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OSPF LSA

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1. LSA Types

OSPF uses different LSA types (Link State Advertisements) to share routing information between routers and build its LSDB (Link State Database). Each LSA type has a specific purpose and flooding scope where it is flooded only within or between areas.

Find next more details about the LSA Types:

LSA Type 1 – “Router” – generated by every router

This LSA is flooded only within the area. Router LSA describes the links and neighbors of the local router.

root@R3> show ospf database area 0 lsa-id 17.0.0.3 router extensive 

    OSPF database, Area 0.0.0.0
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Router  *17.0.0.3         17.0.0.3         0x80000322  1024  0x22 0x7f16 108
  bits 0x3, link count 7
  id 17.0.0.6, data 17.8.4.1, Type PointToPoint (1)
    Topology count: 0, Default metric: 10
  id 17.8.4.0, data 255.255.255.0, Type Stub (3)
    Topology count: 0, Default metric: 10
  id 17.0.0.4, data 17.8.11.1, Type PointToPoint (1)
    Topology count: 0, Default metric: 10
  id 17.8.11.0, data 255.255.255.0, Type Stub (3)
    Topology count: 0, Default metric: 10
  id 128.49.247.24, data 255.255.255.255, Type Stub (3)
    Topology count: 0, Default metric: 0
  id 17.0.0.3, data 255.255.255.255, Type Stub (3)
    Topology count: 0, Default metric: 0
  id 17.0.0.1, data 17.18.1.2, Type Virtual (4)
    Topology count: 0, Default metric: 10
  Topology default (ID 0)
    Type: Virtual, Node ID: 17.0.0.1
      Metric: 10, Bidirectional
    Type: PointToPoint, Node ID: 17.0.0.4
      Metric: 10, Bidirectional
    Type: PointToPoint, Node ID: 17.0.0.6
      Metric: 10, Bidirectional
  Gen timer 00:32:55
  Aging timer 00:42:55
  Installed 00:17:04 ago, expires in 00:42:56, sent 00:17:02 ago
  Last changed 1w1d 03:06:41 ago, Change count: 27, Ours
LSA Type 2 – “Network” – generated only by the DR on the segment

Type 2 LSA is used only for broadcast media (Ethernet by default) and is originated by the Designated Router (DR) of the segment. It is used to represent the segment with all attached routers. The flooding scope is within the area only.

root@R6> show ospf database network extensive    

    OSPF database, Area 0.0.0.2
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Network  17.28.14.2       17.0.0.7         0x80000001   384  0x20 0xe9cd  32
  mask 255.255.255.0
  attached router 17.0.0.7
  attached router 17.0.0.6
  Topology default (ID 0)
    Type: Transit, Node ID: 17.0.0.6
      Metric: 0, Bidirectional
    Type: Transit, Node ID: 17.0.0.7
      Metric: 0, Bidirectional
  Aging timer 00:53:35
  Installed 00:06:23 ago, expires in 00:53:36, sent 00:06:23 ago
  Last changed 00:06:23 ago, Change count: 1
LSA Type 3 – “Summary” – generated by the ABR

Summary LSA is used from the ABR to exchange information from LSA type 1 and LSA type 2 to other attached areas. Its scope is whole OSPF domain if not limited by the area design.

root@R3> show ospf database area 0 netsummary lsa-id 17.0.0.1 extensive 

    OSPF database, Area 0.0.0.0
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Summary  17.0.0.1         17.0.0.1         0x800001cd   346  0x22 0x65e5  28
  mask 255.255.255.255
  Topology default (ID 0) -> Metric: 0
  Aging timer 00:54:14
  Installed 00:05:43 ago, expires in 00:54:14, sent 00:05:41 ago
  Last changed 2w1d 22:54:37 ago, Change count: 1
Summary *17.0.0.1         17.0.0.3         0x8000025f  2026  0x22 0x9715  28
  mask 255.255.255.255
  Topology default (ID 0) -> Metric: 10
  Gen timer 00:16:13
  Aging timer 00:26:13
  Installed 00:33:46 ago, expires in 00:26:14, sent 00:33:44 ago
  Last changed 1w1d 03:20:32 ago, Change count: 1, Ours
LSA Type 4 – “ASBRSum” – generated by ABR to advertise ASBR

