Default Route Redistribution into OSPF

In most enterprise and branch networks, internal routers do not need to know every route on the internet — they only need to know how to reach internal networks and where to send everything else. The default route (0.0.0.0/0) is that "send everything else here" instruction. In an OSPF domain, one border router — typically the one connected to the ISP or upstream network — holds the default route and must share it with all other OSPF routers so they automatically learn where to forward unknown destinations.

The default-information originate command causes an OSPF router to generate a Type 5 External LSA for the 0.0.0.0/0 route and flood it throughout the entire OSPF domain. All routers that receive it install the 0.0.0.0/0 route as O*E2 — marking it as their Gateway of Last Resort. This eliminates the need to manually configure a default route on every router in the OSPF domain.

Before starting, complete OSPF Single-Area Configuration and OSPF Multi-Area Configuration to understand OSPF neighbour adjacency and LSA types. Review Static Route Configuration for default route fundamentals. For OSPF neighbour state details, see OSPF Neighbour States.

1. How Default Route Distribution Works in OSPF

The ASBR Role

When a router runs default-information originate, it becomes an ASBR (Autonomous System Boundary Router) — a router that injects external routes into the OSPF domain. The default route does not originate inside OSPF — it comes from a static route pointing toward an ISP or upstream device. The ASBR takes that external route and packages it into a Type 5 External LSA that floods to every router in every OSPF area.

Element	Role	Details
ASBR	The router that generates and floods the default route LSA	Identified by "It is an autonomous system boundary router" in `show ip ospf`
Type 5 External LSA	The LSA that carries the 0.0.0.0/0 route across the OSPF domain	Flooded to all areas — unlike Type 3 which stays between areas
*OE2 route**	How the default route appears on downstream routers	O = OSPF, * = candidate default (Gateway of Last Resort), E2 = External Type 2
E2 metric	The metric stays fixed at the ASBR's value regardless of topology distance	Default metric for redistributed routes — metric does not accumulate as it crosses routers

default-information originate — Two Modes

Command	Behaviour	When to Use
`default-information originate`	Generates the Type 5 LSA for 0.0.0.0/0 only if a default route already exists in the routing table (static or learned). If the default route disappears (ISP link down), the LSA is withdrawn automatically	✅ Production standard — safer. Downstream routers lose the default route when the ISP link goes down, preventing traffic black holes
`default-information originate always`	Generates the Type 5 LSA for 0.0.0.0/0 unconditionally — even if no default route exists in the routing table	⚠️ Lab use or specific design only — downstream routers keep the default route even when the exit is gone, causing a black hole at the ASBR

default-information originate always — black hole risk: If the ISP link goes down and the static default route is removed from the routing table, always keeps advertising the default. Downstream routers continue forwarding traffic toward the ASBR — which drops it because it has no valid exit. Use always only in lab environments or when a secondary exit exists.

Type 5 LSA vs Type 3 LSA

Feature	Type 3 Summary LSA	Type 5 External LSA
Generated by	ABR	ASBR
Carries	Inter-area OSPF routes	Routes external to OSPF (static, BGP, redistributed)
Flooding scope	Into adjacent areas only	Entire OSPF domain — all areas
Route code	O IA	O E1 or O E2 (O*E2 for default)
Metric behaviour	Cost increases as it crosses routers	E2: fixed metric. E1: accumulates internal path cost

2. Lab Topology & Scenario

NetsTuts_R1 is the edge router connected to a simulated ISP (203.0.113.1). It runs OSPF with R2 and R3 in Area 0. The goal is for R2 and R3 to automatically learn the internet default route from R1 via OSPF — without any static routes configured on R2 or R3.

