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[ScreenOS] Unable to summarize Type 3 LSA with ‘Area range’ command in OSPF

  [KB21655] Show Article Properties


Summary:
This article describes the issue of the inability to summarize type 3 LSA with ‘Area range’ command in OSPF.
Symptoms:
Type 3 Summary LSA's are originated by area border routers and flooded throughout the AS’s via the associated area. Each summary-LSA describes a route to a destination outside the area, yet is still inside the AS (that is an inter-area route).

There are instances where a few Type 3 LSAs are received in a consecutive network and customers want to further summarize them. The following diagram explains the scenario:






Here the device is configured to advertise the summary route for 44.44.0.0/16; however we still see 44.44.44.0/24 is also being advertised:
SSG520(trust-vr)-> get conf
set vrouter "trust-vr"
unset auto-route-export
set protocol ospf
set enable
set area 0.0.0.0 range 44.44.0.0 255.255.0.0 advertise <<<< to summarize 44.44.0.0/16
set area 0.0.0.1
exit
exit
set vrouter "trust-vr"
unset add-default-route
set route 0.0.0.0/0 gateway 10.204.115.254
exit
set interface ethernet0/1 protocol ospf area 0.0.0.0
set interface ethernet0/1 protocol ospf enable
set interface ethernet0/1 protocol ospf retransmit-interval 5
set interface ethernet0/1 protocol ospf cost 1
set interface ethernet0/2 protocol ospf area 0.0.0.1
set interface ethernet0/2 protocol ospf enable
set interface ethernet0/2 protocol ospf retransmit-interval 5
set interface ethernet0/2 protocol ospf cost 1
SSG520(trust-vr)-> get proto ospf data
VR: trust-vr RouterId: 20.20.20.2
----------------------------------

Router LSA(s) for area 0.0.0.0
Link-State-Id Adv-Router-Id Age Sequence# CheckSum
--------------------------------------------------------------------------------
11.11.11.1 11.11.11.1 1355 0x80000006 0x9465
20.20.20.2 20.20.20.2 1334 0x80000004 0x4918

Network LSA(s) for area 0.0.0.0
Link-State-Id Adv-Router-Id Age Sequence# CheckSum
--------------------------------------------------------------------------------
13.13.13.1 11.11.11.1 1355 0x80000001 0xa0f1

Summary LSA(s) for area 0.0.0.0
Link-State-Id Adv-Router-Id Age Sequence# CheckSum
--------------------------------------------------------------------------------
11.11.11.0 11.11.11.1 1590 0x80000002 0xe218
12.12.12.0 20.20.20.2 1336 0x80000001 0xe1fa
44.44.44.0 11.11.11.1 958 0x80000001 0x583c   << Received 44.44.44.0/24 summary LSA

ASBR LSA(s) for area 0.0.0.0
Link-State-Id Adv-Router-Id Age Sequence# CheckSum
--------------------------------------------------------------------------------
11.11.11.2 11.11.11.1 1588 0x80000002 0xd222


Router LSA(s) for area 0.0.0.1
Link-State-Id Adv-Router-Id Age Sequence# CheckSum
--------------------------------------------------------------------------------
20.20.20.2 20.20.20.2 1334 0x80000002 0x d5c
213.237.186.142 213.237.186.142 1340 0x80000003 0xe9e0

Network LSA(s) for area 0.0.0.1
Link-State-Id Adv-Router-Id Age Sequence# CheckSum
--------------------------------------------------------------------------------
12.12.12.2 213.237.186.142 1340 0x80000001 0xba04

Summary LSA(s) for area 0.0.0.1
Link-State-Id Adv-Router-Id Age Sequence# CheckSum
--------------------------------------------------------------------------------
11.11.11.0 20.20.20.2 1336 0x80000001 0x10ce
13.13.13.0 20.20.20.2 1336 0x80000001 0xbd1c
 44.44.0.0 20.20.20.2 673 0x80000001 0x5f45 <Network summarized with Area range command
 44.44.44.0 20.20.20.2 957 0x80000001 0x83f3 << Sumary LSA received in Area 0 is still 
                                                present and was not summarized with 'Area Range’ command
.


ASBR LSA(s) for area 0.0.0.1
Link-State-Id Adv-Router-Id Age Sequence# CheckSum
--------------------------------------------------------------------------------
11.11.11.1 20.20.20.2 1336 0x80000001 0xffda
11.11.11.2 20.20.20.2 1336 0x80000001 0xffd8


AS External LSA(s)
Link-State-Id Adv-Router-Id Age Sequence# CheckSum
--------------------------------------------------------------------------------
2.2.2.1 11.11.11.1 1573 0x80000041 0xa523
2.2.2.2 11.11.11.1 1573 0x80000041 0x9b2c
2.2.2.3 11.11.11.1 1573 0x80000041 0x9135
2.2.2.4 11.11.11.1 1573 0x80000041 0x873e
Solution:
The Area range command can summarize only Type 1 and Type 2 LSAs on an ABR. This is a RFC standard.
Related Links: