An overview of the
OSPF establishes an LSDB by sharing link status information between two routers. The LSDB is used to generate a routing table
The topology
Neighbors to establish
A neighbor relationship is established between two routers to share link status
| TIME(second) | SRC(Source IP address) | DEST (multicast) | Active Neighbor | DR BDR | State of the SRC DEST | Info |
|---|---|---|---|---|---|---|
| 0 | 10.0.12.1 | 224.0.0.5 | 0.0.0.0 | 0.0.0.0 0.0.0.0 | Down Init | Hello Packet |
| 0 | 10.0.12.2 | 224.0.0.5 | 0.0.0.0 | 0.0.0.0 0.0.0.0 | Init Init | Hello Packet |
| After a round of packet exchange, Neighbor is identified | ||||||
| 10 | 10.0.12.2 | 224.0.0.5 | 10.0.12.1 | 0.0.0.0 0.0.0.0 | Init 2Way | Hello Packet |
| 10 | 10.0.12.1 | 224.0.0.5 | 10.0.12.2 | 0.0.0.0 0.0.0.0 | 2Way 2Way | Hello Packet |
| 20 | 10.0.12.1 | 224.0.0.5 | 10.0.12.2 | 0.0.0.0 0.0.0.0 | 2Way 2Way | Hello Packet |
| 20 | 10.0.12.2 | 224.0.0.5 | 10.0.12.1 | 0.0.0.0 0.0.0.0 | 2Way 2Way | Hello Packet |
| 30 | 10.0.12.1 | 224.0.0.5 | 10.0.12.2 | 0.0.0.0 0.0.0.0 | 2Way 2Way | Hello Packet |
| 30 | 10.0.12.2 | 224.0.0.5 | 10.0.12.1 | 0.0.0.0 0.0.0.0 | 2Way 2Way | Hello Packet |
| 40 | 10.0.12.1 | 224.0.0.5 | 10.0.12.2 | 0.0.0.0 0.0.0.0 | 2Way 2Way | Hello Packet |
| 40 | 10.0.12.2 | 224.0.0.5 | 10.0.12.1 | 0.0.0.0 0.0.0.0 | 2Way 2Way | Hello Packet |
| Within 40 seconds, an interface on the router calculates the DR and BDR packets based on all the Hello packets in the received network segment | ||||||
| 40 | 10.0.12.1 | 224.0.0.5 | 10.0.12.2 | 10.0.12.2 0.0.0.0 | 2Way 2Way | Hello Packet |
| 40 | 10.0.12.2 | 224.0.0.5 | 10.0.12.1 | 10.0.12.2 10.0.12.1 | 2Way 2Way | Hello Packet |
| The DR and DBR have been calculated. The LSA packet exchange starts | ||||||
| Although it is calculated by DBR, each router may not be the same, but it does not affect the following LSA packet exchange (with DR interface is ok). | ||||||
| —- The process of exchanging LSA packets is omitted. For details, see the next title | ||||||
| After the baptism of Hello packets, DBRS tend to be the same | ||||||
| 50 | 10.0.12.1 | 224.0.0.5 | 10.0.12.2 | 10.0.12.2 10.0.12.1 | Full Full | Hello Packet |
| 50 | 10.0.12.2 | 224.0.0.5 | 10.0.12.1 | 10.0.12.2 10.0.12.1 | Full Full | Hello Packet |
The adjacency exchanges state information
Routers that establish neighbor relationships exchange information through packets to achieve link state synchronization
| TIME(second) | SRC(Source IP address) | DEST (multicast) | Init More Master | State of the SRC DEST | Sequence | Info |
|---|---|---|---|---|---|---|
| 41 | 10.0.12.1 | 10.0.12.2 | yes yes yes | ExStart ExStart | x | DB Description |
| 42 | 10.0.12.2 | 10.0.12.1 | yes yes yes | Exchange ExStart | x | DB Description |
| After the above operations, determine the primary and secondary relationship. InIt indicates the first packet, More indicates whether there are More packets, and Master indicates whether it is the primary packet | ||||||
| 43 | 10.0.12.1 | 10.0.12.2 | no no no | Exchange Exchange | x | DB Description LSA |
| 43 | 10.0.12.2 | 10.0.12.1 | no no yes | Exchange Exchange | x | DB Description LSA |
| After the preceding operations, brief Lsas are exchanged. | ||||||
| After receiving the brief LSA from the peer, you can send an LS Request immediately after comparing the LSA | ||||||
