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MPLS labels and backbone routing

Hi Dudes,

Gone thro the some docs on MPLS and below are the questions arised out that.

1. when we use label switching protocol how router will assign label# for the prefix as we are not configuring any range or number when we enable label switching.

2. In enterprise MPLS IP VPN design , what protocol service provider use in their backbone is it OSPF or BGP and if it is BGP do they use LSP or BGP PIGGYBACK to carry labels.

3. how service provider backbone routers normally interconnected is it thro IPLC or ATM links for providing Enterprise IP VPN solution.

Thanks in advance




Re: MPLS labels and backbone routing


1. In frame mode MPLS, the labels are assigned from a Per-Platform Label Space, two mechanisms are used for exchanging labels in simple MPLS VPN, LDP labels (exchanged via LDP - Egress PE) and VPN labels (Exchanged via MBGP).

2. Most commonly SP uses an IGP allover the backbone (most commonly ISIS or OSPF) and then BGP between the PEs (to carry the VPN routes and labels).

3. Most of the providers backbone is over ATM, but most of the providers are migrating to IP backbone, as a matter of fact many has gone through this.


Mohammed Mahmoud.

Re: MPLS labels and backbone routing

1) When you use MPLS to switch labelled packets, it uses LDP to assign the labels to the prefixed available in your IGP table.

By default Label Values form 0-15 cannot be used (0-Explicit Null, 1-Router Alert Label, 2-IPV6 Explicit Null,3-Implcit Null & 4-15 are reserved)

And since the label has 20 bits for the value assignment, this gives us total of 1048575 available label values per device.

As the labels are locally significant. So there is no need to configure any range as the LDP automatically when enabled starts assigning labels with values of 16 and upwards till 1048575 per available prefix. But lets say you want to control that the router assigns only lets say labels withing a certain range like, only labels between 5000 and 8000 be assigned, then you can specify the range as 5000 to 8000.

2) SP's mostly uses ISIS or OSPF as their IGP, and for providing MPLS VPNs MPBGP is also required so you would ideally find either of ISIS or OSPF with BGP in a typical SP environment.

3) Most service providers use various media types for their backbone connects, since ATM is close to being obsolete the current and next generation SP networks are using POS/SONET in their backbone more prevalently. (IPLC is more for an enterprise, for internationl long distance SP's use the Submarine Cable Systems (fiber) routed under the sea. For eg: SE-ME-WE-3/4 etc.

Apart from the above answers it would be ideal if you go through the books and link specified in another post here.