Route Leaking And LDP Extension for inter-area lsp (RFC 5283)

Service Providor networks usually will used   link-state  ISIS or OSPF as IGP with hierarchical areas and route summarization (dependent of their size)  .
networks based  ip/mpls  with classic LDP signaling  summary route and the areas broken the Lsps connectivity.
this design provides challenges for  provision  of services end to end.

There are many   techniques to  overcome this situation they can call "Seamless Mpls" one of them  is the RFC 5283 to solve the summarization.

Normally for LDP binding LSR needs to ensure there is an exact match for X in the RIB anything else is dropped,the RFC suggest changing this verification procedure to the longest match if there prefix Y/24 in the RIB and the X/32 is one of the subnet the X/32 keep lable mapping and propagated in LFIB (you need that all LSR in lsp  will enable RFC ),so you keep the  end-to-end connectivity

on the other hand the   RIB and the LIB prefixes are still summarized

the drawbacks of this approach is that the LFIB size growth, now it depend what your prefer the maintaining LFIB is less compared to maintaining IGP databases :-)

    :For example

 R-Pe2 try to built lsp to PE2 but the LDP did not have the prefix 172.20.20.1 in the RIB cause in the area of level-1  have only default route from PE1 in result the ldp did not mapping this prefix .In the other way PE2 try to built lsp to the R-pe2 but in the RIB  there are prefix 10.11.11.0/24 and not the exact prefix 10.11.11.2/32 ,So the ldp did mapping this prefix as well

The solution:


PE1 configure with specific leaking level 2 to level 1 of the loopback of PE2
In result R-pe2 have in the RIB and in the LIB/LFIB the prefix 172.20.20.1/32 and the LSP can be  built.
In the other way the PE1/P/PE2 configure with the support of RFC 5283 so in the LFIB of PE2
The FEC are built in result the lsp built .

Next post I will write about other techniques  

Further Reading:

LDP Extension for Inter-Area Label Switched Paths