Static route with interface and next-hop IP on Ethernet media?

Hi,

in some cases I've seen static routes entered into routers with both an interface and a next-hop IP address. Like so, on IOS 15.2 routers:
ip route 10.0.0.0 255.0.0.0 GigabitEthernet0/0 172.16.1.2

Could anyone elaborate why this is usefull? As I understand it, it's unwise to use interfaces in static routes on broadcast media to avoid unneeded ARP processing and traffic.
The cisco documentation is not very clear on using both parameters at the same time.
In my case the route is also used together with a network statement in BGP to other BGP peers.
Will this way of static routing perhaps disable recursive lookup and route through through other interfaces?

thanks,
W

Answers

  • It's my understanding that using an IP next hop on a directly connected interface eliminates the router from ARPing for every destination address. Also, if the specified interface goes down but the next hop IP is still reachable via a recursive lookup then you can specify both so the next hop would still be reachable.

  • Hi,

    agreed on the above, however in the above case both a destination interface is configured as well as a next-hop IP address. Like most static routes the next-hop is defined as an IP address. What happens if both the interface AND the next-hop IP is specified?
    Does this then disable recursive lookup, or force the recursive next-hop only to be considered over the same specified interface?
    Most common way:
    ip route 10.0.0.0 255.0.0.0 172.16.1.2

    Used on point to point interfaces, or on Ethernet if the next hop IP might change (or is unknown) and depends on ARP to find the next hop MAC address:
    ip route 10.0.0.0 255.0.0.0 GigabitEthernet0/0

    What happens when using both:
    ip route 10.0.0.0 255.0.0.0 GigabitEthernet0/0 172.16.1.2

    I could lab this up however I'm wondering if it would explain the behavior for IOS/XE/XR kind of platforms the same...

    thanks,
    W

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