5

Again, I'm continuing a review of previous Cisco modules. In CCNA 2, Chapter 2.9.1.4, it states,

Discard Route

A common configuration in many networks is to have a static default route on the edge router forwarding packets to the ISP. The ISP router then has a static route pointing to the customer's network.

For example, Customer A has the network address of 172.16.0.0/16, which is subnetted into several /24 subnets. The edge router of Customer A has a static default route forwarding all other traffic to the ISP router:

ip route 0.0.0.0 0.0.0.0 serial 0/0/0

The ISP router has a static default route for forwarding traffic to Customer A's network:

ip route 172.16.0.0 255.255.0.0 serial 0/0/1

Is 172.16.0.0 255.255.0.0 really a static default route? I know it is a static route, but a static default route? I thought all static default routes had an IP and subnet mask of 0.0.0.0 0.0.0.0.

4

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.

3

You are correct, ISP has static routes for the customer subnets and a default route for upstream provider (provided they aren't using BGP).

In the description just before the example it clearly states "The ISP router then has a static route pointing to the customer's network." which is the typical operation for ISP networks.

Even though the example is poorly written, I believe the point is to introduce the idea of a discard route. This is a practice used by ISP's to prevent routing loops by creating a "static default route" for the entire ip block used for customers to an interface such as null0.

  • Good catch. Thanks for pointing that out too! – user3046 Apr 6 '14 at 19:41
0

ip route 172.16.0.0 255.255.0.0 serial 0/0/1

This says: 172.16/16 goes in the outbound que of S0/0/1

ip route 0.0.0.0 0.0.0.0 serial 0/0/0

This says: everything goes in the outbound que of S0/0/0

The first is an example of ENTERING a static route. The second is an example of ENTERING a static default route.

A route that exists in the table may have many states. It may be static or dynamic, held down, etc. "default route" refers only to the fact that it points at everything: (0.0.0.0/0). "static route" refers only to the method by which it was entered into the routing table. They are independent concepts.

A default route, may be static. A static route may be a default route. Neither is required of the other.

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