By default, OSPF route summarization
generate the summary route metric
based on lowest metric of the more specific routes present in its routing table.
Let’s check this
topology
R3 is an Area Border
Router that receives two route from R1 and R2, Assuming that we manually adjust
the OSPF cost of the link towards R1 to 2000 and the link to R2 to 3000.
Let’s check R3 routing
table
R3#show ip route
100.0.0.0/24 is subnetted, 2 subnets
O 100.0.1.0 [110/2001] via 10.1.3.1,
00:00:03, FastEthernet0/0
O 100.0.2.0 [110/3001] via 10.2.3.2,
00:00:03, FastEthernet0/1
3.0.0.0/32 is subnetted, 1 subnets
C 3.3.3.3 is directly connected, Loopback0
10.0.0.0/24 is subnetted, 3 subnets
C 10.1.3.0 is directly connected,
FastEthernet0/0
C 10.2.3.0 is directly connected,
FastEthernet0/1
C 10.3.4.0 is directly connected,
FastEthernet1/0
Now let’s enable
summarization of the 100.0.0.0/8 route from area 5
R3(config)#router
ospf 1
R3(config-router)#area
5 range 100.0.0.0 255.0.0.0
Let’s check R3 routing table again
R3#show ip route
100.0.0.0/8 is variably subnetted, 3
subnets, 2 masks
O 100.0.0.0/8 is a summary, 00:10:01,
Null0
O 100.0.1.0/24 [110/2001] via 10.1.3.1,
00:00:06, FastEthernet0/0
O 100.0.2.0/24 [110/3001] via 10.2.3.2,
00:00:23, FastEthernet0/1
3.0.0.0/32 is subnetted, 1 subnets
C 3.3.3.3 is directly connected, Loopback0
10.0.0.0/24 is subnetted, 3 subnets
C 10.1.3.0 is directly connected,
FastEthernet0/0
C 10.2.3.0 is directly connected,
FastEthernet0/1
C 10.3.4.0 is directly connected,
FastEthernet1/0
The summary route has been generated and its next hop is Null0 since R3
is the originator of the summary route.
Checking the OSPF Database for self-originated summary LSAs
R3#show ip ospf
database summary 100.0.0.0 self-originate
OSPF Router with ID (3.3.3.3)
(Process ID 1)
Summary Net Link States (Area 0)
LS age: 245
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 100.0.0.0 (summary Network
Number)
Advertising Router: 3.3.3.3
LS Seq Number: 80000001
Checksum: 0x876C
Length: 28
Network Mask: /8
TOS: 0
Metric: 2001
It’s seems that The R3 Generated the summary route with the lowest
metric after comparing between the 100.0.1.1/24 and 100.0.2.2/24 metrics.
Now let’s check what R4 is receiving
R4#show ip route
O IA 100.0.0.0/8
[110/2002] via 10.3.4.3, 00:00:05,
FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/2] via 10.3.4.3, 00:40:39,
FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
C 4.4.4.4 is directly connected, Loopback0
10.0.0.0/24 is subnetted, 4 subnets
O IA 10.1.3.0 [110/2001] via 10.3.4.3, 00:40:39,
FastEthernet0/0
C 10.4.5.0 is directly connected,
FastEthernet0/1
O IA 10.2.3.0 [110/3001] via 10.3.4.3, 00:39:51,
FastEthernet0/0
C 10.3.4.0 is directly connected,
FastEthernet0/0
Indeed R4 is receiving the summary route with a metric of 2002
A point that needs to be clarified, this behavior was based on RFC1583,
in a later implementation of RFC2328 the behavior was changed to the highest
metric instead of the lowest metric. You can refer to that in RFC2328 section
3.5
“Area border routers then summarize the area
contents
(for distribution to the backbone) by advertising a
single
route for each address range. The cost
of the route is
the
maximum cost to any of the networks falling in the specified
range.”
You can disable OSPF compatibility to the old RFC1583 and make the
normal behavior as stated in RFC2328
R3(config)#router
ospf 1
R3(config-router)#no
compatible rfc1583
Now let’s check R3 self-originated summary routes again
R3#show ip ospf
database summary 100.0.0.0 self-originate
OSPF Router with ID (3.3.3.3)
(Process ID 1)
Summary Net Link States (Area
0)
LS age: 8
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 100.0.0.0 (summary Network
Number)
Advertising Router: 3.3.3.3
LS Seq Number: 80000004
Checksum: 0xB450
Length: 28
Network Mask: /8
TOS: 0
Metric: 3001
And R4 for a final check
R4#show ip route
Gateway of last
resort is not set
O IA 100.0.0.0/8
[110/3002] via 10.3.4.3, 00:02:16, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA 3.3.3.3 [110/2] via 10.3.4.3, 00:15:31,
FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
C 4.4.4.4 is directly connected, Loopback0
10.0.0.0/24 is subnetted, 4 subnets
O IA 10.1.3.0 [110/2001] via 10.3.4.3, 00:12:36,
FastEthernet0/0
C 10.4.5.0 is directly connected,
FastEthernet0/1
O IA 10.2.3.0 [110/3001] via 10.3.4.3, 00:12:46,
FastEthernet0/0
C 10.3.4.0 is directly connected,
FastEthernet0/0
Hopefully that clears up how OSPF ABR summarization works
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