                    Internet / ISP
                    203.0.113.1
                          |
                     Gi0/0 (WAN)
                     203.0.113.2
               ┌──────────────────┐
               │   NetsTuts_R1    │  ← ASBR
               │   RID: 1.1.1.1   │    (default-information originate)
               └──────────────────┘
              Gi0/1            Gi0/2
           10.0.12.1         10.0.13.1
               |                 |
         10.0.12.0/30       10.0.13.0/30
               |                 |
           10.0.12.2         10.0.13.2
              Gi0/0            Gi0/0
    ┌──────────────┐        ┌──────────────┐
    │ NetsTuts_R2  │        │ NetsTuts_R3  │
    │ RID: 2.2.2.2 │        │ RID: 3.3.3.3 │
    └──────────────┘        └──────────────┘
         Gi0/1                   Gi0/1
    192.168.20.1            192.168.30.1
    192.168.20.0/24         192.168.30.0/24
     (LAN — PC2)              (LAN — PC3)

  All routers: OSPF Area 0, Process 1

Device	Interface	IP Address	Description
NetsTuts_R1	Gi0/0	203.0.113.2 /30	WAN — link to ISP (203.0.113.1)
NetsTuts_R1	Gi0/1	10.0.12.1 /30	OSPF link to R2 — Area 0
NetsTuts_R1	Gi0/2	10.0.13.1 /30	OSPF link to R3 — Area 0
NetsTuts_R2	Gi0/0	10.0.12.2 /30	OSPF link to R1 — Area 0
NetsTuts_R2	Gi0/1	192.168.20.1 /24	LAN — PC2
NetsTuts_R3	Gi0/0	10.0.13.2 /30	OSPF link to R1 — Area 0
NetsTuts_R3	Gi0/1	192.168.30.1 /24	LAN — PC3

3. Step 1 — Configure OSPF on All Routers (Baseline)

First establish OSPF neighbour adjacency across all three routers in Area 0. The WAN interface (Gi0/0) on R1 is not included in OSPF — it connects to the ISP and should not participate in OSPF Hellos.

NetsTuts_R1 — OSPF (without default route yet)

NetsTuts_R1>en
NetsTuts_R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
NetsTuts_R1(config)#router ospf 1
NetsTuts_R1(config-router)#router-id 1.1.1.1
NetsTuts_R1(config-router)#auto-cost reference-bandwidth 1000
NetsTuts_R1(config-router)#network 10.0.12.0 0.0.0.3 area 0
NetsTuts_R1(config-router)#network 10.0.13.0 0.0.0.3 area 0
NetsTuts_R1(config-router)#exit
NetsTuts_R1(config)#end
NetsTuts_R1#wr
Building configuration...
[OK]
NetsTuts_R1#

Note that the WAN interface (203.0.113.2 on Gi0/0) is deliberately excluded from OSPF. The ISP does not run OSPF — this interface only needs a static default route pointing to 203.0.113.1, not an OSPF adjacency. The network statements use wildcard masks to match the correct interfaces.

NetsTuts_R2

NetsTuts_R2>en
NetsTuts_R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
NetsTuts_R2(config)#router ospf 1
NetsTuts_R2(config-router)#router-id 2.2.2.2
NetsTuts_R2(config-router)#auto-cost reference-bandwidth 1000
NetsTuts_R2(config-router)#network 10.0.12.0 0.0.0.3 area 0
NetsTuts_R2(config-router)#network 192.168.20.0 0.0.0.255 area 0
NetsTuts_R2(config-router)#passive-interface GigabitEthernet0/1
NetsTuts_R2(config-router)#exit
NetsTuts_R2(config)#end
NetsTuts_R2#wr
Building configuration...
[OK]
NetsTuts_R2#
%OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on GigabitEthernet0/0 from LOADING to FULL, Loading Done

NetsTuts_R3

NetsTuts_R3>en
NetsTuts_R3#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
NetsTuts_R3(config)#router ospf 1
NetsTuts_R3(config-router)#router-id 3.3.3.3
NetsTuts_R3(config-router)#auto-cost reference-bandwidth 1000
NetsTuts_R3(config-router)#network 10.0.13.0 0.0.0.3 area 0
NetsTuts_R3(config-router)#network 192.168.30.0 0.0.0.255 area 0
NetsTuts_R3(config-router)#passive-interface GigabitEthernet0/1
NetsTuts_R3(config-router)#exit
NetsTuts_R3(config)#end
NetsTuts_R3#wr
Building configuration...
[OK]
NetsTuts_R3#
%OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on GigabitEthernet0/0 from LOADING to FULL, Loading Done

4. Step 2 — Configure Static Default Route on R1

Before OSPF can distribute the default route, R1 must have a default route in its own routing table. The static default route points to the ISP's gateway (203.0.113.1):

! ── Static default route toward ISP ──────────────────────
NetsTuts_R1(config)#ip route 0.0.0.0 0.0.0.0 203.0.113.1
NetsTuts_R1(config)#end
NetsTuts_R1#wr
Building configuration...
[OK]
NetsTuts_R1#

Verify the Default Route Exists on R1

NetsTuts_R1#show ip route static
Codes: S - static

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

Gateway of last resort is 203.0.113.1 — the default route is installed on R1. The S* code means static candidate default. This is the prerequisite for default-information originate — without it, the command generates no LSA (unless always is used). See show ip route for full route code reference.

