📘 CCNA 200-301 v1.1

3.3 Configure and verify IPv4 and IPv6 static routing

3.3.a Default Route

1. What is a Default Route?

A default route is a special type of static route used by a router when it doesn’t have a specific route in its routing table for a destination network.

It acts as a “last resort” path — when no other more specific route matches the destination IP address.

In other words:

• If the router doesn’t know where a packet should go (no match found),
• It will send that packet to the default route.

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2. Purpose of a Default Route

In an IT network, a router usually connects to multiple networks — LANs, WANs, and the internet.
However, it’s not possible (or practical) to have routes for every possible destination network in the world.

So, instead of configuring thousands of routes, we configure one default route that tells the router:

“If you don’t know where to send a packet, send it to this next-hop router or this interface.”

This is very common in:

• Branch routers that send all non-local traffic to a head office or ISP router.
• Edge routers that forward unknown traffic to the internet.

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3. Default Route in IPv4

Command Syntax:

ip route 0.0.0.0 0.0.0.0 <next-hop IP> 

or

ip route 0.0.0.0 0.0.0.0 <exit-interface>

Explanation:

• 0.0.0.0 with mask 0.0.0.0 means “any network, any host”.
• <next-hop IP> is the IP address of the next router to which traffic should be sent.
• <exit-interface> is the local interface from where the packet will leave.

So, 0.0.0.0/0 represents all IPv4 addresses.

This is the default route in IPv4 networks.

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Example:

If your internal router needs to send all unknown traffic to your ISP router:

Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.1

Here:

• 192.168.1.1 = next-hop IP (ISP router interface)
• Any traffic not matching a more specific route will be sent to 192.168.1.1.

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4. Default Route in IPv6

In IPv6, the same concept applies.

Command Syntax:

ipv6 route ::/0 <next-hop IPv6 address>

or

ipv6 route ::/0 <exit-interface>

Explanation:

• ::/0 means all IPv6 addresses (similar to 0.0.0.0/0 in IPv4).
• It tells the router: “If no specific IPv6 route is found, send the packet here.”

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Example:

Router(config)# ipv6 route ::/0 2001:db8:1000::1

Here:

• 2001:db8:1000::1 = next-hop IPv6 address.
• All unknown IPv6 traffic will be sent to that next-hop.

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5. Verifying the Default Route

After configuring, you must verify that the default route is working correctly.

IPv4 Verification:

Use the following commands:

Router# show ip route

Look for this line:

S* 0.0.0.0/0 [1/0] via 192.168.1.1

Explanation:

• S = Static route
• * = Candidate default route
• 0.0.0.0/0 = Default route
• [1/0] = Administrative distance (1) and metric (0)
• via 192.168.1.1 = Next-hop address

You can also test with:

Router# ping <destination IP>
Router# traceroute <destination IP>

If the packet goes through the default route, it means it’s working.

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IPv6 Verification:

Router# show ipv6 route

Look for:

S ::/0 [1/0] via 2001:db8:1000::1

This means the default IPv6 route is installed and active.

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6. How the Router Uses the Default Route

When a router receives a packet:

1. It checks the destination IP address.
2. It searches its routing table for a matching route.
3. If no exact or more specific route is found,
4. It sends the packet through the default route (if configured).

If there’s no default route, and no other match:

• The router drops the packet.
• It might send an ICMP “Destination Unreachable” message back to the sender.

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7. When to Use a Default Route

Situation	Why a Default Route is Used
Router connected to the Internet	To send all unknown traffic to the ISP
Branch office router	To send traffic to the main office router
Small networks	To simplify routing instead of having many static routes
Stub network	A network with only one path to reach other networks

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8. Summary Table

Feature	IPv4	IPv6
Default route address	0.0.0.0/0	::/0
Command	ip route 0.0.0.0 0.0.0.0 <next-hop>	ipv6 route ::/0 <next-hop>
Purpose	Send unknown traffic to a specified gateway	Send unknown traffic to a specified gateway
Shown in routing table as	S* 0.0.0.0/0	S ::/0
Used in	Static, dynamic, or internet-edge networks	IPv6 networks and dual-stack environments

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9. Common Exam Tips

✅ Remember:

• Default route = 0.0.0.0/0 (IPv4) or ::/0 (IPv6)
• It is a static route used for unknown destinations
• Can be configured with next-hop IP or exit interface
• Verify with show ip route or show ipv6 route
• Appears as S* in the routing table
• Common in stub networks and internet connections

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10. Key Takeaway

The default route is the “catch-all” route that allows routers to forward packets for any unknown network.
Without it, routers drop traffic for destinations not explicitly listed in the routing table.

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