How do you advertise a default route into OSPF?
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On the edge router, default-information originate under router ospf. It only advertises a default if
the router itself has one in its routing table, from any source other than OSPF. Adding always advertises
it regardless. The result is a Type 5 LSA, shown as O*E2 with metric 1, and the router becomes an ASBR.
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Example
R1(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.1
R1(config)# router ospf 1
R1(config-router)# default-information originateOn another router:
R2# show ip route ospf | include 0.0.0.0
O*E2 0.0.0.0/0 [110/1] via 10.0.12.1, 00:00:52, GigabitEthernet0/0/0Options
always: advertise even without a default in the table. Useful when the default should exist whatever happens, risky because it attracts traffic to a router that has lost its uplink.metric 10 metric-type 1: with two edge routers, make the default E1 so each router uses the nearest exit.route-map: advertise only when a condition holds, for example a tracked ISP prefix is present.
Other ways to get a default
- Stub and totally stubby areas: the ABR injects a Type 3 default automatically.
- NSSA:
area X nssa default-information-originateon the ABR gives a Type 7 default.
They will ask next
Why is default-information originate without always usually the better choice?
Because the advertisement follows the real default. If the ISP link fails and the static or BGP default disappears, the router stops advertising and traffic shifts to the other exit.
Traps
- Using
redistribute staticto send a default. On IOS that does not carry 0.0.0.0/0 into OSPF.
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