5. Step 3 — Inject Default Route into OSPF

With the static default route confirmed on R1, add default-information originate to R1's OSPF process. This single command makes R1 the ASBR and floods the 0.0.0.0/0 route as a Type 5 External LSA to R2 and R3:

NetsTuts_R1(config)#router ospf 1
NetsTuts_R1(config-router)#default-information originate
NetsTuts_R1(config-router)#end
NetsTuts_R1#wr
Building configuration...
[OK]
NetsTuts_R1#

No other changes are needed on R2 or R3. As soon as R1 generates the Type 5 LSA, it is flooded to all OSPF neighbours and both downstream routers install the default route automatically.

always Keyword — Lab and Special Cases

! ── Force advertise even without a local default route ───
NetsTuts_R1(config-router)#default-information originate always

Use always in lab environments where no actual ISP link exists — the static default route pointing to a real next hop is not required. In production, avoid always unless you have a redundant exit path or accept that the ASBR will black-hole traffic when the ISP link fails.

Optional: Set a Custom Metric on the Default Route

! ── Advertise with a specific metric value ───────────────
NetsTuts_R1(config-router)#default-information originate metric 50 metric-type 1

By default the metric is 1 and metric-type is E2 (fixed). Using metric-type 1 changes it to E1 — the internal OSPF path cost accumulates on top of the external metric as the route crosses routers. E1 is useful when two ASBRs advertise the same default and you want routers to prefer the closer one. E2 (default) keeps the metric uniform — all downstream routers see the same cost regardless of their distance from the ASBR.

Dual ASBR Redundancy — Two Default Routes

In networks with two edge routers (R1 and R1B), both can run default-information originate. Downstream routers receive two Type 5 LSAs for 0.0.0.0/0 and select the best path based on metric. Using metric-type E1 on both ASBRs ensures routers naturally prefer the closest exit:

! ── On R1 (primary ISP) ──────────────────────────────────
NetsTuts_R1(config-router)#default-information originate metric 10 metric-type 1

! ── On R1B (backup ISP, higher metric = less preferred) ──
NetsTuts_R1B(config-router)#default-information originate metric 100 metric-type 1

R1 is preferred (metric 10 + internal path cost). R1B is used only when R1's link fails and its Type 5 LSA is withdrawn. Downstream routers automatically switch to R1B with no manual intervention.

6. Verification

show ip route — R2 Receives O*E2

NetsTuts_R2#show ip route
Codes: C - connected, L - local, O - OSPF, E2 - OSPF external type 2

Gateway of last resort is 10.0.12.1 to network 0.0.0.0

O*E2  0.0.0.0/0 [110/1] via 10.0.12.1, 00:01:22, GigabitEthernet0/0

      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C        10.0.12.0/30 is directly connected, GigabitEthernet0/0
L        10.0.12.2/32 is directly connected, GigabitEthernet0/0
O        10.0.13.0/30 [110/2] via 10.0.12.1, GigabitEthernet0/0

      192.168.20.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.20.0/24 is directly connected, GigabitEthernet0/1
L        192.168.20.1/32 is directly connected, GigabitEthernet0/1
O        192.168.30.0/24 [110/2] via 10.0.12.1, GigabitEthernet0/0

Gateway of last resort is 10.0.12.1 to network 0.0.0.0 — confirmed. The route O*E2 0.0.0.0/0 [110/1] via 10.0.12.1 is the redistributed default route received from R1. Any packet from PC2 destined for the internet will be forwarded to R1 (10.0.12.1) which then sends it to the ISP.

show ip route — R3 Receives O*E2

NetsTuts_R3#show ip route
Codes: O - OSPF

Gateway of last resort is 10.0.13.1 to network 0.0.0.0

O*E2  0.0.0.0/0 [110/1] via 10.0.13.1, 00:01:18, GigabitEthernet0/0

R3 also receives the default route via its own link to R1 (10.0.13.1). Both R2 and R3 point back to R1 as the Gateway of Last Resort — without any static route configuration on R2 or R3. This is the goal.

show ip ospf — R1 Identified as ASBR

NetsTuts_R1#show ip ospf
 Routing Process "ospf 1" with ID 1.1.1.1
 ...
 It is an autonomous system boundary router
 Redistributing External Routes from,
   static, includes subnets in redistribution
 ...

