Detailed explanation of six common LSAs in OSPF

Detailed explanation of six common LSAs in OSPF

• Type 1 LSA
• Type 2 LSA
• Type 3 LSA
• Category 5 LSA
• Category 4 LSA
• Category 7 LSA
• Timers in OSPF

In the OSPF protocol, LSAs are used to transmit routing information and topology information. Therefore, understanding the contents and functions of different LSAs is of great help in understanding the route formation of the OSPF protocol. The OSPF here is the v2 version, which is only for IPv4.
Three elements describe an LSA: ADV Router, link-ID, and LSA type.

1. Type 1 LSA

Name: Router LSA
Function: Routing information and topology information generated by this router for a certain area
Propagation scope: Transmitted within this area
Link-ID: Router-id of the generator
ADV router: Router-id of the generator
Characteristics: In a single Only one Type 1 LSA is generated in each area. If the Type 1 LSA is incomplete on the MA network, routing information and topology information need to be generated in conjunction with Type 2 LSA.
Types of Type 1 LSA: There are 4 types
1. stubnet (stub network) ------ routing information
2. transnet (transmission network is limited to MA network) -- topology information
3. point-to-point - topology Information
4.virtual link (virtual link)—topology information

Use this OSPF topology to analyze LSA.

Analyze this type 1 LSA and it is generated by the router with route-id 91.1.1.1. There are 2 links, one of which is 1.1.1.1 with a cost of 0 and a mask of 32, indicating that it is the routing information of its own loopback address. The second link is 10.1.1.1 with a cost of 1, indicating that it is its own. The interface IP, but without the subnet mask, is the topology information, in this case the MA network type, so a Type 2 LSA is needed to supplement the missing routing information of the Type 1 LSA.

2. Type 2 LSA

Name: network LSA, network LSA
function: used in the MA network to describe the number of routers on this network and the network mask of this MA network. Propagation
range: can only be transmitted within this area, terminated at the ABR
Link ID: DR interface IP address
ADV router: Router-ID of the router where the DR is located
Features: It will only appear in the MA network and is used to supplement type 1 LSA (1. MA network mask 2. Number of MA network routers)

Analyze Type 2 LSA:

Analyze this type 2 LSA, which is generated by the router with route-id 91.1.1.1. It contains the network mask is 24 and the connected router information: 91.1.1.1 and 92.2.2.2 are connected to the two routers. Provide the network mask and number of connected routers.

3. Type 3 LSA

Name: summary LSA Summary LSA
function: transfer routing information between different areas.
Propagation scope: the entire OSPF protocol.
link-id: The network number that delivers the route
ADV router: Defaults to the router-id of the ABR in the area.
Features: When traveling through different areas, it is regenerated by other ABRs (ADV router changes)

Note:
ABR: refers to the router between different areas that runs the OSPF protocol. For example, R2 and R3 in the picture are both ABRs.

View this Type 3 LSA on R1. Mask 24 is passed in it. The ADV Router shows that this Type 3 LSA is generated by R2, with a cost value of 1 and passing through 1 device.

Category 4 and 5 LSA

Here we introduce Type 5 LSA first, because Type 4 LSA serves Type 5 LSA. Without Type 5 LSA, there would be no Type 4 LSA. Name: External LSA Function: Used to
transmit
external routes throughout OSPF (originally not part of OSPF domain)
Propagation scope: propagated throughout the router domain running the OSPF protocol.
link-id: The network number that transmits the external route
ADV router: The router-id that generates the LSA (the router-id of the ASBR that generates this LSA)
Features: When passing across areas, the ADV router will not change and always generates this ADVrouter of the LSA's router.

Analyzing this type 5 LSA, we can see that it is generated by the router 91.1.1.1, the network number passed is 100.1.1.0, the network mask is 24, its type is 2, the cost value is 1, and the FA address is 0.0.0.0 .
Analysis:
type type: When external routes are introduced, the default type type is 2, which can be modified to type 1. The main difference between the two is that when type is 2, the cost value of the type 5 LSA sent along the way is not accumulated, regardless of the transmission For any number of routers, the value is 1. When type is changed to 1, the cost value is accumulated.
Cost value: The cost value here is also called the seed metric value . It can be modified when introducing external routes. The default is 1.
FA address: also called forwarding address. Category 5 LSA and Category 7 LSA have FA addresses. Most of the FA addresses of Category 5 LSA are empty (0.0.0.0). Forwarding When the address is not empty, the forwarding address is added to the routing table.

The routing priority of the OSPF protocol is 10. When external routes are imported into OSPF, the priorities of all imported routes are automatically modified to 150.

5. Category 4 LSA

Name: summary ASBR LSA
Function: In addition to the area where the ASBR is located, it is used to advertise the location of the ASBR.
Link id: The router-id of the ASBR.
ADV router: The router-id of the ABR in the area where the default ASBR is located.
Features: When traversing different areas, the new ABR is regenerated. (Consistent with Type 3 LSA)

Category 6 and 7 LSA

Name: NSSA LSA
function: In the NSSA area, transfer external routes.
Propagation scope: Delivered in NSSA area.
Link id: Pass routing network number
ADV router: The router-id attribute that generates the LSA
: the default is type 2, and the metric value (seed metric value) is 1. It carries the FA address (forwarding address) and generates a default type 7 LSA route to the outside world.

Type 7 LSA is generated in the special area NSSA area. It passes the network mask. The default type is 2, the cost value is 1, and the FA address is not empty. It is the loopback address of the router that generated the LSA.

The FA address of Type 7 LSA is not empty by default. The FA address is the address with the largest loopback address advertised to other areas.
1. By default, Type 7 LSA generates FA address, but Type 5 LSA does not (Type 5 LSA converted from 7 to 5 carries FA address)

2. Type 7 LSA generation rules: The FA address generated by default is the largest loopback interface address of the ASBR that generates Type 7 LSA; if the interface connected to other protocols also runs the OSPF protocol and the network type is BMA, the FA in the generated Type 7 LSA The address is the next hop address corresponding to the connection to other interfaces; if the network type is P2P, the FA address is still the largest IP address among the loopback interfaces.

Type 3.5 LSA FA address rule: not generated by default. If the interface connected to other protocols runs the OSPF protocol and the network type is NBMA, the FA address is the next hop address of the route before re-announcement. If the network type is P2P, it will not Generate FA address.

Timers in OSPF:

1.Hello time, the default is 10s or 30s. Dead time, the default is 40s or 120s; modify the hello time, the dead time changes, modify the dead time, the hell time remains unchanged, the hello time or dead time is different, it will affect the establishment of neighbor relationships. It must be matched, that is, the relationship is 4 times.
Waiting time: waiting for the election of DR or BDR, always consistent with the dead time. 40s, this time is the waiting time of other routers when a new router is added during the DR or BDR election, and is used to elect the DR.