[j-nsp] Joining OPSF & IS-IS areas via 2 ABRs

[Clarke Morledge]

I have an ISIS/OSPF area joining problem that I am trying to solve, 
wondering if JUNOS policy can help.

Let's say that I have a bunch of routers in an IS-IS Level2 domain.  I 
also have a few routers in an OSPF area (area 0.0.0.0).  The two "areas" 
are joined via two Area Border Routers (ABR1 and ABR2).  Eventually, I 
need to bring the routers in the OSPF  area into the IS-IS L2 domain. 
Unfortunately, the current hardware does not support IS-IS on those 
routers.  So I have to live with these OSPF-only routers temporarily.

     OSPF area backbone 0
   X---------------------X   (these routers only support OSPF)
   |                     |
   |                     |
   X  ABR1          ABR2 X   (these routers support OSPF & IS-IS)
   |                     |              |
   |\                   /               V
   | \                /
   X  \-------------/
    IS-IS L2 domain


I also have a requirement to pass traffic for *some* routes between 
these areas in an "active-passive" type of scenario; i.e. if ABR1 is 
available, force all of the traffic for these routes through ABR1, but do 
not allow traffic between areas for these routes via ABR2.   If something 
bad happens to ABR1, then and only then will ABR2 act on the transit 
router for this inter-area traffic.  Any transitions between ABR1 and ABR2 
need to happen "automagically"; i.e without manual intervention, and 
preferably without any static routes.

Currently, I can configure ABR1 and ABR2 to both act as "active" transit 
routers between the two areas.  However, I haven't been able to figure out 
how to make this an "active/passive" arrangement for particular routes.  I 
have explored  various tweaks involving IGP metrics and route preferences 
(administrative distance in Cisco-speak), but I do not have a satisfactory 
solution yet.

Does the very nature of these IGP protocols make it impossible to solve 
this problem, or can JUNOS policy on the ABRs help here?  Any thoughts on 
how to arrive at a solution?

Clarke Morledge
College of William and Mary
Information Technology - Network Engineering
Jones Hall (Room 18)
Williamsburg VA 23187