Conference noted::decnis

3528.0. "I ISIS L1,L2 and partitioned areas" by OHFSS1::MALOTT (Lost in a Maze of DecNis') Fri Jan 31 1997 13:00

    I have a couple questions regarding the following config using 
    I ISIS (link State) and Decnis routers.  L2's are 600's L1' are 
    500's.  All Edge router groups are L1 and Hub site routers are L2.
    
    
    -----                             
      |        |--------|---------|\  Frame Relay
      +--------| Mar52a |           \|------------|-------------|
      |        |--------|---\                     | Edge Router |
      |                      \                    | Group A     |
      |                       \                   |-------------|
      |                        \
      | Hub Site                \
      |   LAN                    \|\  Frame Relay
      |                             \-------------|-------------|
      |                                           | Edge Router |
      |                             /-------------| Group B     |
      |        |--------|---------|/  Frame Relay |-------------|
      +--------| Mar52b |
      |        |--------|
      |
    -----
    
    As can be seen, Edge Group B has redundant paths to Hub site
    through each L2 router (Mar52a or Mar52b).  Let's say Mar52b
    is Primary path for Edge Group B and Mar52a is primary for 
    Edge Group A.  
    
    Problem: If Mar52b looses his connection to local Hub site LAN,
    Will Edge Group B be able to fail over to Mar52a given that ALL 
    traffic through Frame Circuits is L1 only?  Or, will a second 
    L2 path be required between Mar52b and Mar52a?  This second path 
    either being another L2 router in Edge Group B (bad idea), or 
    another LAN connection between Mar52a and Mar52b (better idea).
    
    Thought being that Mar52b would have an L2 path to get outside 
    of Area.  
    
    Consideration: Both IP and DECnet environment but mostly IP.
    
    I have noted this type config in a couple areas within a large 
    Nis network.
    
    Any help greatly appreciated...
    
    Best Regards,
    
    John Malott

Since EGB has L1 links to both Mar52a and Mar52b L2 routers we can deduce that
both MAr52a and Mar52b are in the same area as EGB. Hence there appears to be no
issue, and certainly no paritioned areas. 

The only issue is whether or not Mar52b will detect the failure of its conection
to Hub site LAN. For DECnet it definitely will, since the adjacencies are
determined by hello messages. In the case of IP, the reachability of the
subnet(s) on the LAN will be determined by the detection of a phgysical failure
of the connection. If the connections fails for some undetectable reason then
Mar52b will continue to advertise reachability to the LAN.

	Mike

    Mike,
    
    Obviously, I'm confused here.  Also, all are area 52, I'm sorry for not
    noting that.  
    
    How do EGB routers being all L1 switch to Mar52a when Mar52b looses
    connectivity to LAN?  Does Mar52b advertise this (I ISIS) to them?  
    I thought that L1 routers, only knowing to send 'out-of-area-52' to 
    nearest L2 would not be able to determine that Mar52b had no more 
    connectivity to rest of the world, and thus continue to send 
    'out-of-area-52' traffic to to Mar52b as long as it remained 'alive' 
    based on cost.
    
    Thank you in advance...
    
    Best Regards,
    
    John Malott  

    >How do EGB routers being all L1 switch to Mar52a when Mar52b looses
    >connectivity to LAN?  Does Mar52b advertise this (I ISIS) to them?  
    >I thought that L1 routers, only knowing to send 'out-of-area-52' to 
    >nearest L2 would not be able to determine that Mar52b had no more 
    >connectivity to rest of the world, and thus continue to send 
    >'out-of-area-52' traffic to to Mar52b as long as it remained 'alive' 
    >based on cost.

Once Mar52b looses its LAN connection, it is no longer an 'attached router'.
It advertises this in its LSPs and the L1 routers will stop using it as
(one of) the nearest L2 router.

    Thank you.  That is exactly what I was looking for.