Autonomous System Border Routers are routers that inject external routing information to the OSPF domain. ASBR Summary LSA describe the router ID of ASBR routers. This LSA is generated by an ABR attached to the area in which the ASBR is located to describe the reachability to this ASBR router-id. Its scope is the whole OSPF domain.

root@R3> show ospf database area 0 asbrsummary lsa-id 17.0.0.1 extensive 

    OSPF database, Area 0.0.0.0
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
ASBRSum *17.0.0.1         17.0.0.3         0x80000265  1626  0x22 0x7d28  28
  mask 0.0.0.0
  Topology default (ID 0) -> Metric: 10
  Gen timer 00:22:54
  Aging timer 00:32:54
  Installed 00:27:06 ago, expires in 00:32:54, sent 00:27:04 ago
  Last changed 1w1d 03:26:43 ago, Change count: 1, Ours
LSA Type 5 – “Extern” – external prefixes generated by ASBR

This LSA describes IP routes redistributed from other routing protocol including static routes. These LSAs are generated by ASBRs. They are flooded to all areas and OSPF routers except the areas, which by design do not need them. By default, the metric type used by Junos OS is type 2, which includes only the external cost of the route and ignoring internal cost to reach the ASBR router. Type 1 consists of external cost plus the internal cost to the ASBR, which can be configured.

root@R3> show ospf database area 0 external lsa-id 1.1.0.0 extensive 
    OSPF AS SCOPE link state database
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Extern   1.1.0.0          17.0.0.1         0x80000247   587  0x22 0xb5a0  36
  mask 255.255.0.0
  Topology default (ID 0)
    Type: 2, Metric: 0, Fwd addr: 0.0.0.0, Tag: 0.0.0.0
  Aging timer 00:50:13
  Installed 00:09:44 ago, expires in 00:50:13, sent 00:09:42 ago
  Last changed 2w6d 04:22:50 ago, Change count: 1
LSA Type 7 – “NSSA” – external prefixes inside NSSA Area

NSSA External LSA is similar to External LSA Type 5 as it is representing IP prefixes external to OSPF domain. The difference is that these LSAs are send by ASBRs inside a NSSA area. Type 7 LSAs are translated to an LSA Type 5 by the ABR attached to the NSSA area from where the LSA Type 7 is generated.

root@R6> show ospf database nssa lsa-id 2.2.1.0 extensive 

    OSPF database, Area 0.0.0.2
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
NSSA     2.2.1.0          17.0.0.7         0x80000261   645  0x28 0x5d36  36
  mask 255.255.255.0
  Topology default (ID 0)
    Type: 1, Metric: 0, Fwd addr: 17.0.0.7, Tag: 0.0.0.0
  Aging timer 00:49:14
  Installed 00:10:42 ago, expires in 00:49:15, sent 00:10:40 ago
  Last changed 2w5d 21:59:39 ago, Change count: 1
LSA Type 10 – “Opaque” – traffic engineering purposes

This LSA has an area wide scope and is used for exchanging additional information about the links used for traffic-engineering purposes in MPLS networks. The information from that LSA is used to populate the Traffic Engineering Database (TED), used for the Constrained Shortest Path First (CSPF) algorithm.

