OSPFv2 LSA Type 4 (ASBR Summary LSA)

Lesson Contents

OSPF LSA type 4, also known as the ASBR Summary LSA, is generated by Area Border Routers (ABR) and advertises the address of an Autonomous System Border Router (ASBR). This helps OSPF routers in other areas to find the ASBR. An ASBR is an OSPF router that redistributes routing information from an external source, such as a routing protocol, into OSPF.

Within the area where the ASBR is, routers can identify the ASBR from the type 1 router LSA. This is possible because the ASBR sets the E-bit in the type 1 LSA to indicate that it is an ASBR. Type 1 LSAs, however, never leave the area. LSA type 4 solves this and is flooded into other areas. In this lesson, you will learn how type 4 LSAs are generated, how they propagate across areas, and what they look like on Cisco IOS routers.

Key Takeaways

The type 4 LSA is the ASBR Summary LSA. It tells routers in other areas where to find the ASBR.
Type 4 LSAs are generated by ABRs, not by the ASBR itself.
The Link State ID in a type 4 LSA is the ASBR’s router ID and not a network prefix (unlike type 3).
Type 4 LSAs are regenerated at each ABR, not forwarded. Each ABR creates a new one with its own metric.
Routers in the same area as the ASBR do not need a type 4 LSA. They can locate the ASBR directly from the type 1 LSA.
Stub areas do not receive type 4 LSAs. Since external routes are blocked, there is no need to advertise the ASBR’s location.
If the ASBR and ABR are the same router, no type 4 LSA is generated because the router already has direct knowledge of the ASBR.
The metric in a type 4 LSA is cumulative: each ABR advertises its own total cost to reach the ASBR.

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 multi area: How to configure OSPF with multiple areas.
LSA type 1 (Router LSA): The E-bit in the type 1 LSA is what identifies a router as an ASBR, and it’s what triggers the ABR to generate a type 4 LSA.
LSA type 3 (Summary LSA): The type 4 LSA works similarly to the type 3 LSA. ABRs regenerate it per area. Understanding type 3 first makes type 4 much easier to follow.

Configuration

We’ll use the following topology:

We have four routers. I’m using Cisco IOS Software [Dublin], Linux Software (X86_64BI_LINUX-ADVENTERPRISEK9-M), Version 17.12.1, RELEASE SOFTWARE (fc5) on all routers.

Topology: OSPF Area 0-1-2 and external network

Verification

Let’s look at this in action.

Configurations

Want to take a look for yourself? Here you will find the configuration of each device.

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
 redistribute connected route-map CONNECTED_TO_OSPF
 network 192.168.12.0 0.0.0.255 area 1
!
ip access-list standard R1_L0
 10 permit 11.11.11.11
!
route-map CONNECTED_TO_OSPF permit 10 
 match ip address R1_L0
!
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
!
interface Ethernet0/3
 no ip address
!
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

R1 redistributes the network (11.11.11.11/32) on a loopback interface into OSPF. This makes R1 an ASBR (Autonomous System Border Router). R1 will flip a bit in the router LSA to identify itself as an ASBR. When R2, who is an ABR, receives this router LSA, it will create a type 4 summary ASBR LSA and flood it into area 0. This LSA will also be flooded in all other areas and is required so that all OSPF routers know where to find the ASBR.

Redistributing a network on a locally connected interface might sound weird if you’ve never seen it before, but it works, and it saves us the trouble of adding another router with another routing protocol to the topology.

The type 4 LSA is only needed for routers in areas other than the ASBR. Routers in the same area can locate the ASBR directly from the type 1 LSA. Stub areas don’t receive type 5, so they don’t need or receive type 4 either.

R1

Let’s start with R1:

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: 824
  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: 80000002
  Checksum: 0x6BA6
  Length: 48
  AS Boundary Router
  Number of Links: 2

    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

The output above shows AS Boundary Router. The loopback 11.11.11.11/32 doesn’t appear here as a stub network because it’s not an OSPF network. It will show up as LSA type 5, which we’ll look at in another lesson. What matters here is that R1’s Type 1 LSA now identifies it as an ASBR.

R2

R2 receives the type 1 LSA and can see that R1 is an ASBR:

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: 906
  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: 80000002
  Checksum: 0x6BA6
  Length: 48
  AS Boundary Router
  Number of Links: 2

    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 same type 1 LSA from R1 as seen from R2. Routers within area 1 can use this LSA to locate the ASBR directly.

Routers in other areas, however, will never see this LSA because type 1 LSAs remain in the area. R2 knows about R1 from area 1’s LSDB. Because R1 is flagged as an ASBR, R2 generates a Type 4 ASBR Summary LSA and floods it into area 0. This LSA tells routers in other areas how to reach the ASBR (R1). Here’s what LSA type 4 looks like:

R2#show ip ospf database asbr-summary

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

                Summary ASB Link States (Area 0)

  LS age: 1011
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: 1.1.1.1 (AS Boundary Router address)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000001
  Checksum: 0x75B0
  Length: 28
  Network Mask: /0
        MTID: 0         Metric: 10

This looks similar to a type 3 LSA, but there are two important differences:

The LS type is “Summary Links (AS Boundary Router)” instead of “Summary Links (Network)”.
The link state ID is R1’s router ID (1.1.1.1) instead of a network prefix.

The metric of 10 is the cost of R2 to reach R1.

R3

R3 and any other router in area 0 can now use this LSA to find the path to the ASBR. R3 receives this Type 4 LSA from R2:

R3#show ip ospf database asbr-summary

            OSPF Router with ID (3.3.3.3) (Process ID 1)

                Summary ASB Link States (Area 0)

  LS age: 1044
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: 1.1.1.1 (AS Boundary Router address)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000001
  Checksum: 0x75B0
  Length: 28
  Network Mask: /0
        MTID: 0         Metric: 10 

                Summary ASB Link States (Area 2)

  LS age: 1043
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: 1.1.1.1 (AS Boundary Router address)
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0xBB5C
  Length: 28
  Network Mask: /0
        MTID: 0         Metric: 20

R3 has two LSAs, one for each area that R3 is connected to. The first one is the same type 4 LSA that R2 originated. R3 is in Area 0, so it can use its own SPF tree to find R2, and then add R2’s advertised cost of 10 to calculate the total cost to reach R1 (1.1.1.1).

Just like with type 3 LSAs, an ABR does not simply forward R2’s type 4 LSA into area 2. R3 generates a new type 4 LSA and floods it into area 2. Here’s what you see in this second LSA:

R3 is the advertising router (3.3.3.3).
The metric is 20. That’s the total cost for R3 to reach R1.

R4

R4 now receives the type 4 LSA, which was generated by R3:

R4#show ip ospf database asbr-summary

            OSPF Router with ID (4.4.4.4) (Process ID 1)

                Summary ASB Link States (Area 2)

  LS age: 1130
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: 1.1.1.1 (AS Boundary Router address)
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0xBB5C
  Length: 28
  Network Mask: /0
        MTID: 0         Metric: 20

R4 now knows that R1 (1.1.1.1) is an ASBR and that the cost to reach it is 20 (R3’s advertised metric).

The type 4 LSA only tells routers where the ASBR is. It says nothing about the external prefix 11.11.11.11/32 itself. That information is added in a type 5 External LSA, which we’ll cover in the next lesson.

When R4 wants to reach 11.11.11.11/32, it combines two pieces of information:

Type 4 LSA: tells R4 how to reach ASBR 1.1.1.1
Type 5 LSA: tells R4 that 11.11.11.11/32 was originated by ASBR 1.1.1.1

Together, these two LSAs give R4 everything it needs to install the external route in its routing table.

Packet Capture

Let’s look at some packets.

Key Takeaways

Prerequisites

Configuration

Verification

R1

R2

R3

R4

Packet Capture

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