Summarization

Understand

Inter-area summarization belongs on an ABR, using area range for prefixes from the named source area. It reduces details in other areas while retaining the source area’s topology. ASBR summary-address applies to external redistribution instead; it does not replace area range.

Use aligned ranges that the site owns. A summary can remain advertised while only one component survives, so traffic to missing components may be discarded at the summarizing boundary. A discard route helps prevent loops but does not make nonexistent destinations reachable. Metric defaults vary with implementation/history; specify a cost when the lesson depends on it.

Lab setup

Use a fresh lab for this lesson; changes from other lessons are not cumulative. All outputs are illustrative excerpts, not recorded router tests. Transit cost is explicitly 10 and Loopback0 cost is 1. Interface names may vary by image.

Loopback0 is 10.255.0.<router number>/32; area assignments: R1: area 0, R2: area 0, R3: area 1, R4: area 2.

configure terminal
hostname R1
router ospf 1
 router-id 10.255.0.1
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
interface GigabitEthernet2
 ip address 192.0.2.9 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 1
 no shutdown
 exit
end

configure terminal
hostname R2
router ospf 1
 router-id 10.255.0.2
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
interface GigabitEthernet2
 ip address 192.0.2.5 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 2
 no shutdown
 exit
end

configure terminal
hostname R3
router ospf 1
 router-id 10.255.0.3
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.3 255.255.255.255
 ip ospf 1 area 1
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.10 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 1
 no shutdown
 exit
end

configure terminal
hostname R4
router ospf 1
 router-id 10.255.0.4
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.4 255.255.255.255
 ip ospf 1 area 2
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.6 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 2
 no shutdown
 exit
end

Apply

Add two /24 components on R3 in area 1, then summarize them at R1. Loopback network type preserves /24 advertisement.

R3

configure terminal
interface Loopback10
 ip address 10.10.0.1 255.255.255.0
 ip ospf network point-to-point
 ip ospf cost 1
 ip ospf 1 area 1
 exit
interface Loopback11
 ip address 10.10.1.1 255.255.255.0
 ip ospf network point-to-point
 ip ospf cost 1
 ip ospf 1 area 1
 exit
router ospf 1
 passive-interface Loopback10
 passive-interface Loopback11
end

R1 · ABR

configure terminal
router ospf 1
 area 1 range 10.10.0.0 255.255.254.0 cost 20
end

Verify and interpret

R2# show ip route ospf | include 10.10.
O IA 10.10.0.0/23 [110/30] via 192.0.2.1, GigabitEthernet1
R1# show ip route 10.10.0.0 255.255.254.0
Routing entry for 10.10.0.0/23
  Known via "ospf 1", distance 110, metric 20
  * directly connected, via Null0

R2 adds cost 10 to the explicit summary metric 20. R1 still has more-specific component routes toward R3.

Shut only R3 Loopback10: 10.10.1.0/24 keeps the /23 alive, while traffic for the failed component can hit the discard route. Shut Loopback11 too: with no active components in this range, the ABR withdraws the summary. Restore both interfaces and verify the /23 returns. The independent 10.255.0.3/32 is outside the range and does not keep it alive.

R3 · component failure test

configure terminal
interface Loopback10
 shutdown
 exit
interface Loopback11
 shutdown
end

R3 · restore

configure terminal
interface Loopback10
 no shutdown
 exit
interface Loopback11
 no shutdown
end

Technical references

OSPFv2 standard · Router configuration guide