[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [RRG] ALT's strong aggregation often leads to *very* long paths



At 07:33 PM 1/28/2008, Robin Whittle wrote:
By mandating the structure of the ALT network be strongly driven by
address aggregation, this means the connections between one router
and the next in the hierarchy will have little or no relation to
geography.  Therefore, the average length of inter-router distance
will be far longer than for ordinary BGP routers, where the network
structure is based primarily on linking to geographic neighbours,
and not at all on address aggregation.

I am attaching a slide which provides a diagram similar to
the scenario that Robin has used; it shows a sequence of LISP ITRs
that a packets traverses from source host to the destination host,
while passing through several levels of aggregation hierarchy.
I do need to read the LISP-ALT document carefully, but my
quick question for now is:
Referring to the diagram, once the EID space x/24 has been mapped
to locator ETR1 and this mapping information is known,
then why can't ITR4 have a route entry that points to ITR8
(pink arrow) as the next hop towards destination x/24 (locator ETR1).
Note that ITR4 and ITR8 are have a direct link (topologically).
If the packet can cut across from ITR4 to ITR8 at mid-level
in the hierarchy of ITRs (i.e., when topology allows it), then the
very long path (shown by the sequence of black arrows) can be avoided.
What does LISP-ALT gain in principle by preventing ITR4 from having
this route entry (i.e., ITR8 as next hop for EID space x/24 at ETR1)?

Sriram

K. Sriram
E-Mail: ksriram@nist.gov
Web:  http://www.antd.nist.gov/~ksriram/

Attachment: strong_aggregation.ppt
Description: Binary data