OSPF Reliable Conditional Default Routing

I was doing the workbook and I was at the OSPF Reliable Conditional Default Routing taks:

The requirementa are as follows:

  • Configure an IP SLA instance on R8 to check ICMP reachability to R10's VLAN 108 IP address every five seconds.
    • R8 should advertise a default route into OSPF, but only if the SLA monitoring reports its status as OK.

In the solution this is awnsered:

ip route 169.254.0.1 255.255.255.255 Null0 track 1
ip prefix-list PLACEHOLDER seq 5 permit 169.254.0.1/32

I'm kinda wondering should it not be ip route 0.0.0.0 0.0.0.0 null0 track 1????

Am I missing something?



 

Comments

  • Hello Ashwin [:)]

     

    The route-map is matching the route and not the tracking state. For this reason the 169. route is used which is will be installed when the tracking state is up. The default route will be advertised when the prefix is in the table.

    If you configure or learn another default route that will be used for the traffic it will use that one also in the route-map and still advertise the default route in case the tracking object is down.

    With the placeholder route you connect the reachability of the IP and advertising of the default to eachother.

    When using the placeholder route the default route in the routing table is unrelated to the tracking state.

     

  • I remember this lab. All that static route he's put there for is so that there is an entry in the routing table for a route that exists.

     

    Basically this is how it works:

    router ospf 1
     default-information originate route-map TRACK_PLACEHOLDER
    
    


    The above states that "you must advertise a default route providing the route-map condition is met". Next, we look at the route-map

    ip prefix-list PLACEHOLDER seq 5 permit 169.254.0.1/32
    !
    route-map TRACK_PLACEHOLDER permit 10
     match ip address prefix-list PLACEHOLDER


    This says, if the address 169.254.0.1 exists in the routing table, then you are allowed to advertise a default route. 

     

    The final part is this:

    ip route 169.254.0.1 255.255.255.255 Null0 track 1
    <br />


    This is to put the route 169.254.0.1 into the routing table, providing the tracking is up (which is just your SLA config).

     

    So the route is not what will be advertised, it is simply there to meet a condition set in the route map. Once the route-map condition is met, then the default route is allowed to be advertised.

  • Hi Ashwin and All,

    I like the your static route to null0, as it is good practices to have a "discard route" covering default route being advertised.  That is, for most topologies this is good practice.

    As you probably already know, 169.254.0.0/24 is link local network.  Routing 169.254.0.0 to null0 adds no  functionality.

    Both methods trigger conditional advertisement of a default route.

    Your method I think is better, as it introduces a discard route.

  • Thanks guy.

    The explanations were clear.

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