MPLS Label Filtering

[rich_foster]

Please help, I'm having trouble understanding the reason for MPLS label filtering, specifically the use of the match mpls-label and set mpls-label commands.

My understanding is that (using these commands in a route map) you can filter MPLS tagged IPv4 routes (not VPNv4) advertised by LDP?  So this would apply only to service provider routes, i.e. not customer tagged routes?

In what scenario would you actually use this, and are MPLS forwarding tables not usually built by an IGP?

Also how can I see the mpls-label that I am matching using the route map commands, and why do IPv4 BGP routes carry MPLS tags in the first place?

Thanks

Rich

[Giuseppe Larosa · Accepted solution]

Hello Richard,

the scenarios where these features are useful are those of interconnection of service providers networks in order to provide MPLS services to common customers.

Two possible models can be used:

inter -AS : it is a relationship between two providers at their edge and they have customers in common that want to have their VPN sites interconnected (and the sites are spread over both provider networks)

Carrier Supporting Carrier :  in this scenario the TOP level provider has a customer that it is an MPLS service provider.

The label space is each node is unique but different protocols can assign labels: LDP, RSVP TE, and also BGP with labels.

LDP can be used only within a single AS domain. BGP with labels can be used on links between two providers in order to advertise labels.

These labels are needed to build MPLS label switched paths LSPs that span over the two providers networks.

These LSPs are then used to perform forwarding of inter-AS MPLS L3 VPNs or inter-AS MPLS L2 VPNs (packets have two labels or more, internal labels represent services like VPN sites, the LSPs provide the external label that changes at each router hop including border between providers)

Internally on each AS, LDP can be used to propagate labels as usual and another key command is next-hop-self towards iBGP peers.

This allows to join the LSP section in provider A with the LSP section in provider B.

the use of route-maps with match mpls-label or set-label allows for selective sending or receiving of BGP prefixes with an MPLS label attribute as described in the document

unless you work in a service provider environment these features are not used

Hope to help

Giuseppe