OSPF Router LSA (Type 1 LSA)

Hello everyone and welcome to Network Playroom! In this video, we're going to look at the OSPF LSA Type 1 or the router LSA in detail. Router LSAs are originated by every router. This is the most fundamental LSA and it lists all of a router’s links, the state and outgoing cost of each link, and any known OSPF neighbors on the link. All of the router's links to the area must be described in a single router-LSA. These LSAs are flooded only within the area in which they are originated. Each LSA has a flooding scope and router LSA stays inside the area boundaries. There are other LSAs, which can cross area boundaries, but we'll discuss those at another time. A quick recap, each LSA type has a common 20-byte header. The Type field and the Link State ID will depend on the OSPF LSA type. The Type for the Router LSA is 1. The Link State ID for the Router LSAs is the originating router’s Router ID. The V, or Virtual Link Endpoint bit, is set to 1 when the originating router is an endpoint of at least one fully adjacent virtual link. The V-bit is signaled in Type-1 LSA only if the router is the endpoint of one or more fully adjacent virtual links. The area through which you configure the virtual link is known as a transit area. When the V-bit is set this could change path calculation preference between intra-area and inter-area routes. See: https://www.cisco.com/c/en/us/support... The E, or External bit, is set to 1 when the originating router is an ASBR. The ASBR sits at the border of the OSPF domain and another routing domain. The B, or Border bit, is set to 1 when the originating router is an ABR. An ABR is router that has interfaces in two or more areas and one of those areas is the backbone area. Number of Links specifies the number of router links the LSA describes. The Router LSA must describe all of the originating router’s links, or interfaces, to the area in which the LSA is flooded. The following fields in the Router LSA describe each link and appear one or more times, corresponding to the number in the Number of Links field. Understanding link type is important because the descriptions of the Link ID and Link Data fields vary according to the value of the Link Type field. Link Type describes the general type of connection the link provides. 1 - Point-to-point connection to another router 2 - Connection to a transit network 3 - Connection to a stub network 4 - Virtual link Link ID identifies the object to which the link connects. This is dependent on the link type. Note when connecting to an object that also originates an LSA (i.e., another router or a transit network), the Link ID is the same as the Link State ID in the header of the neighboring router’s LSA. This provides the key for looking up the neighboring LSA in the link state database during the routing table calculation. This value is used to find the neighbor’s LSA in the link-state database. 1 - Neighboring router’s Router ID 2 - IP address of the DR’s interface 3 - IP network or subnet address 4 - Neighboring router’s Router ID Link Data also depends on the value of the Link Type field. For connections to stub networks, Link Data specifies the network mask. For point-to-point connections, it specifies the interface's ifIndex value. For the other link types it specifies the router interface's IP address. Number of TOS specifies the number of Type of Service metrics listed for this link. Although TOS is no longer supported in RFC 2328, the TOS fields are still included for backward compatibility with earlier OSPF implementations. If no TOS metrics are associated with a link, this field is set to 0x00. Metric is the cost of the link. This is an important field. The metric you will see on the routing table is calculated from these values. The next two fields are associated with a link corresponding to the number of TOS field. If the number of TOS = 0, there will be no instances of these fields. Note that Cisco supports only TOS = 0. TOS specifies the Type of Service to which the following metric refers. TOS Metric is the metric associated with the specified TOS value. The command show ip ospf database router will list all of the Router LSAs in a database. A single router LSA is observed by specifying the router’s ID. The complete LSA is recorded in the link-state database. One line you will notice the example is the statement “Routing Bit Set on this LSA.” The routing bit is not a part of the LSA itself; it is an internal maintenance bit used by IOS indicating that the route to the destination advertised by this LSA is valid. So when you see “Routing Bit Set on this LSA,” it means that the route to this destination is in the routing table. Thank you for watching! OSPF LSA Types packet capture: https://www.cloudshark.org/captures/0... OSPF point-to-point adjacencies packet capture: https://www.cloudshark.org/captures/0...