"It is an autonomous system boundary router" — IOS explicitly confirms R1 has become an ASBR. The line "Redistributing External Routes from, static" confirms the source of the redistributed default route.

show ip ospf database external — Type 5 LSA

NetsTuts_R1#show ip ospf database external

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Type-5 AS External Link States

  LS age: 87
  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: 0x7D61
  Length: 36
  Network Mask: /0
  Metric Type: 2 (Larger than any link state path)
  Metric: 1
  Forward Address: 0.0.0.0
  External Route Tag: 0

The Type 5 LSA for the default route is visible. Key fields: Link State ID: 0.0.0.0 (the default route destination), Advertising Router: 1.1.1.1 (R1 is the ASBR generating this LSA), Metric Type: 2 (E2 — fixed metric), Metric: 1 (the cost advertised). This LSA is flooded to every router in the OSPF domain.

show ip ospf database external — on R2

NetsTuts_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: 112
  Advertising Router: 1.1.1.1
  Link State ID: 0.0.0.0
  Network Mask: /0
  Metric Type: 2
  Metric: 1

R2 holds the same Type 5 LSA in its LSDB — the Advertising Router field confirms it came from R1 (1.1.1.1). R2 did not generate this LSA — it received it via OSPF flooding from R1.

ping 8.8.8.8 from R2 — End-to-End Verification

NetsTuts_R2#ping 8.8.8.8 source GigabitEthernet0/1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 8.8.8.8, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 18/20/22 ms
NetsTuts_R2#

End-to-end internet reachability confirmed from R2's LAN interface. R2 has no static default route — all internet-bound traffic is forwarded via the O*E2 default route pointing to R1 (10.0.12.1), which then forwards to the ISP (203.0.113.1).

Verification Command Summary

Command	What to Look For	Run On
`show ip route`	"Gateway of last resort is..." and *OE2 0.0.0.0/0** entry	R2, R3 (downstream)
`show ip route static`	*S 0.0.0.0/0** — confirms the prerequisite default route exists on the ASBR	R1 (ASBR)
`show ip ospf`	"It is an autonomous system boundary router" — confirms ASBR role	R1 (ASBR)
`show ip ospf database external`	Type 5 LSA for 0.0.0.0, Advertising Router = ASBR's RID, Metric Type 2	Any router in the domain
`show ip ospf neighbor`	All neighbours FULL — prerequisite for LSA flooding to work	All routers
`ping [public IP]`	Successful pings confirm end-to-end internet reachability via OSPF default	R2, R3 (downstream)

7. Troubleshooting Default Route Distribution Issues

Problem	Symptom	Cause	Fix
O*E2 route not appearing on downstream routers	`show ip route` on R2/R3 shows no Gateway of Last Resort and no 0.0.0.0/0 entry	No static default route on R1 — `default-information originate` (without `always`) requires a default route to exist first	Add `ip route 0.0.0.0 0.0.0.0 [next-hop]` on R1. Verify with `show ip route static` — confirm S* 0.0.0.0/0 exists. Then re-check downstream routers.
Default route present but internet traffic fails	O*E2 route exists on R2/R3 but pings to 8.8.8.8 fail	R1's WAN interface is down or the ISP next hop (203.0.113.1) is unreachable	Verify R1's WAN interface: `show ip interface brief`. Ping 203.0.113.1 from R1. Check ISP link status.
Default route withdrawn when ISP link flaps	O*E2 disappears from R2/R3 during ISP instability	Expected behaviour — `default-information originate` (without `always`) withdraws the LSA when the static default route disappears	This is correct production behaviour — prevents black holes. If stability is needed regardless, use `always` with a secondary exit path.
Two ASBRs both advertising default — wrong one selected	Traffic exits via the backup ISP when primary is up	Both ASBRs using E2 (fixed metric) — downstream routers see identical cost and may use either ASBR	Switch both to metric-type E1 with different base metrics. Primary ASBR uses lower metric. Internal OSPF path cost accumulates, naturally directing traffic to the closer/preferred ASBR.
R1 not identified as ASBR	`show ip ospf` on R1 does not say "autonomous system boundary router"	`default-information originate` was not entered, or was entered but no default route exists (and `always` was not used)	Verify the command exists in `show running-config` `\| section router ospf`. Check that `ip route 0.0.0.0 0.0.0.0` is in the routing table.
Default route not reaching routers in non-backbone areas	Routers in Area 1 or Area 2 have no O*E2 default route	The area is configured as a stub area — stub areas block Type 5 External LSAs by design	Stub areas receive a Type 3 default route instead. If the area must receive the specific ASBR's default, change it from stub to normal area. See OSPF Multi-Area Configuration.

