Conference noted::decnis

3576.0. "Question on OSPF subnet filtering." by GADWAL::W_MCGAW () Wed Mar 19 1997 14:03

    Hi,
    
    I was wondering if someone familiar with subnet filtering under OSPF
    could look at this information from my customer and tell me if they see
    anything obviously wrong with his commands.  He is using DECnis V3.0. 
    The compile goes o.k. but he gets a preocess failure message on the
    enable command.
    
    Why does the following ncl code fail with a "process failure" message      
    on the enable command? (Commands are attempting to filter the route to     
    subnet 31.16.3.0 within previously defined ospf area ospf-area-31-16.      
    An address domain is defined, then a filter with an action of "block"      
    on routes matching the domain and received from OSPF.) There are no        
    errors or warnings encountered in the compile prior to the enable          
    failure.                                                                  
                                                                              
    In _extra_create file:                                                    
    CREATE ROUTING NETWORK PROTOCOL IP DOMAIN Sub_31_16_3                     
    CREATE ROUTING CONTROL PROTOCOL ospf-area-31-16 -                         
     RECEIVE DESTINATION Sub_filter FILTER ACTION BLOCK, -                    
      SOURCE ROUTE ATTRIBUTES {OSPF Intra Area}                               
                                                                              
    In _extra_set file:                                                       
    
    SET ROUTING NETWORK PROTOCOL IP DOMAIN sub_31_16_3 -                      
     IP ADDRESS {{address = 31.16.3.0, mask = 255.255.255.0}}                 
    SET ROUTING CONTROL PROTOCOL ospf-area-31-16 RECEIVE DESTINATION -        
     sub_filter DOMAIN {{ROUTING NETWORK PROTOCOL IP DOMAIN sub_31_16_3}}     
    SET ROUTING CONTROL PROTOCOL ospf-area-31-16 RECEIVE DESTINATION -        
     sub_filter ADDRESS MATCH TYPE PREFIX                                      
                                                                              
    Following command in _extra_enable file fails with "process failure"      
    message                                                                   
    ENABLE ROUTING CONTROL PROTOCOL ospf-area-31-16 -                          
     RECEIVE DESTINATION sub_filter                                            
    
    
    Thank you in advance,
    
    Walt McGaw
    Digital Network Services Unit
    USCSC/CS

Hi Walt,

I think you may have an ordering problem. The receive destination filter
must be enabled before the OSPF area gets enabled, so your extra enable
commands should look something like:

 ENABLE ROUTING CONTROL PROTOCOL ospf-area-31-16 -
     RECEIVE DESTINATION sub_filter

 enable ROUTING CONTROL PROTOCOL ospf-area-31-16

Hope this helps,

        Colin.

    Hi Colin,
    
    I'll have the customer try that and see what happens.  I'll give you an
    updaate either way.
    
    Thanks!
    Walt

    Hi Colin,
    
    Here is an update from the customer on your recommendation.  It appears
    it worked but he is mentioning something about a "black hole"?
    
    Walt-                                                                      
                                                                              
    Thanks for the follow-up.                                                  
                                                                               
                                                                               
    Route filtering works after placing the "enable filter" command in the     
    extra file preceding the "enable ospf-area" commands as DEC advised.       
                                                                               
    o OSPF route filters do create "black holes". The router advertises a      
    route even though this route is blocked. Packet addressed to blocked       
    route is sent up to the next hop (filtering) router but the router         
    just drops the packet.                                                      
    
    

>    o OSPF route filters do create "black holes". The router advertises a      
>    route even though this route is blocked. Packet addressed to blocked       
>    route is sent up to the next hop (filtering) router but the router         
>    just drops the packet. 

	This looks like an extract from the manual.  Its simply stating the
	fact that when you block routes using RPF they are prevented from
	being inserted into the forwarding table but they are still advertised
	by the protocol.  So other routers will still think that you have
	a route to the blocked network(s).  This is normal behaviour and is
	a characteristic of all route filtering mechanisms.

	Tony

    Thank you for the explanation Tony!
    
    Walt