MPLS VPN ISIS Loopback leaking

[4everlearning]

Hi Guys,

Can someone explain to me why it is recommended to leak loopback addresses to ISIS in MPLS VPN environment ?

In Cisco website it says this "In MPLS VPN environments, reachability information for the loopback addresses of every Provider Edge (PE) router is needed. Leaking routes for the PE loopback addresses allows a multiarea hierarchy to be used in MPLS VPN implementations".

I don't think i understand the above statement. 

Thanks,

[Peter Paluch]

Hi,

I must admit I also had my share of troubles understanding that convoluted statement.

I believe what the author wanted to say is that a service provider's network itself can be split into multiple areas. With IS-IS, routes in one area do not leak into other areas in general. Only L2-capable routers know about all prefixes in all areas but L1 routers only know about prefixes in their own area.

Imagine now that the service provider's network is indeed large and is split into several areas, causing some PE routers to be in one area while other PE routers are in other areas. If these PE routers do not also happen to be L2-capable routers then none of PE routers in one area has information about the loopback addresses of PE routers in other areas.

This situation can be saved by configuring route leaking in IS-IS that actually allows the loopback addresses of PE routers known to all L2-capable routers to be leaked to L1 routers in all areas. Thanks to this, PE routers will know each other, even if placed into different areas, and the MPLS and related VPN services will be able to work again.

Would this help a little? Please feel welcome to ask further!

Best regards,
Peter