root@R6# run show ospf database opaque-area area 2 extensive                   

    OSPF database, Area 0.0.0.2
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
OpaqArea*1.0.0.1          17.0.0.6         0x80000001   756  0x20 0x40c2  28
Opaque LSA
  RtrAddr (1), length 4:
    17.0.0.6
  Gen timer 00:36:21
  Aging timer 00:47:23
  Installed 00:12:36 ago, expires in 00:47:24, sent 00:12:36 ago
  Last changed 00:12:36 ago, Change count: 1, Ours
OpaqArea 1.0.0.1          17.0.0.7         0x80000001   571  0x20 0x44bc  28
Opaque LSA
  RtrAddr (1), length 4:
    17.0.0.7
  Aging timer 00:50:28
  Installed 00:09:30 ago, expires in 00:50:29, sent 00:09:30 ago
  Last changed 00:09:30 ago, Change count: 1
OpaqArea*1.0.0.3          17.0.0.6         0x80000001    70  0x20 0xb0f3 136
Opaque LSA
  Link (2), length 112:
    Linktype (1), length 1:
      1
    LinkID (2), length 4:
      17.0.0.7
    LocIfAdr (3), length 4:
      17.28.14.1
    RemIfAdr (4), length 4:
      17.28.14.2
    TEMetric (5), length 4:
      10
    MaxBW (6), length 4:
      1000Mbps
    MaxRsvBW (7), length 4:
      1000Mbps
    UnRsvBW (8), length 32:
        Priority 0, 1000Mbps
        Priority 1, 1000Mbps
        Priority 2, 1000Mbps
        Priority 3, 1000Mbps
        Priority 4, 1000Mbps
        Priority 5, 1000Mbps
        Priority 6, 1000Mbps
        Priority 7, 1000Mbps
    LinkLocalRemoteIdentifier (11), length 8:
      Local 360, Remote 0
    Color (9), length 4:
      2
  Gen timer 00:48:11
  Aging timer 00:58:49
  Installed 00:01:10 ago, expires in 00:58:50, sent 00:01:10 ago
  Last changed 00:01:10 ago, Change count: 1, Ours, TE Link ID: 2147483649
OpaqArea 1.0.0.3          17.0.0.7         0x80000001    38  0x20 0x2895 136
Opaque LSA                              
  Link (2), length 112:
    Linktype (1), length 1:
      1
    LinkID (2), length 4:
      17.0.0.6
    LocIfAdr (3), length 4:
      17.28.14.2
    RemIfAdr (4), length 4:
      17.28.14.1
    TEMetric (5), length 4:
      10
    MaxBW (6), length 4:
      1000Mbps
    MaxRsvBW (7), length 4:
      1000Mbps
    UnRsvBW (8), length 32:
        Priority 0, 1000Mbps
        Priority 1, 1000Mbps
        Priority 2, 1000Mbps
        Priority 3, 1000Mbps
        Priority 4, 1000Mbps
        Priority 5, 1000Mbps
        Priority 6, 1000Mbps
        Priority 7, 1000Mbps
    LinkLocalRemoteIdentifier (11), length 8:
      Local 335, Remote 0
    Color (9), length 4:
      2
  Aging timer 00:59:21
  Installed 00:00:37 ago, expires in 00:59:22, sent 00:00:37 ago
  Last changed 00:00:37 ago, Change count: 1

2. Wireshark

First, let’s see the encapsulation of an OSPF packet: 

Frame 155: Packet, 210 bytes on wire (1680 bits), 210 bytes captured (1680 bits)
Ethernet II, Src: 56:04:0d:00:75:8b (56:04:0d:00:75:8b), Dst: IPv4mcast_05 (01:00:5e:00:00:05)
Internet Protocol Version 4, Src: 17.8.1.2, Dst: 224.0.0.5
    0100 .... = Version: 4
    .... 0101 = Header Length: 20 bytes (5)
    Differentiated Services Field: 0xc0 (DSCP: CS6, ECN: Not-ECT)
    Total Length: 196
    Identification: 0xcd0e (52494)
    000. .... = Flags: 0x0
    ...0 0000 0000 0000 = Fragment Offset: 0
    Time to Live: 1
    Protocol: OSPF IGP (89)
    Header Checksum: 0xf903 [validation disabled]
    [Header checksum status: Unverified]
    Source Address: 17.8.1.2
    Destination Address: 224.0.0.5
    [Stream index: 2]
Open Shortest Path First
    OSPF Header
        Version: 2
        Message Type: LS Update (4)
        Packet Length: 176
        Source OSPF Router: 17.0.0.3
        Area ID: 0.0.0.0 (Backbone)
        Checksum: 0x2e6b [correct]
        Instance ID: Base IPv4 Unicast Instance (0)
        Auth Type: Null (0)
        Auth Data (none): 0000000000000000
    LS Update Packet
        Number of LSAs: 5
        LSA-type 5 (AS-External-LSA (ASBR)), len 36
        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 3 (Summary-LSA (IP network)), len 28

