OSPFv2 LSA Type 3 (Summary LSA)

OSPF LSA type 3 summary LSAs allow routers in one area to learn about networks in other areas. Without them, OSPF routers would only know about routes within their own area.  Within an area, routers generate type 1 LSAs (router LSA) to advertise their prefixes. This LSA, however, stays within the area. An Area Border Router (ABR) does not forward type 1 LSAs to other areas. Instead, an ABR generates a type 3 LSA, which contains information from type 1 LSAs, and floods it into the other area. When you look at a router and see O IA entries, those are your inter-area routes, and they come from type 3 summary LSAs.

In this lesson, you will learn how type 3 Summary LSAs are generated by ABRs, how they propagate across areas, and how to verify them in the OSPF link-state database and routing table using Cisco IOS.

Key Takeaways

• Type 3 summary LSAs carry inter-area prefix information, and they allow routers in one OSPF area to learn about networks in another area.
• Only ABRs generate type 3 LSAs.
• Type 3 LSAs are regenerated, not forwarded. Each ABR creates a brand-new type 3 LSA instead of forwarding one they received from another ABR.
• OSPF does not summarize anything by default. Each prefix gets its own type 3 LSA unless manual summarization is configured.
• Inter-area routes appear as O IA in the routing table, which tells you the route was learned via a Type 3 Summary LSA.
• The metric is cumulative. Each ABR advertises the total cost to reach the prefix from its own perspective, and downstream routers add their local link cost on top.

Prerequisites

To understand this lesson, you should be familiar with the following topics:

• OSPF fundamentals: How OSPF builds neighbor adjacencies, exchanges LSAs, and runs SPF.
• OSPF areas: What the backbone area (area 0) is, what non-backbone areas are, and why inter-area routing requires an ABR.
• LSA type 1 (Router LSA): The type 1 LSA is what the ABR reads to generate the type 3 LSA, and it stays within its own area.

Configuration

To see this in action, we’ll use this topology:

We have four routers in three areas. We’ll track a type 1 LSA that is created on R1 all the way to R4. All routers run Cisco IOS Software [Dublin], Linux Software (X86_64BI_LINUX-ADVENTERPRISEK9-M), Version 17.12.1, RELEASE SOFTWARE (fc5).

hostname R1
!
ip cef
!
interface Loopback0
 ip address 11.11.11.11 255.255.255.255
!
interface Ethernet0/1
 ip address 192.168.12.1 255.255.255.0
 ip ospf network point-to-point
!
router ospf 1
 router-id 1.1.1.1
 network 11.11.11.11 0.0.0.0 area 1
 network 192.168.12.0 0.0.0.255 area 1
!
end

hostname R2
!
ip cef
!
interface Ethernet0/1
 ip address 192.168.12.2 255.255.255.0
 ip ospf network point-to-point
!
interface Ethernet0/2
 ip address 192.168.23.2 255.255.255.0
 ip ospf network point-to-point
!
router ospf 1
 router-id 2.2.2.2
 network 192.168.12.0 0.0.0.255 area 1
 network 192.168.23.0 0.0.0.255 area 0
!
end

hostname R3
!
ip cef
!
interface Ethernet0/1
 ip address 192.168.23.3 255.255.255.0
 ip ospf network point-to-point
!
interface Ethernet0/2
 ip address 192.168.34.3 255.255.255.0
 ip ospf network point-to-point
!
router ospf 1
 router-id 3.3.3.3
 network 192.168.23.0 0.0.0.255 area 0
 network 192.168.34.0 0.0.0.255 area 2
!
end

hostname R4
!
ip cef
!
interface Ethernet0/1
 ip address 192.168.34.4 255.255.255.0
 ip ospf network point-to-point
!
router ospf 1
 router-id 4.4.4.4
 network 192.168.34.0 0.0.0.255 area 2
!
end

Topology: OSPF Area 0-1-2

Verification

We’ll start with R1, which has a loopback interface with network 11.11.11.11/32, and we’ll follow it all the way to R4. This is what it looks like:

R1#show ip ospf database router self-originate

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

                Router Link States (Area 1)

  LS age: 1337
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 1.1.1.1
  Advertising Router: 1.1.1.1
  LS Seq Number: 800000F7
  Checksum: 0xDE02
  Length: 60
  Number of Links: 3

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 11.11.11.11
     (Link Data) Network Mask: 255.255.255.255
      Number of MTID metrics: 0
       TOS 0 Metrics: 1

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 2.2.2.2
     (Link Data) Router Interface address: 192.168.12.1
      Number of MTID metrics: 0
       TOS 0 Metrics: 10

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 192.168.12.0
     (Link Data) Network Mask: 255.255.255.0
      Number of MTID metrics: 0
       TOS 0 Metrics: 10

This is the type 1 router LSA, which shows up as a stub network with a /32 mask and a metric of 1. This type 1 LSA stays inside area 1. R2 receives it, but it does not forward it into area 0.

We can verify that R2 learns this:

R2#show ip ospf database router adv-router 1.1.1.1

            OSPF Router with ID (2.2.2.2) (Process ID 1)

                Router Link States (Area 1)

  LS age: 1353
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 1.1.1.1
  Advertising Router: 1.1.1.1
  LS Seq Number: 800000F8
  Checksum: 0xDC03
  Length: 60
  Number of Links: 3

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 11.11.11.11
     (Link Data) Network Mask: 255.255.255.255
      Number of MTID metrics: 0
       TOS 0 Metrics: 1

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 2.2.2.2
     (Link Data) Router Interface address: 192.168.12.1
      Number of MTID metrics: 0
       TOS 0 Metrics: 10

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 192.168.12.0
     (Link Data) Network Mask: 255.255.255.0
      Number of MTID metrics: 0
       TOS 0 Metrics: 10

This is the exact same type 1 LSA from R1, but as seen from R2. It shows up in the routing table like this: