[c-nsp] MPLS and VRF

[Bruce Pinsky]

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Eric A Louie wrote:
> I've been reading and studying but I'm still not quite getting a few concepts 
> down around it.
> 
> I'm looking for the relationship between the labels and transparency through 
> non-MPLS routers, if there is any, and the relationship between VRFs and 
> labels.  Anyone have any guidance for me?
> 

Labels are used to forward packets through the MPLS network.  If there are
any non-MPLS routers in the path, then you don't have an end-to-end LSP
(Label Switch Path) and the packets will get dropped.

In L3VPN over MPLS, there are two labels attached to the packet.  One
represents which PE the packet should be forwarded to and the other
represents which VPN on that PE the packet belongs to.

VRFs are simply the logical separation of routing/forwarding tables on the
PE. Labels are used to represent which VRF (VPN) a packets belongs in.  The
label information for VPNs is transmitted as part of the VPNv4 address
family in BGP.  Here is an example of how a packet flows through the MPLS
network:

http://www.google.com/imgres?hl=en&client=firefox-a&hs=FKB&sa=X&rls=org.mozilla:en-US:official&biw=1600&bih=699&tbm=isch&prmd=imvns&tbnid=rrTrsJp0TSWTVM:&imgrefurl=http://cisco-vpn.org.ua/1587051044/ch06lev1sec1.html&docid=Ikb3Z8KNnYKLLM&imgurl=http://cisco-vpn.org.ua/1587051044/images/06fig22_alt.jpg&w=1000&h=427&ei=dHSIUKe_BanWyQH96oDYDg&zoom=1&iact=rc&dur=453&sig=115233268954071552763&page=2&tbnh=117&tbnw=274&start=23&ndsp=30&ved=1t:429,r:14,s:20,i:180&tx=203&ty=32


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bep

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