Next, let’s see how different LSA Types look like in Wireshark:

LSA Type 1 - Router 
        LSA-type 1 (Router-LSA), len 84
            .000 0000 0000 0011 = LS Age (seconds): 3
            0... .... .... .... = Do Not Age Flag: 0
            Options: 0x20, (DC) Demand Circuits
                0... .... = DN: Not set
                .0.. .... = (O) Opaque: Not set
                ..1. .... = (DC) Demand Circuits: Supported
                ...0 .... = (L) LLS Data block: Not Present
                .... 0... = (N) NSSA: Not supported
                .... .0.. = (MC) Multicast: Not capable
                .... ..0. = (E) External Routing: Not capable
                .... ...0 = (MT) Multi-Topology Routing: No
            LS Type: Router-LSA (1)
            Link State ID: 17.0.0.7
            Advertising Router: 17.0.0.7
            Sequence Number: 0x80000005
            Checksum: 0xd0f4
            Length: 84
            Flags: 0x02, (E) AS boundary router
                0... .... = (H) Host: No
                ..0. .... = (S) Shortcut-capable ABR: No
                ...0 .... = (N) NSSA translation: No
                .... 0... = (W) Wild-card multicast receiver: No
                .... .0.. = (V) Virtual link endpoint: No
                .... ..1. = (E) AS boundary router: Yes
                .... ...0 = (B) Area border router: No
            Number of Links: 5
            Type: Stub     ID: 17.8.5.0        Data: 255.255.255.0   Metric: 1000
                Link ID: 17.8.5.0 - 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: 1000
            Type: Stub     ID: 17.8.14.0       Data: 255.255.255.0   Metric: 1000
                Link ID: 17.8.14.0 - 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: 1000
            Type: Stub     ID: 17.11.22.7      Data: 255.255.255.255 Metric: 0
                Link ID: 17.11.22.7 - IP network/subnet number
                Link Data: 255.255.255.255
                Link Type: 3 - Connection to a stub network
                Number of Metrics: 0 - TOS
                0 Metric: 0
            Type: Stub     ID: 17.0.0.7        Data: 255.255.255.255 Metric: 0
                Link ID: 17.0.0.7 - IP network/subnet number
                Link Data: 255.255.255.255
                Link Type: 3 - Connection to a stub network
                Number of Metrics: 0 - TOS
                0 Metric: 0
            Type: Stub     ID: 128.49.228.243  Data: 255.255.255.255 Metric: 0
                Link ID: 128.49.228.243 - IP network/subnet number
                Link Data: 255.255.255.255
                Link Type: 3 - Connection to a stub network
                Number of Metrics: 0 - TOS
                0 Metric: 0
LSA Type 2 - Network 
        LSA-type 2 (Network-LSA), len 28
            .000 0000 0000 0001 = LS Age (seconds): 1
            0... .... .... .... = Do Not Age Flag: 0
            Options: 0x20, (DC) Demand Circuits
                0... .... = DN: Not set
                .0.. .... = (O) Opaque: Not set
                ..1. .... = (DC) Demand Circuits: Supported
                ...0 .... = (L) LLS Data block: Not Present
                .... 0... = (N) NSSA: Not supported
                .... .0.. = (MC) Multicast: Not capable
                .... ..0. = (E) External Routing: Not capable
                .... ...0 = (MT) Multi-Topology Routing: No
            LS Type: Network-LSA (2)
            Link State ID: 17.8.14.1
            Advertising Router: 17.0.0.6
            Sequence Number: 0x80000003
            Checksum: 0xc423
            Length: 28
            Netmask: 255.255.255.0
            Attached Router: 17.0.0.6
LSA Type 3 - Summary 
        LSA-type 3 (Summary-LSA (IP network)), len 28
            .000 0000 0000 0001 = LS Age (seconds): 1
            0... .... .... .... = Do Not Age Flag: 0
            Options: 0x20, (DC) Demand Circuits
                0... .... = DN: Not set
                .0.. .... = (O) Opaque: Not set
                ..1. .... = (DC) Demand Circuits: Supported
                ...0 .... = (L) LLS Data block: Not Present
                .... 0... = (N) NSSA: Not supported
                .... .0.. = (MC) Multicast: Not capable
                .... ..0. = (E) External Routing: Not capable
                .... ...0 = (MT) Multi-Topology Routing: No
            LS Type: Summary-LSA (IP network) (3)
            Link State ID: 17.0.0.1
            Advertising Router: 17.0.0.6
