Octet

Say this first

With summary-address under router ospf on the ASBR that redistributes them. The ASBR originates one Type 5 (or Type 7 in an NSSA) for the summary instead of one per prefix. Like area range, IOS adds a Null0 discard route. Area range does not work on externals, because they are not tied to an area.

Go deeper

Example

R3 redistributes static routes 172.16.0.0/24 to 172.16.3.0/24 toward a partner.

Cisco IOS

router ospf 1
 redistribute static
 summary-address 172.16.0.0 255.255.252.0

Output

R3# show ip route | include 172.16.0.0/22
O        172.16.0.0/22 is a summary, 00:00:41, Null0
R1# show ip route | include 172.16.0.0/22
O E2     172.16.0.0/22 [110/20] via 10.0.13.3, 00:00:41, GigabitEthernet0/0/1

Two places it works

On the ASBR that redistributes the routes.
On an NSSA ABR, which can summarize Type 7 routes while it translates them into Type 5.

Options

summary-address 172.16.0.0 255.255.252.0 not-advertise suppresses the summary and its components.
tag <n> sets a route tag on the summary, which is handy in two-way redistribution.

They will ask next

Can a normal ABR summarize Type 5 routes that pass through it?
No. Type 5 LSAs flood unchanged through the whole domain. Only the originating ASBR, or an NSSA ABR doing translation, can summarize externals.

Traps

Saying area range works for redistributed routes. It does not.

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