> ## Content Index > Fetch the complete content index at: https://www.pinglabz.com/llms.txt > Use this file to discover other available public pages before exploring further. # How to Advertise a Default Route in OSPF - URL: https://www.pinglabz.com/advertise-default-route-ospf/ - Published: 2026-03-24T03:16:22.000Z - Updated: 2026-08-02T05:13:20.000Z - Description: The default-information originate command injects 0.0.0.0/0 as a Type 5 LSA and turns the router into an ASBR. Here is the syntax, the always option, and verification. - Author: Jaime - Tags: OSPF, #Import 2026-08-01 19:54 ## The `default-information originate` Command **Format:** (This article is part of the PingLabz OSPF series - the [full OSPF guide](https://www.pinglabz.com/ospf/) maps the whole cluster in reading order.) ``` Router(config-router)# default-information originate [always] [metric value] [metric-type {1 | 2}] ``` **What it does:** - Advertises `0.0.0.0/0` into OSPF as a **Type 5 External LSA** - Marks the router as an **ASBR** (Autonomous System Boundary Router) ## Basic Configuration ### Scenario: Edge router (R1) has Internet connectivity via `0.0.0.0/0` static route. Advertise it to internal routers. **R1 Configuration:** ``` interface gi0/0 description Internet link ip address dhcp ip route 0.0.0.0 0.0.0.0 gi0/0 router ospf 1 default-information originate ``` **Result:** - R1 advertises `0.0.0.0/0` into OSPF - Other OSPF routers install the default route pointing to R1 ## Verification ### Check Default Route on Internal Routers **R2 (internal router):** ``` R2# show ip route Gateway of last resort is 10.1.1.1 to network 0.0.0.0 O*E2 0.0.0.0/0 [110/1] via 10.1.1.1, 00:05:23, GigabitEthernet0/0 ``` **Key indicators:** - **O**\* - OSPF default route - **E2** \- External Type 2 (default metric type) - **via 10.1.1.1** \- Points to R1 ### Check OSPF Database **R2:** ``` R2# show ip ospf database external OSPF Router with ID (2.2.2.2) (Process ID 1) Type-5 AS External Link States LS age: 123 Options: (No TOS-capability, DC, Upward) LS Type: AS External Link Link State ID: 0.0.0.0 (External Network Number) Advertising Router: 1.1.1.1 LS Seq Number: 80000001 Checksum: 0xABCD Length: 36 Network Mask: /0 Metric Type: 2 (Larger than any link state path) MTID: 0 Metric: 1 Forward Address: 0.0.0.0 External Route Tag: 0 ``` **Key fields:** - **Link State ID: 0.0.0.0** \- Default route - **Advertising Router: 1.1.1.1** \- R1 is advertising it - **Metric Type: 2** \- E2 route ## The `always` Keyword **Problem:** By default, `default-information originate` only works if the router **already has** a default route in its routing table. **Scenario:** R1's Internet link goes down. Static default route is removed from routing table. OSPF stops advertising `0.0.0.0/0`. **Result:** Internal routers lose their default route and can't reach the Internet. ### Solution: Use `always` ``` Router(config-router)# default-information originate always ``` **What `always` does:** - Advertises `0.0.0.0/0` into OSPF **even if** the router doesn't have a default route - Useful when: - You want to force a default route for traffic engineering - You're using route tracking (advanced) ⚠️ **Warning:** Use carefully. If R1 advertises `0.0.0.0/0` but has no Internet connectivity, traffic black-holes. ## Setting the Metric **Default metric:** 1 **To change it:** ``` Router(config-router)# default-information originate metric 100 ``` **Use case:** You have two edge routers. One is primary (metric 10), one is backup (metric 100). Routers prefer the primary. **Example:** **R1 (primary):** ``` router ospf 1 default-information originate metric 10 ``` **R2 (backup):** ``` router ospf 1 default-information originate metric 100 ``` **Result:** Internal routers prefer R1 (lower cost). If R1 fails, they fail over to R2. ## Metric Type: E1 vs E2 OSPF external routes can be **Type 1 (E1)** or **Type 2 (E2)**. ### E2 (Default) - **Cost