            Sequence Number: 0x80000003
            Checksum: 0x98b7
            Length: 28
            Netmask: 255.255.255.255
            TOS: 0
            Metric: 3000
LSA Type 4 - ASBR 
        LSA-type 4 (Summary-LSA (ASBR)), len 28
            .000 0000 0000 0011 = LS Age (seconds): 3
            0... .... .... .... = Do Not Age Flag: 0
            Options: 0x22, (DC) Demand Circuits, (E) External Routing
                0... .... = DN: Not set
                .0.. .... = (O) Opaque: Not set
                ..1. .... = (DC) Demand Circuits: Supported
                ...0 .... = (L) LLS Data block: Not Present
                .... 0... = (N) NSSA: Not supported
                .... .0.. = (MC) Multicast: Not capable
                .... ..1. = (E) External Routing: Capable
                .... ...0 = (MT) Multi-Topology Routing: No
            LS Type: Summary-LSA (ASBR) (4)
            Link State ID: 17.0.0.7
            Advertising Router: 17.0.0.5
            Sequence Number: 0x80000001
            Checksum: 0xd34e
            Length: 28
            Netmask: 0.0.0.0
            TOS: 0
            Metric: 1000
LSA Type 5 - Extern 
        LSA-type 5 (AS-External-LSA (ASBR)), len 36
            .000 0000 0000 0100 = LS Age (seconds): 4
            0... .... .... .... = Do Not Age Flag: 0
            Options: 0x22, (DC) Demand Circuits, (E) External Routing
                0... .... = DN: Not set
                .0.. .... = (O) Opaque: Not set
                ..1. .... = (DC) Demand Circuits: Supported
                ...0 .... = (L) LLS Data block: Not Present
                .... 0... = (N) NSSA: Not supported
                .... .0.. = (MC) Multicast: Not capable
                .... ..1. = (E) External Routing: Capable
                .... ...0 = (MT) Multi-Topology Routing: No
            LS Type: AS-External-LSA (ASBR) (5)
            Link State ID: 2.2.2.0
            Advertising Router: 17.0.0.6
            Sequence Number: 0x80000002
            Checksum: 0xbaa0
            Length: 36
            Netmask: 255.255.255.0
            1... .... = External Type: Type 2 (metric is larger than any other link state path)
            .000 0000 = TOS: 0
            Metric: 0
            Forwarding Address: 17.11.22.7
            External Route Tag: 0
LSA Type 7 - NSSA 
        LSA-type 7 (NSSA AS-External-LSA), len 36
            .000 0000 0000 0001 = LS Age (seconds): 1
            0... .... .... .... = Do Not Age Flag: 0
            Options: 0x28, (DC) Demand Circuits, (P) Propagate
                0... .... = DN: Not set
                .0.. .... = (O) Opaque: Not set
                ..1. .... = (DC) Demand Circuits: Supported
                ...0 .... = (L) LLS Data block: Not Present
                .... 1... = (P) Propagate: Set
                .... .0.. = (MC) Multicast: Not capable
                .... ..0. = (E) External Routing: Not capable
                .... ...0 = (MT) Multi-Topology Routing: No
            LS Type: NSSA AS-External-LSA (7)
            Link State ID: 2.2.2.0
            Advertising Router: 17.0.0.7
            Sequence Number: 0x80000001
            Checksum: 0x4013
            Length: 36
            Netmask: 255.255.255.0
            1... .... = External Type: Type 2 (metric is larger than any other link state path)
            .000 0000 = TOS: 0
            Metric: 0
            Forwarding Address: 17.11.22.7
            External Route Tag: 0
LSA Type 10 - Opaque 
    LSA-type 10 (Opaque LSA, Area-local scope), len 28
        .000 0000 0000 1000 = LS Age (seconds): 8
        0... .... .... .... = Do Not Age Flag: 0
        Options: 0x22, (DC) Demand Circuits, (E) External Routing
            0... .... = DN: Not set
            .0.. .... = (O) Opaque: Not set
            ..1. .... = (DC) Demand Circuits: Supported
            ...0 .... = (L) LLS Data block: Not Present
            .... 0... = (N) NSSA: Not supported
            .... .0.. = (MC) Multicast: Not capable
            .... ..1. = (E) External Routing: Capable
            .... ...0 = (MT) Multi-Topology Routing: No
        LS Type: Opaque LSA, Area-local scope (10)
        Link State ID Opaque Type: Traffic Engineering LSA (1)
        Link State ID TE-LSA Reserved: 0
        Link State ID TE-LSA Instance: 1
        Advertising Router: 17.0.0.3
        Sequence Number: 0x80000001
        Checksum: 0x16f0
        Length: 28

