Unanswered Question
Oct 10th, 2007
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Hai all,

Can any one help me in understanding LDP (How label swapping is happening in MPLS ) /Tag switching ... and how vrf is created ... How customer data is traversed via MPLS Backbone ..(VRF information is attached with the label ... pls help me regarding this .. to have a better understanding

Thanks in Advance


Bharti Airtel


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bvsnarayana03 Wed, 10/10/2007 - 04:10
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In MPLS, data packets are tagged with Labels & through out the ISP packets are switched by label swapping rather than traditional IP lookup on each & every hop.

There are 2 methods of tagging labels to data packets: LDP (IEEE standard), TAG switching (cisco Propreitary).

VRF: There is a limitation to the No. of routing instances that can be run on a single router e.g. 1 instance of RIP & BGP, 30 instances of OSPF per router etc. VRF helps overcome this limitation of limited routing instances, thus help in creating multiple instances of single protocol on a router, & thereby helping ISP to connect multiple customers to a single router.

MPLS is a big technology, & the questions put by you needs explanation of it as a whole. This was just an attempt to brief the terms specifically asked by you. I'm searching for appropriate link to explain the entire technology, will paste once found.

CSCO10758684 Thu, 10/11/2007 - 10:38
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Namasthe Narayan,

Thanks for your valuable reply ...i am working for Airtel (Blr) .. so some eagerness to to know what exctly is happening in Our Backbone netowrk ...(we dont have access to check the TDP/LDP status ) .... i knew that in simply line MPLS cannt be explaned .... some eagerness to know the basics ...

Pls share the the links if you find ...

Once again thanks a lot

Mohamed Sobair Wed, 10/10/2007 - 04:14
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what you are asking about is too much of information to be described in a slite reply or definition, but here are Very BASIC and Simple information about MPLS:

MPLS: is a multi-protocol label Switching defined as Control Plane Technology to Optimize Network behaviour Of mapping Layer-3 Data Flow to Layer-2 traffic between Network Nodes.

MPLS Label Format is consist Of 32 Bit , 20 bit is the Label (SHIM) + 3 bit is Exprimental bit 1 bit + 6 bits for TTL

The TAG & LDP describes the Control Plane in an MPLS, LDP is IETF standard and TDP is Cisco Proprietary, an interface must be enabled for any of those to start forward and recognize MPLS packet, However , Still there is RSVP and they are differ.

An MPLS Network based differ from an IP-Network base interms of each protocol behaviour for finding the best path to reach specific destination , Decisions in MPLS based On Label since all MPLS routers builts LIB (Label information base) On top of the routing protocol used. Unlike Normal Routing lookup, However MPLS built all LIB/LFIB based On the routing protocol.

VRF: Its Virtual routing forwarding, Multiple VRFs is like Multiple logical routers , each with its own routing table.

One Of the Main biggest advantage Of MPLS is MPLS-VPN, MPLS-VPN allows VPN connectivities over an MPLS infrastructure, By carrying the VRFs through an MP-BGP through an extended communities.

(MPLS - VPN label) contains extra added label for each specific VPN + the LDP label, so that when the original VRF routes carried is forwarded to the correst customer part of the VPN and to the correct customer interface.

Finally, The P , PE , CE defines , Provider routers (Performs Only label Swapping) , PE provider edge router (the ingress and egress nodes for each Label Switch path) , and Cutomer Edge router attached to ur PE and the type of service differ based on Customer requirment.

I would suggest u go through some Documents to more understand MPLS.


Mohamed Sobair

CSCO10758684 Thu, 10/11/2007 - 10:49
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hai Sobair,

Thanks a lot .. i expcted only a glass of water .. but i got .. full ocean ..Jus kidding .. thanks for your update .. i know the terms but its not clear some were ..would be helpfull for me if you could tell which ... Document can help me regarding this .. i menan from the basic ...

Pls share if you hav any link for this ..

Once again thanks a lot

gaurav_thapar79 Wed, 10/10/2007 - 05:02
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Hi Lijesh,

MPLS is indeed a big topic to discuss. Few basics needs to be cleard before you embark on MPLS switching fundamentals like LDP/TDP and Traffic engineering.

VRF - VPN routing and forwarding tables.

Label switching instead on conventional Frame/Cell switching and IP switching.

Cisco IOS CEF actually generates fixed length Labels and gets mapped onto IP packet at PE.Then PE-toP and vice-versa happens via label switching. This lable table is called FEC.

3 basic operation happens in MPLS as follows:-

CE-To-PE = IP packet

At PE:- PUSH (Label mapped onto IP packet)

PE-ToP - SWAP (Label switching happend between PE router and LSR -Label switching router.

then, P-PE = SWAP process happens again.

PE-CE = POP process happens where in Lables gets removed and results back into IP packet.

For rest of the operation, please refer cisco.com - documentation on MPLS.

gaurav_thapar79 Wed, 10/10/2007 - 06:46
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2nd Point:-

VRF information is attached with the label

Response:- This actually does not happen as you have thought.

There are two things which works in MPLS such as RT/RD.

RT= Route Target

RD = Route Distinguisher.

The RT/RD values actually decides on how to traverse the path over MPLS LSR backbone end-to-end.

Hopefully, i have managed to anwser your queries.

CSCO10758684 Thu, 10/11/2007 - 11:10
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Hellow Sir,

Thanks a lot ... i think i have to start from the bigining .. i was digging from Cisco.com only .. ..trying to find very basic doc...

Pleas share if you have any ...(From lotus mail you can typr Lijesh.NC .. if possible pls share

Thanks a lot once again

Mohamed Sobair Fri, 10/12/2007 - 10:26
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My MPLS experience is On Juniper routers, as we dont have Cisco Routers Platform running MPLS. But the Concept is the same Only CLI is diffrent.

I have recently ordered two great books to guide through Cisco Platforms as well and they are:

1- MPLS Fundementals. By (Luc De Ghein)

2- MPLS VPN Architecture. By (Ivan & Jim)

I also have (KnwoledgeNet Campus) instructed by (Dan Young & Anthony Sequeira).

For Cisco Documentation, you might visit Technical Support & Documentaion), please check the bellow:



Mohamed Sobair

CSCO10758684 Fri, 10/12/2007 - 19:29
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hai ,

Thanks a lot Sobair ... now i am behind this ...i got few meterials .... if you have meterials on juniper CLI for MPLS and normal routers .. kindly share the same...in Bharti we have only few juniper routers (Only for internet rest all cisco higher end series)

Pls share the same ... (ncliju@gmail.com)

Thanks a lot



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