OSPF Type-3 Network-Summary-LSA

Network Setup for Type-3 Network-Summary-LSA

Type-3 Network-Summary-LSAs are originated from an ABR to advertise the subnets in an area (omitting the information of Type-1 and Type-2 LSAs) to neighboring routers outside the area. Type-1 and Type-2 LSAs stay within an area, when an ABR receives a Type-1 or Type-2 LSA, it generates a Type-3 LSA for the network learnt via the Type-1 or Type-2 LSA to other areas.

The Type-3 Network-Summary-LSA and Type-4 ASBR-Summary LSA have the same format, with the differences in the contents of the LS Type and Link-State ID fields in the LSA header. For Type-3 LSAs, the Link-State ID is the IP address of the subnet that is being advertised; while for Type-4 LSAs, the Link-State ID is the Router ID of the ASBR that is being advertised.

OSPF Summary-LSA (Type-3 Network-Summary-LSA and Type-4 ASBR-Summary-LSA) Format

Below lists the fields in the OSPF Summary-LSA:

Field	Description
Network Mask	For Type-3 LSAs, it indicates the subnet mask of the network that is being advertised. For Type-4 LSAs, this field has no meaning and is set to 0.0.0.0. Note: If a Type-3 LSA is advertising a default route, both the Link-State ID and Network Mask fields will be 0.0.0.0.
Metric	Indicates the cost of the route to the destination. Note: Cisco supports only TOS 0 therefore there are no TOS and TOS Metric fields in the Cisco Summary-LSAs.

Type-3 Network-Summary-LSAs are not being summarized and therefore do not contain summary routes by default. All subnets in an area will be advertised by default.

OSPF does not perform autosummarization as like RIPv2 and EIGRP. Route summarization in OSPF requires manual configuration. Route summarization and filtering at the ABRs should always be considered due to the nature that a Type-3 LSA will be advertised into the backbone area for every network in the originating area, which can cause significant flooding problems. Route summarization and filtering on ABRs provide greater scalability for OSPF.

Note: Type-3 and Type-4 Summary-LSAs do not carry topological information; they carry only IP prefixes or subnets. Summary-LSAs are originated by an ABR with the following rules:
 From a non-backbone area to the backbone area, Summary-LSAs are generated for connected routes and intra-area routes. Only intra-area routes are advertised into the backbone area to avoid routing loops. If there are any inter-area routes coming from a non-backbone area it means that the backbone is discontiguous, and OSPF does not allow discontiguous backbone.
 From the backbone area to a non-backbone area, Summary-LSAs are generated for connected routes, intra-area routes, and inter-area routes.

The Advertising Route field in the LSA header of a Summary-LSA contains the Router ID of the ABR that generates the Summary-LSA.

Below shows the routing table and Type-3 Network-Summary-LSAs received on RT4:

RT4#sh ip route

Gateway of last resort is not set

     20.0.0.0/30 is subnetted, 1 subnets
O IA    20.20.20.0 [110/128] via 30.30.30.1, 00:00:21, Serial0/0
     10.0.0.0/30 is subnetted, 1 subnets
O IA    10.10.10.0 [110/192] via 30.30.30.1, 00:00:18, Serial0/0
O IA 192.168.1.0/24 [110/193] via 30.30.30.1, 00:00:18, Serial0/0
     30.0.0.0/30 is subnetted, 1 subnets
C       30.30.30.0 is directly connected, Serial0/0
RT4#
RT4#sh ip ospf database

            OSPF Router with ID (44.44.44.44) (Process ID 100)

                Router Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum Link count
33.33.33.33     33.33.33.33     34          0x80000001 0x005CDA 2
44.44.44.44     44.44.44.44     31          0x80000002 0x00A663 2

                Summary Net Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum
10.10.10.0      33.33.33.33     19          0x80000001 0x0031EB
20.20.20.0      33.33.33.33     29          0x80000001 0x0045F9
192.168.1.0     33.33.33.33     19          0x80000001 0x00F9D2
RT4#
RT4#sh ip ospf database summary

            OSPF Router with ID (44.44.44.44) (Process ID 100)

                Summary Net Link States (Area 2)

  Routing Bit Set on this LSA
  LS age: 19
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 10.10.10.0 (summary Network Number)
  Advertising Router: 33.33.33.33
  LS Seq Number: 80000001
  Checksum: 0x31EB
  Length: 28
  Network Mask: /30
        TOS: 0  Metric: 128

  Routing Bit Set on this LSA
  LS age: 29
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 20.20.20.0 (summary Network Number)
  Advertising Router: 33.33.33.33
  LS Seq Number: 80000001
  Checksum: 0x45F9
  Length: 28
  Network Mask: /30
        TOS: 0  Metric: 64

  Routing Bit Set on this LSA
  LS age: 19
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 192.168.1.0 (summary Network Number)
  Advertising Router: 33.33.33.33
  LS Seq Number: 80000001
  Checksum: 0xF9D2
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 129

RT4#

OSPF routers must perform SPF calculations upon the intra-area topology changes (Type-1 and Type-2 LSAs), as these changes impact the selection of best paths towards destination within an area. Changes upon Type-3 LSAs do not trigger an SPF calculation, as the changes do not affect the topology between an OSPF router and the ABR.