Friday, May 15, 2009

Loopback Summarization Is Now Possible

It is always recommended that not to perform the summarization on loopbacks because it does the black holing in the cloud. I have already explained in the post "Why Sumamrization Is Not Recommended On Loopbacks". But the problem of not doing summarization on loopbacks leads to the inconsistency of OSPF hierarchy. As a consequence, RIB & LFIB will contain all the loopbacks as 32 which leads to the memory problem. As per RFC 3036, section LDP recommends that the IP address of the FEC Element should exactly match an entry in the IP Routing Information Base.
To overcome this problem, a new solution has been proposed which is alluded in RFC 5283. As per Proposed solution of Longest-Match Label Mapping Message Procedure which depicts a new label mapping procedure for LDP. Excerpt from the RFC 5283 is given below
"With this new Longest-Match Label Mapping Procedure, an LSR receiving
a Label Mapping message from a neighbor LSR for a Prefix Address FEC
Element FEC1 SHOULD use the label for MPLS forwarding if its routing
table contains an entry that matches the FEC Element FEC1 and the
advertising LSR is a next hop to reach FEC1. If so, it SHOULD
advertise the received FEC Element FEC1 and a label to its LDP peers.

By "matching FEC Element", one should understand an IP longest match.
That is, either the LDP FEC element exactly matches an entry in the
IP RIB or the FEC element is a subset of an IP RIB entry. There is
no match for other cases (i.e., if the FEC element is a superset of a
RIB entry, it is not considered a match).

Note that LDP re-advertises to its peers the specific FEC element
FEC1, and not the aggregated prefix found in the IP RIB during the
longest-match search.

Note that with this Longest-Match Label Mapping Procedure, each LSP
established by LDP still strictly follows the shortest path(s)
defined by the IGP.

In future, if vendors implement the solution suggested by RFC 5283, will maintain the OSPF hieracy and save lot of RIB memory.

The main disadvantage of adopting this solution is that service provider need to run BGP on P routers.

shivlu jain

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1 comment:

aman said...

i like this post,u have done hard work,so i appriciate u.