OSPF - Type 1 LSA

OSPF uses various types of Link State Advertisements (LSAs) to share network information and to populate their OSPF link state database (LSDB). Type 1 LSAs are those that are flooded to all OSPF routers within an OSPF area.

Within a Type1 LSA, you will find a list with all the directly connected links of this router. Each link is identified with two elements:

  • A Link ID which is the the network prefix in question
  • A Link Type which describes the nature of the specific link

The following is a Wireshark packet capture of a Type1 LSA:

Frame 15: 434 bytes on wire (3472 bits), 434 bytes captured (3472 bits)
Ethernet II, Src: c2:04:55:cc:04:01 (c2:04:55:cc:04:01), Dst: c2:00:46:ad:00:00 (c2:00:46:ad:00:00)
Internet Protocol Version 4, Src: 192.168.12.1, Dst: 192.168.12.2
Open Shortest Path First
    OSPF Header
        Version: 2
        Message Type: LS Update (4)
        Packet Length: 400
        Source OSPF Router: 1.1.1.1
        Area ID: 0.0.0.20
        Checksum: 0xd794 [correct]
        Auth Type: Null (0)
        Auth Data (none): 0000000000000000
    LS Update Packet
        Number of LSAs: 11
        LSA-type 1 (Router-LSA), len 48
            .000 0001 1011 1110 = LS Age (seconds): 446
            0... .... .... .... = Do Not Age Flag: 0
            Options: 0x22, (DC) Demand Circuits, (E) External Routing
            LS Type: Router-LSA (1)
            Link State ID: 5.5.5.5
            Advertising Router: 5.5.5.5
            Sequence Number: 0x80000004
            Checksum: 0x7caa
            Length: 48
            Flags: 0x00
            Number of Links: 2
            Type: Stub     ID: 192.168.120.0    Data: 255.255.255.0   Metric: 10
                Link ID: 192.168.120.0 - IP network/subnet number
                Link Data: 255.255.255.0
                Link Type: 3 - Connection to a stub network
                Number of Metrics: 0 - TOS
                0 Metric: 10
            Type: Transit  ID: 192.168.12.2      Data: 192.168.12.2       Metric: 10
                Link ID: 192.168.12.2 - IP address of Designated Router
                Link Data: 192.168.12.2
                Link Type: 2 - Connection to a transit network
                Number of Metrics: 0 - TOS
                0 Metric: 10
        LSA-type 1 (Router-LSA), len 36
        LSA-type 2 (Network-LSA), len 32
        LSA-type 3 (Summary-LSA (IP network)), len 28
        LSA-type 3 (Summary-LSA (IP network)), len 28
        LSA-type 3 (Summary-LSA (IP network)), len 28
        LSA-type 4 (Summary-LSA (ASBR)), len 28
        LSA-type 5 (AS-External-LSA (ASBR)), len 36
        LSA-type 5 (AS-External-LSA (ASBR)), len 36
        LSA-type 5 (AS-External-LSA (ASBR)), len 36
        LSA-type 5 (AS-External-LSA (ASBR)), len 36

This OSPF update has two Type1 LSAs. The first has a link ID of 192.168.20.0 and has a stub link type, while the second has a link ID of 10.0.20.2 and a link type of transit.

There are four link types:

Notice that the destination address of the IP packet containing this LSA is unicast. This is the case because this particular OSPF LS update packet contains multiple LSA types, some of which need to be sent in unicast. However, when an LS Update contains only Type 1 LSAs, the LSA is indeed flooded to the whole area. Notice the following packet:

Frame 15: 110 bytes on wire (880 bits), 110 bytes captured (880 bits)
Ethernet II, Src: c2:00:56:ab:00:00 (c2:00:56:ab:00:00), Dst: IPv4mcast_05 (01:00:5e:00:00:05)
Internet Protocol Version 4, Src: 10.0.20.2, Dst: 224.0.0.5
Open Shortest Path First
    OSPF Header
        Version: 2
        Message Type: LS Update (4)
        Packet Length: 76
        Source OSPF Router: 5.5.5.5
        Area ID: 0.0.0.20
        Checksum: 0x3b48 [correct]
        Auth Type: Null (0)
        Auth Data (none): 0000000000000000
    LS Update Packet
        Number of LSAs: 1
        LSA-type 1 (Router-LSA), len 48
            .000 0000 0000 0001 = LS Age (seconds): 1
            0... .... .... .... = Do Not Age Flag: 0
            Options: 0x22, (DC) Demand Circuits, (E) External Routing
            LS Type: Router-LSA (1)
            Link State ID: 5.5.5.5
            Advertising Router: 5.5.5.5
            Sequence Number: 0x80000005
            Checksum: 0x0a40
            Length: 48
            Flags: 0x00
            Number of Links: 2
            Type: Stub     ID: 192.168.20.0    Data: 255.255.255.0   Metric: 10
                Link ID: 192.168.20.0 - IP network/subnet number
                Link Data: 255.255.255.0
                Link Type: 3 - Connection to a stub network
                Number of Metrics: 0 - TOS
                0 Metric: 10
            Type: Stub     ID: 10.0.20.0       Data: 255.255.255.252 Metric: 10
                Link ID: 10.0.20.0 - IP network/subnet number
                Link Data: 255.255.255.252
                Link Type: 3 - Connection to a stub network
                Number of Metrics: 0 - TOS
                0 Metric: 10

This LS Update contains only Type 1 LSAs, and thus the destination address is the Multicast address for all OSPF routers.

https://networklessons.com/ospf/ospf-lsa-types-explained

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