if we have to enable LDP?

Correct Answer by Harold Ritter about 8 years 8 months ago

Montiean,


You are correct. You do not need to enable LDP in the core in the case all of your PEs have TE tunnels between each other. The TE signaled LSP is used in such a scenario to forward traffic from one PE to another.


This has some scaling limitations has the number of PEs grows. In some cases, people use an MPLS TE full mesh between the P routers and run LDP on top of the TE tunnels so that PE routers will use LDP to signal LSPs between each others.


So in short, you can use MPLS TE, LDP or a mix of both to create LSPs between the PEs in your network.


Regards,

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shivlu jain Tue, 09/02/2008 - 03:57
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Actually your MP-BGP comes up but without LDP the traffic of VPN will not flow. For the flow you need to enable the LDP.


regards

shivlu

m.noktes Tue, 09/02/2008 - 06:27
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Thanks shivlu replied back. Anyway, I did tested in my lab and the flow works just fine without ldp enable in the P routers.


Here is my network topology.


CE1--PE1--P1--P2--PE2--CE2


Below was some result i can post since i can't have access to my lab. Do i misunderstanding something here?


What is your thought?


CE1#show ip route eigrp

99.0.0.0/32 is subnetted, 1 subnets

D EX 99.99.99.99 [170/10064896] via 172.16.12.2, 00:01:25, Ethernet0/0

172.16.0.0/24 is subnetted, 2 subnets

D 172.16.1.0 [90/284160] via 172.16.12.2, 00:01:25, Ethernet0/0

22.0.0.0/24 is subnetted, 1 subnets

D 22.22.22.0 [90/412160] via 172.16.12.2, 00:01:25, Ethernet0/0

CE1#ping 22.22.22.22


Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 22.22.22.22, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 16/28/36 ms



PE1#show mpls forwarding-table

Local Outgoing Prefix Bytes Label Outgoing Next Hop

Label Label or VC or Tunnel Id Switched interface

16 No Label 10.10.203.0/30 0 Fa0/1 10.10.102.2

17 No Label 10.10.202.0/30 0 Fa0/1 10.10.102.2

18 No Label 2.2.2.2/32 0 Fa0/1 10.10.102.2

19 No Label 3.3.3.3/32 0 Fa0/1 10.10.102.2

20 No Label 5.5.5.5/32 0 Fa0/1 10.10.102.2

21 Pop Label [T] 6.6.6.6/32 0 Tu1 point2point

22 No Label 10.10.99.0/30 0 Fa0/1 10.10.102.2

23 No Label 10.10.99.4/30 0 Fa0/1 10.10.102.2

24 No Label 10.10.99.8/30 0 Fa0/1 10.10.102.2

25 Pop Label [T] 4.4.4.4/32 0 Tu0 point2point

26 No Label 11.11.11.0/24[V] 0 Fa0/0 172.16.12.1

27 Aggregate 172.16.12.0/24[V] 570 app1

28 Aggregate 88.88.88.1/32[V] 0 app2


[T] Forwarding through a TSP tunnel.

View additional labelling info with the 'detail' option



P1#show mpls forwarding-table

Local Outgoing Prefix Bytes Label Outgoing Next Hop

Label Label or VC or Tunnel Id Switched interface

16 No Label 5.5.5.5/32 0 Se1/0 point2point

17 No Label 6.6.6.6/32 0 Se1/0 point2point

18 No Label 10.10.99.4/30 0 Fa0/1 10.10.203.2

19 No Label 10.10.99.8/30 0 Se1/0 point2point

20 No Label 3.3.3.3/32 0 Fa0/1 10.10.203.2

21 No Label 10.10.202.0/30 0 Fa0/1 10.10.203.2

22 Pop Label 6.6.6.6 0 [20] 982 Fa0/0 10.10.102.1

23 17 1.1.1.1 1 [9] 1504 Se1/0 point2point

24 Pop Label 4.4.4.4 0 [8] 2280 Fa0/0 10.10.102.1

25 24 1.1.1.1 0 [11] 2076 Fa0/1 10.10.203.2

26 No Label 1.1.1.1/32 0 Fa0/0 10.10.102.1

27 No Label 4.4.4.4/32 0 Fa0/1 10.10.203.2

P1#show mpls ldp ne

P1#show mpls ldp neighbor


P1#




Correct Answer
Harold Ritter Tue, 09/02/2008 - 06:44
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Montiean,


You are correct. You do not need to enable LDP in the core in the case all of your PEs have TE tunnels between each other. The TE signaled LSP is used in such a scenario to forward traffic from one PE to another.


This has some scaling limitations has the number of PEs grows. In some cases, people use an MPLS TE full mesh between the P routers and run LDP on top of the TE tunnels so that PE routers will use LDP to signal LSPs between each others.


So in short, you can use MPLS TE, LDP or a mix of both to create LSPs between the PEs in your network.


Regards,

Harold Ritter Tue, 09/02/2008 - 06:48
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  • Cisco Employee,

Just as a clarification, when I said "you do not need to enable LDP in the core" I meant that LDP would not need to be enabled on neither the P and PE routers. Again, this is assuming that you have TE tunnels between all PEs.


Regards,

m.noktes Tue, 09/02/2008 - 08:22
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Thanks Harold! This clear my mind a lot of this label concept now.

sailwinthu Wed, 09/03/2008 - 02:31
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Hi,dude


I am not sure my answer suitable for your questions or not.

If you have full mesh MPLS TE between PEs you can by pass LDP in your network.

But if you have only 1 way TE from ingressPE to egressPE E and don't have return TE LSP.So you have to enable LDP on your network.

The above that i mentioned earlier is for TE from ingressPE to egressPE.

If your TE is from ingressPE to P.so you might need to enable LDP on Tunnel. Other wise your P router at tail end of TE can't route the packet correctly to the egressPE.



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