Key Points & Exam Tips

default-information originate causes the OSPF router to generate a Type 5 External LSA for 0.0.0.0/0 — flooding the default route to every router in the OSPF domain.
The command requires a default route to already exist in the routing table (S* 0.0.0.0/0). Without it, no LSA is generated. Use always to override this requirement. See Static Route Configuration for how to add the prerequisite default route.
The router running default-information originate becomes an ASBR — confirmed by "It is an autonomous system boundary router" in show ip ospf.
Downstream routers install the default route as O*E2 0.0.0.0/0. The asterisk marks it as the Gateway of Last Resort. The E2 metric stays fixed regardless of hop count. Verify with show ip route.
default-information originate always advertises 0.0.0.0/0 even without a local default route — useful in labs but risky in production as it can create black holes when the ISP link is down.
Use metric-type 1 (E1) instead of the default E2 when two ASBRs both advertise the default route — E1 accumulates internal path cost, directing each router to naturally prefer the closest exit.
The WAN interface connecting to the ISP should not be included in the OSPF network statement — it does not participate in OSPF Hellos or LSAs. Use passive-interface on LAN-only interfaces.
show ip ospf database external confirms the Type 5 LSA exists — key fields are Link State ID (0.0.0.0), Advertising Router (ASBR's RID), and Metric Type (1 or 2).
Stub areas block Type 5 External LSAs — routers in stub areas cannot receive the OSPF default route via default-information originate. They receive a Type 3 summary default instead.
On the CCNA exam: know the difference between default-information originate (conditional) and default-information originate always (unconditional), and what O*E2 means in the routing table. For OSPF troubleshooting, see Troubleshooting OSPF Neighbour Adjacency.

Next Steps: With the default route distributed across the OSPF domain, continue to DHCP Server Configuration to dynamically assign IP addresses including the correct default gateway to hosts on each subnet. For filtering which routes OSPF distributes, explore OSPF Single-Area Configuration and route filtering with distribute-lists. To verify all OSPF route sources and metrics in one view, use show ip protocols. For inter-area route efficiency, see Route Summarisation and Aggregation. For deeper OSPF troubleshooting, see Troubleshooting OSPF Neighbour Adjacency.

TEST WHAT YOU LEARNED

1. An engineer configures `default-information originate` on R1 but R2 and R3 do not receive a default route. `show ip route static` on R1 shows no output. What is the cause?

A. The OSPF process ID on R1 does not match R2 and R3 B. The command needs to be entered on all routers, not just R1 C. No static default route exists on R1 — default-information originate without always only generates a Type 5 LSA when a default route (0.0.0.0/0) is already present in the routing table D. R1 must be the DR on all interfaces before it can originate external LSAs

Correct answer is C. default-information originate is conditional — it checks whether a default route (0.0.0.0/0) exists in the local routing table before generating the Type 5 LSA. If show ip route static returns no output, there is no static default route. The fix is to add ip route 0.0.0.0 0.0.0.0 [ISP-next-hop] on R1. Alternatively, use default-information originate always to bypass the check — though this is not recommended in production.

