OSPF summary LSA (Type 3 LSA)

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Summary LSA can be of two flavors: type 3 and type 4. In this tutorial we will focus on type 3 LSA. Type 3 LSAs are generated by ABR. They represent networks from an area and are sent to the rest of the areas in OSPF domain. Type 1 LSAs don’t cross area boundary, so Area Border Router (ABR) uses type 3 LSA to inform other areas about networks learned in its area. Type 3 LSA uses network address as Link ID and router id of advertising router as ADV Router in OSPF database. 

By default a type 3 LSA contains information about only one network learned in the area, so, ABR (Area Border Router) will generate one type 3 LSA per network learned via type 1 LSA. This can cause serious problems in case there are a lot of networks to advertise, because a lot of LSA will be generated by ABRs. Area border routers can generate Type 3 LSAs that advertise summarized networks instead of single networks. Summarization is highly recommended in large OSPF domains, it can significantly  improve the performance of the network. If you need a guide on how to perform summarization, have a look at OSPF internal route summarization tutorial. For now let’s consider following topology:

Type-3 LSA

Here we have two ABRs. ABR1 will generate type 3 LSAs for network 192.168.0.0/24 in area 10 and send it in area 0. Also it will generate type 3 LSA for networks 192.168.1.0/24 and 192.168.2.0/24 in area 0 and will send them in area 10. ABR2 will perform a similar operation, i.e. it will generate type 3 link state advertisement in area 20 for network 192.168.3.0/24 and send it in area 0, and for networks 192.168.1.0/24 and 192.168.2.0/24 in area 0 that will send in area 20.

Routers in the scenario are configured according to data provided in the picture, with router IDs labeled on routers. Let’s have a look at the OSPF database on BB router. For this purpose use “show ip ospf database

BB#sh ip ospf database 

... output omitted ...

                Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.0.0     2.2.2.2         1025        0x8000000A 0x00EDCA
192.168.3.0     4.4.4.4         966         0x8000000A 0x00901D
BB#

In the output you see that we have two summary LSAs, network 192.168.0.0 is advertised by ABR1 with router id 2.2.2.2 and network 192.168.3.0 is advertised by ABR2 which has router id 4.4.4.4. That’s expectable. Now I will issue the same command on R1 that resides in area 10, here you might see something unexpected.

R1#sh ip ospf database 

... output omitted ...

                Summary Net Link States (Area 10)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.1.0     2.2.2.2         1556        0x8000000A 0x00E2D4
192.168.2.0     2.2.2.2         1556        0x80000009 0x003E6F
192.168.3.0     2.2.2.2         1556        0x80000009 0x00970B
R1#

Here you have all summary LSAs generated by ABR1. Everything is ok but why is 192.168.3.0 summary LSA generated by ABR1 and not by ABR2? Well, it was generated by ABR 2 with router id 4.4.4.4 as you have seen previously, but ABR1 being a border router changes advertiser router id to its router id. Use “show ip ospf database summary” to get a detailed view about database.

R1#show ip ospf database summary 

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Summary Net Link States (Area 10)

  Routing Bit Set on this LSA
  LS age: 872
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 192.168.1.0 (summary Network Number)
  Advertising Router: 2.2.2.2
  LS Seq Number: 8000000B
  Checksum: 0xE0D5
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 10 

  Routing Bit Set on this LSA
  LS age: 872
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 192.168.2.0 (summary Network Number)
  Advertising Router: 2.2.2.2
  LS Seq Number: 8000000A
  Checksum: 0x3C70
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 20 

  Routing Bit Set on this LSA
  LS age: 873
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 192.168.3.0 (summary Network Number)
  Advertising Router: 2.2.2.2
  LS Seq Number: 8000000A
  Checksum: 0x950C
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 30 

R1#
One important thing to mention about summary LSAs is that the routers will not run the SPF algorithm once they receive a summary LSA, they will just add or remove learned route in routing table .
R1#show ip route 

... output omitted ...

Gateway of last resort is not set

C    192.168.0.0/24 is directly connected, FastEthernet0/0
O IA 192.168.1.0/24 [110/20] via 192.168.0.1, 05:27:17, FastEthernet0/0
O IA 192.168.2.0/24 [110/30] via 192.168.0.1, 05:27:07, FastEthernet0/0
O IA 192.168.3.0/24 [110/40] via 192.168.0.1, 05:27:07, FastEthernet0/0
R1#

The routes learned from type 3 summary LSAs are marked as “IA” which means that they belong to another area (inter area routes).

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Comments (2)

  • saeed

    |

    Hi Dear friend

    thanks

    Reply

  • aldo

    |

    very well explained! thanks

    Reply

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