LA bit setting in IA for virtual link

[vanitha <vanitha at FUTURE.FUTSOFT.COM>]

Hi,

        As per the section 3.4.3.7 in RFC 2740, "If RTX has one or more virtual
links configured through the area, it includes one of its site-local or
global scope IPv6 interface addresses in the LSA (if it hasn't already),
setting the LA-bit in the PrefixOptions field, and setting the PrefixLength
to 128 and the Metric to 0. This information will be used later in the
routing calculation so that the two ends of the virtual link can discover
each other's IPv6 addresses. "

Setting LA bit to any of the interface prefix is not sufficient to
handle multiple virtual links. The prefix should belong to one of the
interfaces in
transit area.
Consider the below situation

        |-------------|         1       |-----------|
        |       R1      |---------------|   R2      |--------------R3
      |         |---------------|           |   3
        --------------          2       ------------


Link 1 is in area 1 with metric as 1 at R2
Link 2 is in area 2  with metric as 10 at R2
Link 3 is in area backbone.
virtual link is configured between R1 and R2 through the transit area 2.
(Say virtual Link 1)
one more virtual link is configured between R1 and R2 through the transit
area 1. (Say virtual link 2)

If R1 adds link 1 prefix with LA bit set in both the transit areas, then
Route at R2 for the LA bit set prefix will be through the link 1  because of
the lower cost.
For both the virtual link, the packet will be send on link 1.

The packets will be always associated with the virtual link 2 because of the
validation procedures told in 8.2 section of RFC2328.
"The receiving interface must also attach to the virtual link's configured
Transit area. If all of these checks succeed, the packet is accepted and is
from now on associated with the virtual link "
So virtuallink1 will never come up.

Instead if we mandate that the LA bit should be set for a prefix of one of
the interfaces of transit area, then things will work fine.
If there is no prefix in the transit area, virtual link will fail. This is
in line with OSPFv2.
Reference--> Section 15 Virtual Links. "Note that when one (or both) of the
virtual link endpoints connect to the Transit area via an unnumbered
point-to-point link, it may be impossible to calculate either the virtual
interface's IP address and/or the virtual neighbor's IP address, thereby
causing the virtual link to fail. "

Regards,
Vanitha N.





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