2. What does the code **O*E2** mean when it appears in a router's routing table?

A. O = OSPF, * = candidate default (Gateway of Last Resort), E2 = External Type 2 — a route redistributed into OSPF from outside the domain via a Type 5 LSA, with a fixed metric that does not increase as it crosses routers B. O = OSPF, * = backup route, E2 = advertised by an ABR from Area 2 C. O = OSPF, * = equal-cost path exists, E2 = the route has a metric of exactly 2 D. O = OSPF external, * = route is active, E2 = route is in Area 0 only

Correct answer is A. Breaking down O*E2: O identifies the source as OSPF. The asterisk (*) marks this specific route as the candidate default — the Gateway of Last Resort — meaning packets with no more specific match are forwarded using this route. E2 means External Type 2, indicating the route was redistributed from outside the OSPF AS (in this case from a static route via default-information originate) and the metric remains constant at the value set by the ASBR, regardless of internal path cost.

3. What is the risk of using `default-information originate always` in a production network?

A. It causes OSPF to advertise all connected routes as external, flooding the LSDB B. It permanently sets the metric to 0, making the route less preferred than any other default C. It causes the ASBR to generate two Type 5 LSAs — one for 0.0.0.0/0 and one for the static route D. If the ISP link fails and the static default route is removed, the ASBR continues advertising 0.0.0.0/0. Downstream routers keep forwarding internet traffic to the ASBR — which drops it because the exit no longer exists, creating a black hole

Correct answer is D. The always keyword bypasses the prerequisite check — it unconditionally advertises the default route regardless of whether a valid exit path exists. In production, when the ISP link goes down, the static default route is typically removed from the routing table. Without always, the Type 5 LSA is withdrawn and downstream routers know there is no exit — they drop packets appropriately. With always, they keep forwarding to the ASBR which silently drops everything — a black hole that is harder to diagnose.

4. Why should the WAN interface (connecting R1 to the ISP) be excluded from OSPF network statements?

A. Including the WAN interface would cause R1 to advertise the ISP's network into OSPF, consuming extra bandwidth B. The ISP does not run OSPF — including the WAN interface would cause R1 to send Hello packets toward the ISP with no response, waiting for a neighbour that never arrives. It also risks advertising the WAN subnet into OSPF unnecessarily C. The WAN interface must be excluded because OSPF cannot run on interfaces with public IP addresses D. Including the WAN interface would prevent default-information originate from working correctly

Correct answer is B. The ISP is an external entity — it does not participate in the customer's OSPF process. If the WAN interface IP is included in an OSPF network statement, R1 starts sending OSPF Hello packets out the WAN interface toward the ISP. This serves no purpose (the ISP router won't respond with OSPF Hellos), wastes resources, and potentially advertises the WAN subnet (203.0.113.0/30) as an OSPF internal route — information that is irrelevant to internal routers. Exclude the WAN interface and use a static default route to reach the ISP instead.

5. A router in an OSPF stub area is not receiving the O*E2 default route from the ASBR. What is the reason?

A. Stub areas require default-information originate always — the standard command does not work B. The ASBR must be inside the stub area for the default route to reach it C. O*E2 routes have a higher AD in stub areas and are filtered by the ABR's routing table D. Stub areas block all Type 5 External LSAs by design — including the default route Type 5. Instead, the ABR automatically injects a Type 3 Summary LSA for 0.0.0.0/0 into the stub area

Correct answer is D. One of the defining characteristics of OSPF stub areas is that Type 5 External LSAs are not allowed inside them. This blocks all externally redistributed routes — including the default route generated by default-information originate. To compensate, the ABR automatically generates a Type 3 Summary LSA for 0.0.0.0/0 into the stub area, providing a default route to internal routers. This stub-area default appears as O IA 0.0.0.0/0 rather than O*E2.

6. Two ASBRs (R1 and R1B) both run `default-information originate` with the default E2 metric of 1. How do downstream routers choose between the two default routes?

A. With identical E2 metrics (both = 1), OSPF falls back to the lowest OSPF cost to reach each ASBR. If costs are still equal, the higher ASBR Router ID is used as a tiebreaker — neither is deterministically preferred B. The router with the higher Router ID always wins as the preferred ASBR C. OSPF load-balances between both default routes equally regardless of path cost D. The most recently added ASBR takes precedence — last writer wins

Correct answer is A. With E2 metrics identical, OSPF uses the cost to reach the forwarding address or ASBR as a secondary tiebreaker. If costs are also equal, the higher ASBR Router ID wins. This makes E2 multi-ASBR scenarios unpredictable — routers may not prefer the "better" exit. The solution is to use metric-type E1 with different base metrics: the primary ASBR uses a lower metric so downstream routers naturally prefer it, and the accumulated internal path cost automatically directs each router toward its closest exit when both ASBRs have equal base metrics.

