📘 CCNA 200-301 v1.1
3.1 Interpret the components of routing table
3.1.g Gateway of last resort
What is a Gateway of Last Resort?
The Gateway of Last Resort is the router’s “default route” — it is the route the router will use when there is no specific match in the routing table for a destination IP address.
Think of it as a backup route or a final option for forwarding packets that don’t match any other route.
🔹 Why is it needed?
A router uses its routing table to decide where to send packets.
Each route in the table tells the router which next hop or exit interface to use for a specific network.
But sometimes:
- The router receives a packet with a destination network not listed in its routing table.
Instead of dropping that packet, the router can forward it to the Gateway of Last Resort — if one is configured.
So, the Gateway of Last Resort prevents unnecessary packet drops and helps ensure connectivity to unknown or external networks (like the internet).
🔹 How it appears in the routing table
When you run the command:
show ip route
You may see a line like:
Gateway of last resort is 192.168.1.1 to network 0.0.0.0
This means:
- 192.168.1.1 is the next-hop IP address the router will use for all destinations not found in its routing table.
- 0.0.0.0/0 is the default route — it represents “any IP address”.
🔹 How the Gateway of Last Resort is set
The Gateway of Last Resort is set automatically or manually, depending on how you configure the router.
1. Manually – Using a Static Default Route
You can configure a default route using this command:
ip route 0.0.0.0 0.0.0.0 <next-hop-IP or exit-interface>
Example:
ip route 0.0.0.0 0.0.0.0 192.168.1.1
- This tells the router:
“If you don’t know where to send a packet, send it to 192.168.1.1.” - Once configured, it appears in the routing table as:
S* 0.0.0.0/0 [1/0] via 192.168.1.1- S means static route.
- * (star symbol) indicates that this route is the Gateway of Last Resort.
2. Automatically – From a Dynamic Routing Protocol
Some dynamic routing protocols (like EIGRP, RIP, or OSPF) can advertise a default route to other routers.
Example with EIGRP:
If you configure one router with:
ip route 0.0.0.0 0.0.0.0 10.1.1.1
and then advertise it using:
router eigrp 100
redistribute static
Then other routers running EIGRP can learn this route dynamically and set their own Gateway of Last Resort to that router.
🔹 How the router uses it (Decision Process)
When a router receives a packet:
- The router checks the destination IP address in the packet.
- It looks for a longest match in the routing table (the route with the most specific network prefix).
- If no route matches, it checks if a default route (0.0.0.0/0) exists.
- If the default route exists → it uses the Gateway of Last Resort.
- If not → the router drops the packet and may send an ICMP “Destination Unreachable” message back to the sender.
🔹 How to verify it
You can use:
show ip route
and look for:
Gateway of last resort is <IP> to network 0.0.0.0
Or, to see only the default route:
show ip route 0.0.0.0
You may see:
S* 0.0.0.0/0 [1/0] via 192.168.1.1
🔹 Important Notes for the CCNA Exam
| Concept | Description |
|---|---|
| 0.0.0.0/0 | Represents the default route (all IP addresses). |
| S* | Static route that acts as the Gateway of Last Resort. |
| Gateway of Last Resort | The next-hop IP or interface used when no other route matches. |
| Set manually | Using ip route 0.0.0.0 0.0.0.0 <next-hop>. |
| Set dynamically | Through routing protocols that advertise default routes. |
| If not configured | Packets with unknown destinations are dropped. |
🔹 Example: Routing Table Interpretation
R1# show ip route
Codes: C - connected, S - static, R - RIP, D - EIGRP, O - OSPF, * - candidate default
Gateway of last resort is 10.0.0.2 to network 0.0.0.0
S* 0.0.0.0/0 [1/0] via 10.0.0.2
C 10.0.0.0/24 is directly connected, GigabitEthernet0/0
C 192.168.1.0/24 is directly connected, GigabitEthernet0/1
Interpretation:
- The router will send packets for 10.0.0.0/24 and 192.168.1.0/24 directly to those interfaces.
- If a packet arrives for any other network (for example, 8.8.8.8), the router will use the Gateway of Last Resort (10.0.0.2) to forward it.
✅ Summary
| Feature | Description |
|---|---|
| Purpose | Used when no specific route exists in the routing table. |
| Represents | The router’s default route (0.0.0.0/0). |
| Configured by | Static route command or learned from routing protocols. |
| Display command | show ip route |
| Symbol | S* (Static Default Route) |
| Without it | Router drops packets for unknown destinations. |
In short:
The Gateway of Last Resort is the default path the router uses to forward packets when it does not know where the destination network is.
It is shown as a default route (0.0.0.0/0) in the routing table and is an essential concept for ensuring continuous connectivity — especially toward external or unknown networks.
