One of OSPF’s scalability features is stub routing. When OSPF stub routing is enabled, the ABR no longer floods Type 5 LSAs into the area, minimizing the routing table and SPF churn. Following are two forms of stub routing:
■ Not-so-stubby routing: Permits external routes.
■ Totally stub routing: Does not enable external routes.
Stub routing enables intra-area routes to be learned and flooded throughout the area and all external routes. A not-so-stubby-area (NSSA) behaves in much the same way as a stub area with the exception that you can configure the NSSA to allow redistribution of exter-nal routes as Type 7 LSAs. Although not commonly deployed in the data center, there are use cases for the OSPF stub. A common model for environments is where end-host sys-tems such as servers or mainframes are capable of routing via multiple interfaces via OSPF benefit from stub routing to minimize the amount of routing table state main-tained. Another example that could leverage the OSPF stub would be when a merger or acquisition is made and complex routing can be simplified with the OSPF stub.
Stub routing is configured under the main routing process, as demonstrated in Example 3-42 for the network topology illustrated in Figure 3-7.
Example 3-42 OSPF Stub Routing Using a Numeric Process ID
Congo# confi t
Enter configuration commands, one per line. End with CNTL/Z.
Congo(config)# router ospf 100 Congo(config-router)# area 51 stub
124 NX-OS and Cisco Nexus Switching: Next-Generation Data Center Architectures
192.168.1.36/30
Libya 192.168.1.3/32 – Lo0
Congo 192.168.1.1/32 – Lo0 router ospf DataCenter 1
area 51 stub
Egypt 192.168.1.2/32 – Lo0
1/18 1/18
Po10
192.168.1.40/30
vPC Peer Keep Alive OSPF Area 0
OSPF Area 51
Figure 3-7 OSPF Stub
Congo# confi t
Enter configuration commands, one per line. End with CNTL/Z.
Congo(config)# router ospf DataCenter1 Congo(config-router)# area 51 stub
You can identify the stub status of a neighbor from the show ip ospf detail command output, as demonstrated in Example 3-44.
Example 3-44 Verification of OSPF Stub Neighbor
Congo#show ip ospf
Routing Process DataCenter1 with ID 192.168.1.1 VRF default Stateful High Availability enabled
Graceful-restart is configured Grace period: 60 state: Inactive Last graceful restart exit status: None Supports only single TOS(TOS0) routes Supports opaque LSA
This router is an area border Administrative distance 110 Reference Bandwidth is 40000 Mbps
Initial SPF schedule delay 200.000 msecs, minimum inter SPF delay of 1000.000 msecs, maximum inter SPF delay of 5000.000 msecs Initial LSA generation delay 0.000 msecs,
minimum inter LSA delay of 5000.000 msecs,
Example 3-43 demonstrates configuring OSPF stub routing using an alphanumeric tag.
Example 3-43 OSPF Stub Routing Using an Alphanumeric Process ID
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maximum inter LSA delay of 5000.000 msecs Minimum LSA arrival 1000.000 msec
Maximum paths to destination 8
Number of external LSAs 0, checksum sum 0 Number of opaque AS LSAs 0, checksum sum 0 Number of areas is 2, 1 normal, 1 stub, 0 nssa Number of active areas is 2, 1 normal, 1 stub, 0 nssa
Area BACKBONE(0.0.0.0)
Area has existed for 00:03:13
Interfaces in this area: 2 Active interfaces: 2 Passive interfaces: 0 Loopback interfaces: 0 No authentication available
SPF calculation has run 5 times Last SPF ran for 0.000502s Area ranges are
Number of LSAs: 6, checksum sum 0x33cd3 Area (0.0.0.51) (Inactive)
Area has existed for 00:03:13
Interfaces in this area: 1 Active interfaces: 1 Passive interfaces: 1 Loopback interfaces: 0 This area is a STUB area
Generates stub default route with cost 1 No authentication available
SPF calculation has run 5 times Last SPF ran for 0.000076s Area ranges are
192.168.128.0/20 Passive (Num nets: 0) Advertise Number of LSAs: 7, checksum sum 0x446f2
Example 3-45 demonstrates the syntax for configuring a totally stubby area.
Example 3-45 Configuration of a Totally Stub Area Using a Numeric Process ID
Congo# confi t
Enter configuration commands, one per line. End with CNTL/Z.
Congo(config)# router ospf 100
Congo(config-router)# area 51 stub no-summary
Example 3-46 demonstrates configuring a totally stubby area using an alphanumeric tag for the routing process ID.
Example 3-46 Configuration of a Totally Stub Area Using an Alphanumeric Process ID
Congo# confi t
126 NX-OS and Cisco Nexus Switching: Next-Generation Data Center Architectures
Enter configuration commands, one per line. End with CNTL/Z.
Congo(config)# router ospf DataCenter1 Congo(config-router)# area 51 stub no-summary
You can identify the stub status of a neighbor from the show ip ospf detail command output, as demonstrated in Example 3-47. You can notice an additional line that indicates Summarization is disabled to reflect the totally stub area status.
Example 3-47 Verification of Totally Stub Status on a Neighbor
Congo(config-router)# show ip ospf
Routing Process DataCenter1 with ID 192.168.1.1 VRF default Stateful High Availability enabled
Graceful-restart is configured Grace period: 60 state: Inactive Last graceful restart exit status: None Supports only single TOS(TOS0) routes Supports opaque LSA
This router is an area border Administrative distance 110 Reference Bandwidth is 40000 Mbps
Initial SPF schedule delay 200.000 msecs, minimum inter SPF delay of 1000.000 msecs, maximum inter SPF delay of 5000.000 msecs Initial LSA generation delay 0.000 msecs,
minimum inter LSA delay of 5000.000 msecs, maximum inter LSA delay of 5000.000 msecs Minimum LSA arrival 1000.000 msec
Maximum paths to destination 8
Number of external LSAs 0, checksum sum 0 Number of opaque AS LSAs 0, checksum sum 0 Number of areas is 2, 1 normal, 1 stub, 0 nssa Number of active areas is 2, 1 normal, 1 stub, 0 nssa
Area BACKBONE(0.0.0.0)
Area has existed for 00:15:47
Interfaces in this area: 2 Active interfaces: 2 Passive interfaces: 0 Loopback interfaces: 0 No authentication available
SPF calculation has run 7 times Last SPF ran for 0.000484s Area ranges are
Number of LSAs: 6, checksum sum 0x33ad4 Area (0.0.0.51) (Inactive)
Area has existed for 00:15:47
Chapter 3: Layer 3 Support and Configurations 127
Interfaces in this area: 1 Active interfaces: 1 Passive interfaces: 1 Loopback interfaces: 0
This area is a STUB area Generates stub default route with cost 1
Summarization is disabled No authentication available
SPF calculation has run 7 times Last SPF ran for 0.000074s Area ranges are
192.168.128.0/20 Passive (Num nets: 0) Advertise Number of LSAs: 2, checksum sum 0x17359