7. What command on R1 confirms it has become an ASBR after `default-information originate` is configured?

A. show ip route ospf — an ASBR marker appears next to 0.0.0.0/0 B. show ip ospf neighbor — the ASBR flag appears in the neighbour state C. show ip ospf — explicitly states "It is an autonomous system boundary router" and "Redistributing External Routes from, static" D. show ip ospf database — the ASBR status is shown in the router LSA header

Correct answer is C. show ip ospf is the command that explicitly identifies a router's role in the OSPF hierarchy. For an ASBR, it states "It is an autonomous system boundary router" and lists the source of the redistributed routes ("Redistributing External Routes from, static"). This is the definitive confirmation command — analogous to how the same command says "It is an area border router" for ABRs. show ip ospf database external then shows the actual Type 5 LSA being generated.

8. What is the difference between metric-type E1 and metric-type E2 for a redistributed default route?

A. E1 routes have AD 110 and E2 routes have AD 120 — E1 is more trusted B. E2 (default) keeps the same metric as set at the ASBR regardless of distance — all routers see the same cost. E1 accumulates the internal OSPF path cost on top of the external metric — routers closer to the ASBR see a lower total cost C. E1 routes are preferred in stub areas and E2 routes in normal areas D. E1 uses classful routing and E2 uses classless — only E2 supports VLSM

Correct answer is B. E2 (the default) has a fixed metric — it does not change as the route traverses routers. Every router in the OSPF domain sees the same cost (e.g., 1) regardless of how far they are from the ASBR. E1 adds the internal OSPF path cost to the external metric — a router 5 hops from the ASBR sees a higher cost than a router 1 hop away. This makes E1 useful for dual-ASBR scenarios where you want each router to naturally prefer the nearest exit point, while E2 is simpler when path cost proximity doesn't matter.

9. R2 has O*E2 0.0.0.0/0 via 10.0.12.1 in its routing table. The link between R1 and R2 (10.0.12.0/30) goes down. What happens to the default route on R2?

A. R2 keeps the route for 180 seconds (OSPF dead timer) then removes it B. R2 marks the route as suspect but keeps forwarding through it until a new path is found C. R2 immediately removes the default route and triggers a full SPF recalculation across the domain D. OSPF detects the link failure via Hello/Dead timers. R2 removes the neighbour adjacency with R1, re-runs SPF, and if no alternative path to R1 exists, the O*E2 default route is removed — R2 has no Gateway of Last Resort until connectivity is restored

Correct answer is D. When the direct link between R1 and R2 goes down, OSPF detects the failure when R2's Dead timer expires (40 seconds by default on Ethernet) or immediately if R2 detects the interface going down at Layer 1. R2 removes the neighbour adjacency with R1 and re-runs SPF. Since the only path to R1 was the direct link (in this topology), R2 has no remaining path to the ASBR — the O*E2 default route is removed. R2's Gateway of Last Resort disappears until connectivity is restored or an alternative path exists.

10. Which command on a downstream router confirms that the Type 5 External LSA for the default route was originated by R1 (RID 1.1.1.1) and not generated locally?

A. show ip route 0.0.0.0 — displays the originating router's address B. show ip ospf database external — the "Advertising Router" field shows 1.1.1.1, confirming R1 generated the LSA, not the local router C. show ip ospf neighbor — the ASBR is marked with an E flag next to the neighbour ID D. show ip ospf interface brief — external LSA source is shown per interface

Correct answer is B. show ip ospf database external displays all Type 5 External LSAs in the LSDB. The Advertising Router field explicitly identifies which router generated the LSA — in this case 1.1.1.1 (R1). This is the definitive way to confirm the ASBR source of a redistributed route. If the Advertising Router field shows the local router's RID, it would mean the local router itself is the ASBR. This command is essential for multi-ASBR troubleshooting to identify which exit point each default route advertisement comes from.

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