3. Summary

LSA Types summary:

Type Name LSA ID Data
1 Router RID V,E,B
bits		 Link Type
p2p
Transig
Stub
Virtual Link		 Link ID
Neighbor RID
DR int IP
Network
Neighbor RID		 Link Data
Local int IP
Local int IP
Mask
Local int IP		 Metric
2 Network DR IP Mask Attached Router
(repeatead for all routers)
3 Summary Network Mask Metric
4 ASBR Summary ASBR RID Mask (0.0.0.0) Metric
5 AS External Network Metric E bit
(Metric type)		 Metric Forwarding
Address		 Tag
7 NSSA External Network Metric E bit Metric Forwarding
Address		 Tag
10 Opaque Locally
significant ID		 Traffic engineering

4. OSPF3 (IPv6)

OSPF3 was developed for IPv6, but supports both IPv6 and IPv4.

The OSPF encapsulation and LSA types are similar (but not identical). Unsurprisingly, we see that OSPF3 exchanges LSA using Link Local IPv6 addressing, so family inet6 must be activated on all OSPF3 interfaces (even when using only realm ipv4).

Frame 361: Packet, 354 bytes on wire (2832 bits), 354 bytes captured (2832 bits)
Ethernet II, Src: 56:04:0d:00:01:c7 (56:04:0d:00:01:c7), Dst: IPv6mcast_05 (33:33:00:00:00:05)
Internet Protocol Version 6, Src: fe80::5604:dff:fe00:1c7, Dst: ff02::5
    0110 .... = Version: 6
    .... 1100 0000 .... .... .... .... .... = Traffic Class: 0xc0 (DSCP: CS6, ECN: Not-ECT)
    .... 0000 0000 0000 0000 0000 = Flow Label: 0x00000
    Payload Length: 300
    Next Header: OSPF IGP (89)
    Hop Limit: 1
    Source Address: fe80::5604:dff:fe00:1c7
    Destination Address: ff02::5
    [Source SLAAC MAC: 54:04:0d:00:01:c7 (54:04:0d:00:01:c7)]
    [Stream index: 1]
Open Shortest Path First
    OSPF Header
        Version: 3
        Message Type: LS Update (4)
        Packet Length: 300
        Source OSPF Router: 17.0.0.7
        Area ID: 0.0.0.0 (Backbone)
        Checksum: 0x0ac3 [correct]
        Instance ID: Base IPv4 Unicast AF (64)
        Reserved: 00
    LS Update Packet
        Number of LSAs: 6
        LSA-type 9 (Intra-Area-Prefix-LSA), len 64
        LSA-type 8 (Link-LSA), len 52
        LSA-type 2 (Network-LSA), len 32
        LSA-type 9 (Intra-Area-Prefix-LSA), len 40
        LSA-type 8 (Link-LSA), len 52
        LSA-type 1 (Router-LSA), len 40

OSPF3 will be covered in a dedicated post.

External Resources:

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