BDP and LDP

Answered Question
Apr 9th, 2008

Hi people,

Is their a difference between BGP labels and LDP labels? Or they the samething? If different, can someone please explain the difference...

Thanks.

I have this problem too.
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Correct Answer by Jon Marshall about 8 years 8 months ago

It will be present but the P routers know nothing about it ie.

H1 -> CE1 -> PE1 -> P1 -> P2 -> PE2 -> CE2 -> H2

H1 and H2 are members of the same VPN. H1 sends an IP packet to H2.

Packet goes from H1 -> CE1 -> PE1.

PE1 has a route to destination (H2 subnet) pointing to PE2. It knows this is part of the same VPN so it attaches a VPN label to the packet.

PE1 now needs to send the packet to PE2 and to do this it needs another label. Remember the VPN label is only important to the PE routers.

PE1 attaches another label on top of the VPN label and sends the packet to P1. P1 strips the top label off, does a label lookup on this in it's mpls forwarding table and then puts a new label in the packet that will switch the packet to P2. Note that at no time does P1 try and interpret the VPN label. It doesn't look at it all.

P2 receives the packet and again it only looks at the topmost label. Due to PHP (Penultimate Hop popping) P2 then sends the packet to PE2 without a new label attached ie. only the VPN label is attached to the packet. PE2 receives the packet, removes the VPN label and forwards the packet to CE2 which then forwards it on to H2.

Key thing to remember.

PE routers use 2 sets of labels -

VPN label which is only relevant to other PE routers.

LDP label which is relevant to PE & P routers. It is the LDP labels that allow a PE router to talk to another PE router across the MPLS network.

Jon

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Jon Marshall Wed, 04/09/2008 - 23:12

The 2 things are quite different.

LDP is one of the protocols used to exchange labelled routes between P & PE routers within an MPLS network. These labels are then used by the PE and P router to transport the packets across the MPLS network rather than doing a traditional IP lookup at each hop along the path.

BGP doesn't really use labels as such, communities are the closest thing to this. BGP is the standard Exterior Gateway Protocol in use today. It is used to exchange routes between Autonmous systems. An extension of BGP called MP-BGP (Multi-protocol BGP) is used by PE routers in an MPLS network to support such things as MPLS VPN's.

BGP is not just used in an MPLS environment, LDP is.

Jon

cisco_trouble Thu, 04/10/2008 - 00:03

Jon,

Within a MPLS VPN environment, which distributes the VPN labels, LDP or BGP(iBGP)? To my understanding, LDP adds a label(PE & P routers) and iBGP adds a label (VPN)...is this correct?

Jon Marshall Thu, 04/10/2008 - 00:06

Yes you are correct.

LDP is used to distribute labels to allow PE & P router to swithc packets across the MPLS network.

BGP or more specifically MP-BGP is used to distribute the VPN labels between PE routers. Note that the P routers have no visibility of the VPN label at all.

Jon

cisco_trouble Thu, 04/10/2008 - 00:26

That's an important note; could you elaborate on that a bit. By no visibility, do you mean a mp-bgp (vpn) label will not be present between a P and PE router?

Correct Answer
Jon Marshall Thu, 04/10/2008 - 00:42

It will be present but the P routers know nothing about it ie.

H1 -> CE1 -> PE1 -> P1 -> P2 -> PE2 -> CE2 -> H2

H1 and H2 are members of the same VPN. H1 sends an IP packet to H2.

Packet goes from H1 -> CE1 -> PE1.

PE1 has a route to destination (H2 subnet) pointing to PE2. It knows this is part of the same VPN so it attaches a VPN label to the packet.

PE1 now needs to send the packet to PE2 and to do this it needs another label. Remember the VPN label is only important to the PE routers.

PE1 attaches another label on top of the VPN label and sends the packet to P1. P1 strips the top label off, does a label lookup on this in it's mpls forwarding table and then puts a new label in the packet that will switch the packet to P2. Note that at no time does P1 try and interpret the VPN label. It doesn't look at it all.

P2 receives the packet and again it only looks at the topmost label. Due to PHP (Penultimate Hop popping) P2 then sends the packet to PE2 without a new label attached ie. only the VPN label is attached to the packet. PE2 receives the packet, removes the VPN label and forwards the packet to CE2 which then forwards it on to H2.

Key thing to remember.

PE routers use 2 sets of labels -

VPN label which is only relevant to other PE routers.

LDP label which is relevant to PE & P routers. It is the LDP labels that allow a PE router to talk to another PE router across the MPLS network.

Jon

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