In large scale OSPF networks, configuring route summarization can effectively reduce the size of routing table and free up system resources while maintaining system performance. In addition, if a link within the aggregated IP address range frequently goes up and down, the change will not be notified to the upstream devices. OSPF is a CPU intensive protocol especially in large deployments. Constant route flapping can impact the performance of a router significantly. Route summarization will shield all the upstream routers from constantly processing and updating routing information caused by route flapping.

Configuring ABR Route Summarization

The following commands can be used to configure OSPF route summarization.

set protocols ospf [instance-id <instance-id> | vrf <vrf-name>] area {<ipv4>|<area-id>} range <prefix/mask> [advertise <true | false> | cost <cost> | substitute <prefix/mask>]

set protocols ospf6 [vrf <vrf-name>] area {<ipv4>|<area-id>} range <prefix/mask> [advertise <true | false> | cost <cost>]

Route summarization is configured on ABRs. Since ABRs share routing LSAs with other regions, if there are large number of destinations in an area, flooding all this information individually using separate LSA for each destination can easily overwhelm routers processing these LSAs. Instead, we can specify a range of routing subnets to be summarized or grouped into a single segment and shared with other regions. This can save both router processing cycles and also minimize the size of the routing table on routers receiving the summary LSAs.

The following commands configure route summarization in OSPF area 1.

admin@Xorplus# set protocols ospf area 1 range 10.42.0.0/16
admin@Xorplus# commit

Configuring ASBR Route Summarization

When the imported routes have the same routing prefix, you can aggregate these introduced routes and publish them as one aggregated route by using the ASBR route summarization. By configuring route summarization, you can reduce the routing information and the size of the routing table to improve the performance of the device.

The following commands can be used to configure ASBR route summarization.

set protocols ospf [vrf <vrf-name>] summary-address <prefix/mask> [no-advertise | tag <tag-value>]

set protocols ospf [vrf <vrf-name>] aggregation timer <aggregation-timer>

The following commands configure OSPF route summarization for ASBR.

admin@Xorplus# set protocols ospf summary-address 10.2.0.0/16 tag 2
admin@Xorplus# set protocols ospf aggregation timer 100
admin@Xorplus# commit

You can use the command run show ospf summary-address to show the configuration for display all configured summary routes with matching external LSA information.

admin@Xorplus# run show ospf summary-address 
Summary-address     Metric-type     Metric     Tag       External_Rt_count
aggregation delay interval :100(in seconds)
10.2.0.0/16           E2              20        2        0

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