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