Default Routing

Mental model

A router’s routing table is a list of “if destination matches X, send to Y” rules. The most specific match wins. So if you have:

10.0.0.0/24 via R2
0.0.0.0/0 via R-ISP

A packet to 10.0.0.5 matches the first rule (more specific). A packet to 8.8.8.8 doesn’t match the first, falls through to the second.

The second rule — 0.0.0.0/0 — matches everything. It’s the catch-all. Almost every edge router needs one, because no edge router knows every public-internet route (that’s hundreds of thousands of entries).

A default route is also called the gateway of last resort.

Two ways to install a default route

Method 1 — static default route

The classic. Just type it in:

R1(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.1

Read aloud: “for any destination, send to 203.0.113.1.” Simple, reliable, used on 99% of branch and home routers.

Verify:

R1# show ip route 0.0.0.0
Gateway of last resort is 203.0.113.1 to network 0.0.0.0
S*    0.0.0.0/0 [1/0] via 203.0.113.1

The S* means “static route, candidate default.” The * is what matters — it’s the candidate of last resort.

Method 2 — let a routing protocol distribute it

In larger networks, only the edge router (which faces the ISP) has the real default. Internal routers learn it via OSPF or EIGRP.

With OSPF:

R-edge(config)# router ospf 1
R-edge(config-router)# default-information originate

This makes R-edge tell its OSPF neighbors “I have a default route, send me anything unknown.” Internal routers install the default with O*E2 in their tables.

With EIGRP:

R-edge(config)# router eigrp 100
R-edge(config-router)# redistribute static

Or, on older IOS:

R-edge(config)# ip default-network 192.168.1.0

Default route vs summary route

These two get confused:

• Default route = 0.0.0.0/0 — matches everything when nothing else does.
• Summary route = aggregating many specific routes into one less-specific entry. E.g. 10.0.0.0/8 summarizing 10.1.0.0/24 + 10.2.0.0/24 + …

Both are less specific than alternatives, but a default matches everything not otherwise routed. A summary still only matches a defined range.

Floating default — the backup

If your primary internet path can die, install a second default with higher administrative distance — it kicks in only when the primary disappears:

R1(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.1                     ! primary, AD 1
R1(config)# ip route 0.0.0.0 0.0.0.0 198.51.100.1 100                ! backup, AD 100

While the primary is up, only it shows in show ip route. When it fails, the backup automatically appears.

Verification

R1# show ip route
R1# show ip route 0.0.0.0
R1# show ip route 8.8.8.8        ! shows which route actually matches a specific destination

The third one is the most useful for troubleshooting: ask the router exactly which route it would use for a specific destination IP.

Common mistakes

1. Pointing the default to an unreachable next-hop. If 203.0.113.1 isn’t reachable from R1, the default appears in show running-config but never installs into show ip route. Test reachability first.

2. Multiple defaults with same AD = unintentional load-balancing. Two ip route 0.0.0.0 0.0.0.0 statements with AD 1 each, pointing at different ISPs → traffic load-balances. Sometimes wanted, sometimes not. Specify distinct AD for active/backup.

3. Default route loops. R1’s default points to R2. R2’s default points to R1. Any unknown destination ping-pongs between them until TTL expires. Always trace defaults end-to-end.

4. Forgetting default-information originate in OSPF. Putting a static default on the edge router doesn’t automatically share it via OSPF. Add the originate command, or internal routers won’t know.

5. Wrong subnet/mask spelling. ip route 0.0.0.0 0.0.0.0 ... is the default. ip route 0.0.0.0 255.0.0.0 ... is not a default — it’s a static for the 0.0.0.0/8 range, which doesn’t really exist. Easy typo, hard to spot.

6. Default route in MPLS L3VPN customer-facing. When the provider runs OSPF/EIGRP/BGP with you, your default should come from them, not from you originating one to them. Coordination matters.

Lab to try tonight

1. One router connected to a “fake ISP” (another router with a public-IP loopback). Internal LAN behind R1.
2. Configure a static default route on R1 pointing to the ISP. Verify with show ip route 0.0.0.0.
3. From the LAN, ping the ISP’s public IP. Should work.
4. Remove the default. Ping again. Should fail with “Destination host unreachable.”
5. Restore the default. Add a backup default with AD 100 pointing to a different ISP. Verify both appear in running-config; only the primary is in show ip route.
6. Shut the primary’s outgoing interface. Watch the backup default appear in show ip route. Bring back the primary, watch the backup disappear.
7. Bonus: configure OSPF area 0 between R1 and an internal router R2. On R1, add default-information originate. Verify R2 learns the default with O*E2.

Cheat strip

Concept	Plain English
0.0.0.0/0	The default route. Matches anything.
Gateway of last resort	Same thing, different name
Most specific match wins	More specific routes always beat the default
S*	Static candidate default in show ip route
O*E2	OSPF-learned default (external type 2)
default-information originate	OSPF command to share your default with neighbors
Floating default	Backup default with higher AD
AD (Admin Distance)	Lower = more trustworthy. Static=1, OSPF=110, RIP=120.