Cisco – Static routes: numerical next-hop vs interface

ciscorouterrouting

I'm reviewing old Cisco modules, and in CCNA 2, Module 2.5.1.2, it states,

Under certain conditions, the network administrator will not want to configure the static route with an exit interface but with the next-hop IP address.

Why would you prefer routes with a next-hop IP address instead of an exit interface?

Best Answer

If you point a static route to a broadcast interface, the route is inserted into the routing table only when the broadcast interface is up. This configuration is not recommended because when the next hop of a static route points to an interface, the router considers each of the hosts within the range of the route to be directly connected through that interface. For example, ip route 0.0.0.0 0.0.0.0 Ethernet0.

With this type of configuration, a router performs Address Resolution Protocol (ARP) on the Ethernet for every destination the router finds through the default route because the router considers all of these destinations as directly connected to Ethernet 0.

This kind of default route, especially if it is used by a lot of packets to many different destination subnets, can cause high processor utilization and a very large ARP cache (along with attendant memory allocation failures).

Specifying a numerical next hop on a directly connected interface prevents the router from performing ARP or each destination address. However, if the interface with the next hop goes down and the numerical next hop is reachable through a recursive route, you should specify both the next hop IP address and the interface through which the next hop should be found. For example, ip route 0.0.0.0 0.0.0.0 Serial 3/3 192.168.20.1.

Excerpt from Specifying a Next Hop IP Address for Static Routes at cisco.com

Related Solutions

Cisco PBR verify-availability with interface instead of next-hop

As discussed in chat, PBX / SIP traffic is unique to an IP host route in your case. Therefore, you can remove PBR and use tracking objects on overlapping static routes, which go out different dialer interfaces to solve the problem.

ip route 89.123.45.10 255.255.255.255 Dial0 track 1 1 name PBX_Pri
ip route 89.123.45.10 255.255.255.255 Dial1 track 2 10 name PBX_Bak
ip route 0.0.0.0 0.0.0.0 Dial 1 track 2 1 name Data_Pri
ip route 0.0.0.0 0.0.0.0 Dial 0 track 1 10 name Data_Bak

Cisco – IP Address and Subnet Mask of Static Default Route

I think you're getting your verbiage off, but not wrong.

A common configuration in many networks is to have a static default route on the edge router forwarding packets to the ISP.

A LAN that has no other exits will have a default route (i.e. static route) that points all unknown traffic to the edge. Since routers match on the longest prefix matching the destination address, this is your gateway of last resort.

The ISP router then has a static route pointing to the customer's network.

An ISP will never have a default route pointing to a customer network. They will have a static route for the networks they service.

Is 172.16.0.0 255.255.0.0 really a static default route?

That is not a default route. You were right in your assumption below.

I thought all static default routes had an IP and subnet mask of 0.0.0.0 0.0.0.0.

Best Answer

Related Solutions

Cisco PBR verify-availability with interface instead of next-hop

Cisco – IP Address and Subnet Mask of Static Default Route

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