Cisco – IP Address and Subnet Mask of Static Default Route

cisco

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.

Best Answer

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.

Related Solutions

Cisco – Configuring redundancy using floating static

As you mentioned, your static routes break, because there is no existing way to force failover to the WAN link at both sites when one site looses the uplink to the internet.

The best solution to this problem is to set up some form of routed tunnel (sourced from the ISP-link addresses on R1 and R2 in the diagram) through the internet, run a dynamic routing protocol (OSPF / EIGRP) through your tunnel and backup WAN, and set the routing metrics for the WAN link to be much higher.

HQ-Backup tunnel

Since you source the tunnel from the internet links on both sides, if either side looses internet connectivity the tunnel breaks (and the IGP recoverges to your WAN link). If you do this, be sure that PMTUD works through the tunnel so the tunnel overhead is accounted for. This kind of inter-office encrypted VPN is quite common, so it's a familiar topology for anyone who has to support this network in the future.

The issue you face with your existing design is that all your inter-office traffic traffic seems to be unencrypted (at least if I read your question at face-value). If you are not already doing so, I would strongly recommend that you encrypt all traffic between your corporate sites, if it's going through the internet.

Cisco 877 – Advertise OSPF default route only when PPP is up

Change your static route to use the interface as well as the next hop. That way, when the interface goes down, the route will be removed and no longer advertised over OSPF.

no ip route 0.0.0.0 0.0.0.0 218.101.y.y  

ip route 0.0.0.0 0.0.0.0 218.101.y.y Dialer0

You should also remove the redistribute command -- it's not needed

Best Answer

Related Solutions

Cisco – Configuring redundancy using floating static

Cisco 877 – Advertise OSPF default route only when PPP is up

Related Topic