calculation:** Fixed external cost (doesn't add internal OSPF cost) - **Example:** If external metric is 10, cost is 10 everywhere ### E1 - **Cost calculation:** External cost + internal OSPF cost - **Example:** If external metric is 10 and internal cost to ASBR is 5, total cost is 15 **Configuration:** ``` Router(config-router)# default-information originate metric-type 1 ``` **When to use E1:** - Multiple exit points - You want routers to pick the closest exit point **When to use E2 (default):** - Single exit point - Simpler metric calculation ## Dual Default Route (Redundancy) ### Scenario: Two edge routers for redundancy. **R1 (primary):** ``` router ospf 1 default-information originate metric 10 ``` **R2 (backup):** ``` router ospf 1 default-information originate metric 100 ``` **Internal router view:** ``` R3# show ip route O*E2 0.0.0.0/0 [110/10] via 10.1.1.1, GigabitEthernet0/0 [110/100] via 10.1.2.1, GigabitEthernet0/1 ``` **Result:** - Primary path via R1 (cost 10) - Backup path via R2 (cost 100) - If R1 fails, R3 uses R2 ## Conditional Default Route Advertisement (Advanced) **Scenario:** Only advertise default route if Internet is reachable. **Uses IP SLA tracking:** **Step 1: Create IP SLA probe** ``` ip sla 1 icmp-echo 8.8.8.8 source-interface gi0/0 frequency 30 ip sla schedule 1 life forever start-time now ``` **Step 2: Track the probe** ``` track 1 ip sla 1 reachability ``` **Step 3: Conditional default route** ``` ip route 0.0.0.0 0.0.0.0 gi0/0 track 1 router ospf 1 default-information originate ``` **Result:** - If 8.8.8.8 is reachable, static default route exists - OSPF advertises it - If 8.8.8.8 is unreachable, static route removed, OSPF stops advertising ## Common Mistakes ### Mistake 1: Using `always` Without Understanding **Problem:** ``` router ospf 1 default-information originate always ``` (Router has no Internet connectivity) **Impact:** Router advertises `0.0.0.0/0`, but traffic black-holes. **Fix:** Only use `always` if you understand the implications. Otherwise, rely on static default route existence. ### Mistake 2: Multiple ASBRs with Same Metric **Problem:** Both R1 and R2 advertise default route with metric 1. **Impact:** ECMP load-balancing. If one path is slower, traffic suffers. **Fix:** Set different metrics to prefer one router. ### Mistake 3: Advertising Default into Stub Areas **Problem:** You manually configure `default-information originate` on an ABR in a stub area. **Impact:** Stub areas already get a default route automatically from the ABR. Manual config is redundant. **Fix:** Let stub area ABR auto-generate the default route. ## Best Practices ### 1\. Use Conditional Advertisement (IP SLA) Prevent black-holing by only advertising when Internet is up. ### 2\. Set Metrics for Redundancy Primary: low metric Backup: high metric ### 3\. Document in Config ``` router ospf 1 ! Advertise default route for Internet access default-information originate metric 10 ``` ### 4\. Monitor Default Route Set up alerts if default route disappears from internal routers. ## Summary Now you know: ✅ **Why default routes matter** \- Internet connectivity for internal routers ✅ **default-information originate** \- Injects 0.0.0.0/0 into OSPF ✅ **always keyword** \- Advertises even without default route in table ✅ **Metric control** \- Prefer primary over backup ✅ **E1 vs E2** \- Metric calculation differences ✅ **Conditional advertisement** \- IP SLA tracking for reliability **Next Step:** You've mastered single-area OSPF and default routes. Ready to scale? Learn [**How to Configure Multi-Area OSPF**](https://www.pinglabz.com/configure-multi-area-ospf-cisco/) next. **Internal Links:** - ← [OSPF Passive Interfaces](https://www.pinglabz.com/ospf-passive-interface/) - → [Configuring Multi-Area OSPF](https://www.pinglabz.com/configure-multi-area-ospf-cisco/) - → [OSPF Redistribution](https://www.pinglabz.com/ospf-redistribution-configuration/)