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OSPF Router with Multiple Areas and Summarization

Example Scenario

This recipe demonstrates two routers which handle traffic for multiple local networks. Though it is a simple configuration, multiple areas are used so that routes for each site may be summarized.

Summarization reduces the number of routes that each neighbor must advertise and reduces the number of routes that each neighbor must maintain in its local database. As networks grow, this becomes an important factor when resources are constrained. This example allows for significant future expansion with little or no increase in OSPF database complexity for peers.

In modern networking environments, most implementations like TNSR are capable of handling many thousands of routes in a single area. Even so, using multiple areas with summarization can be easier for administrators to manage and troubleshoot.

Since each of these routers is connected to more than one area, each becomes an Area Border Router (ABR). As such, they are capable of route summarization using Type 3 Link State Advertisement (LSA) messages.

Note

This example ignores external connectivity, only focusing on the relationship between two routers and their component networks.

Additionally, since each of these routers is not connected to other routers outside the backbone network, their local areas can be considered stub areas and the local interfaces can be configured as passive interfaces.

See also

For a simpler example involving a single area, see OSPF Example.

Scenario Topology

../../_images/diagram-ospf-abr.png

TNSR OSPF with ABR

Scenario Information

Shared Information

Item

Value

OSPF Backbone Area

0.0.0.0

Backbone Network

172.16.0.0/24
Router 1 Information

Item

Value

Backbone Address

172.16.0.2/24

Local Router ID

10.2.0.1

Local OSPF Area

0.0.0.2

Active Interfaces (Cost)

TenGigabitEthernet6/0/0 (5)

Passive Interfaces

TenGigabitEthernet6/0/1, TenGigabitEthernet8/0/0

Local Networks

10.2.0.0/24, 10.2.1.0/24

Local Network Summary

10.2.0.0/16
Router 2 Information

Item

Value

Backbone Address

172.16.0.25/24

Local Router ID

10.25.0.1

Local OSPF Area

0.0.0.25

Active Interfaces (Cost)

GigabitEthernet3/0/0 (5)

Passive Interfaces

GigabitEthernet0/13/0, GigabitEthernet0/14/0

Local Networks

10.25.0.0/24, 10.25.1.0/24

Local Network Summary

10.25.0.0/16

TNSR Configuration Steps

Configure Interfaces on R1

r1 tnsr# conf
r1 tnsr(config)# interface TenGigabitEthernet6/0/0
r1 tnsr(config-interface)# description "To Backbone"
r1 tnsr(config-interface)# ip address 172.16.0.2/24
r1 tnsr(config-interface)# mtu 1500
r1 tnsr(config-interface)# enable
r1 tnsr(config-interface)# exit
r1 tnsr(config)#
r1 tnsr(config)# interface TenGigabitEthernet6/0/1
r1 tnsr(config-interface)# description "Local Network 1"
r1 tnsr(config-interface)# ip address 10.2.0.1/24
r1 tnsr(config-interface)# enable
r1 tnsr(config-interface)# exit
r1 tnsr(config)#
r1 tnsr(config)# interface TenGigabitEthernet8/0/0
r1 tnsr(config-interface)# description "Local Network 2"
r1 tnsr(config-interface)# ip address 10.2.1.1/24
r1 tnsr(config-interface)# enable
r1 tnsr(config-interface)# exit
r1 tnsr(config)#

Configure Interfaces on R2

r2 tnsr# conf
r2 tnsr(config)# interface GigabitEthernet3/0/0
r2 tnsr(config-interface)# description "To Backbone"
r2 tnsr(config-interface)# ip address 172.16.0.25/24
r1 tnsr(config-interface)# mtu 1500
r2 tnsr(config-interface)# enable
r2 tnsr(config-interface)# exit
r2 tnsr(config)#
r2 tnsr(config)# interface GigabitEthernet0/13/0
r2 tnsr(config-interface)# description "Local Network 1"
r2 tnsr(config-interface)# ip address 10.25.0.1/24
r2 tnsr(config-interface)# enable
r2 tnsr(config-interface)# exit
r2 tnsr(config)#
r2 tnsr(config)# interface GigabitEthernet0/14/0
r2 tnsr(config-interface)# description "Local Network 2"
r2 tnsr(config-interface)# ip address 10.25.1.1/24
r2 tnsr(config-interface)# enable
r2 tnsr(config-interface)# exit
r2 tnsr(config)#

Configure OSPF on R1

r1 tnsr(config)# route dynamic ospf
r1 tnsr(config-frr-ospf)# server
r1 tnsr(config-ospf)# ospf router-id 10.2.0.1
r1 tnsr(config-ospf)# passive-interface TenGigabitEthernet6/0/1
r1 tnsr(config-ospf)# passive-interface TenGigabitEthernet8/0/0
r1 tnsr(config-ospf)# interface TenGigabitEthernet6/0/1
r1 tnsr(config-ospf-if)# ip address * area 0.0.0.2
r1 tnsr(config-ospf)# interface TenGigabitEthernet8/0/0
r1 tnsr(config-ospf-if)# ip address * area 0.0.0.2
r1 tnsr(config-ospf)# interface TenGigabitEthernet6/0/0
r1 tnsr(config-ospf-if)# ip address * cost 5
r1 tnsr(config-ospf-if)# ip address * area 0.0.0.0
r1 tnsr(config-ospf-if)# exit
r1 tnsr(config-ospf)# area 0.0.0.2
r1 tnsr(config-ospf-area)# stub
r1 tnsr(config-ospf-area)# range 10.2.0.0/16
r1 tnsr(config-ospf-area)# exit
r1 tnsr(config-ospf)# exit
r1 tnsr(config-frr-ospf)# enable
r1 tnsr(config-frr-ospf)# exit
r1 tnsr(config)#

Configure OSPF on R2

r2 tnsr(config)# route dynamic ospf
r2 tnsr(config-frr-ospf)# server
r2 tnsr(config-ospf)# ospf router-id 10.25.0.1
r2 tnsr(config-ospf)# passive-interface GigabitEthernet0/13/0
r2 tnsr(config-ospf)# passive-interface GigabitEthernet0/14/0
r2 tnsr(config-ospf)# interface GigabitEthernet0/13/0
r2 tnsr(config-ospf-if)# ip address * area 0.0.0.25
r2 tnsr(config-ospf-if)# exit
r2 tnsr(config-ospf)# interface GigabitEthernet0/14/0
r2 tnsr(config-ospf-if)# ip address * area 0.0.0.25
r2 tnsr(config-ospf-if)# exit
r2 tnsr(config-ospf)# interface GigabitEthernet3/0/0
r2 tnsr(config-ospf-if)# ip address * cost 5
r2 tnsr(config-ospf-if)# ip address * area 0.0.0.0
r2 tnsr(config-ospf-if)# exit
r2 tnsr(config-ospf)# area 0.0.0.25
r2 tnsr(config-ospf-area)# stub
r2 tnsr(config-ospf-area)# range 10.25.0.0/16
r2 tnsr(config-ospf-area)# exit
r2 tnsr(config-ospf)# exit
r2 tnsr(config-frr-ospf)# enable
r2 tnsr(config-frr-ospf)# exit

Notes

This scenario can easily be adjusted to connect with other local routers handling additional networks inside the local ranges. To do so, remove the stub configuration for the local area and passive-interface directives for interfaces which will communicate with local routers. Then configure the other routers as needed.