| Specific packets are shown in the following tables | ||||||
| 50 | 10.0.12.1 | 10.0.12.2 | no no no | Full Full | x+1 | DB Description ACK |
| The Slave sends an ACK to the Master. Assume that LSDB synchronization is complete, so both routers are in Full state |
| Time | SRC | DEST | Content | Info |
|---|---|---|---|---|
| 44 | 10.0.12.2 | 10.0.12.1 | 【 Advert Router=10.0.1.1】 【 Advert Router=10.0.1.1】 | LS Request |
| 44 | 10.0.12.1 | 10.0.12.2 | 【 Advert Router=10.0.1.2】 【 Advert Router=10.0.1.2】 | LS Request |
| 44 | 10.0.12.2 | 224.0.0.5 | [router-LSA (1)] = {Links = [{Type: Sub, ID: 10.0.12.0, Data: 255.255.255.0, Metric: 1}]} | LS Update |
| 44 | 10.0.12.1 | 224.0.0.5 | [router-LSA (1)] = {Links = [{Type: Sub, ID: 10.0.12.0, Data: 255.255.255.0, Metric: 1}]} | LS Update |
| 44s Both ends send a Request and actively update the CURRENT LSA. The router status changes from Exchange to Loading | ||||
| The current LSA describes its link status. Each router has only one link or interface, whose type is Sub | ||||
| Sub network: ID indicates the network number, Data indicates the subnet mask, and Metric indicates the Metric value | ||||
| 45 | 10.0.12.1 | 10.0.12.2 | [Router-LSA(1)] = {Links = [{Type: Transit, ID: 10.0.12.2, Data: 10.0.12.1, Metric: 1}]} | LS Update |
| 45 | 10.0.12.2 | 10.0.12.1 | [Router-LSA(1)] = {Links = [{Type: Transit, ID: 10.0.12.2, Data: 10.0.12.2, Metric: 1}]} | LS Update |
| The 45S Update mainly changes the Sub network in Links to Transit type | ||||
| Because of the 44s multicast Update, each router updates its Sub network to the Transit network | ||||
| Transit network: ID indicates the ADDRESS of the DR interface, Data indicates the address of the interface that sends packets, and Metric indicates the Metric value | ||||
| 46 | 10.0.12.2 | 224.0.0.5 | [Network-LSA(2)] = {Link State ID: 10.0.12.2, Advert Router: 10.0.1.2, NetMask: 255.255.255.0, Attached Router: [10.0.1.2, 10.0.1.1]} | LS Update |
| The 46S is sent by the router on the DR interface. The router calculates a Network based on the Transit interface and proactively broadcasts the 46S. Only the router on the DR interface has the right to send network-LSA packets | ||||
| The following are some LS Ack packets. The router status changes from Loading to Full |
Generate the SPT shortest path tree
After multiple cycles of switching, a router always holds link state information about the entire network. The LSDB of R01 is as follows:
| Type | LinkState ID | AdvRouter | Age | Len | Seq | Metric | Self |
|---|---|---|---|---|---|---|---|
| Router | 10.0.1.1 | 10.0.1.1 | 1514 | 36 | 001 | 1 | yes |
| Router | 10.0.1.2 | 10.0.1.2 | 1152 | 36 | 002 | 1 | no |
| Network | 10.0.12.2 | 10.0.1.2 | 1152 | 32 | 003 | 0 | no |
The LSDB of router R02 is as follows:
| Type | LinkState ID | AdvRouter | Age | Len | Seq | Metric | Self |
|---|---|---|---|---|---|---|---|
| Router | 10.0.1.2 | 10.0.1.2 | 1514 | 36 | 001 | 1 | yes |
| Network | 10.0.12.2 | 10.0.1.2 | 1152 | 32 | 003 | 0 | yes |
| Router | 10.0.1.2 | 10.0.1.2 | 1152 | 36 | 002 | 1 | no |
Generate routing
The LSDB contents of each router can be built into a tree. A Router is a node and a Network is an edge
LSDB content can also be understood as a matrix. Router can be understood as each element, and Network can be understood as a binary unordered group, which can form an unordered graph, that is, a matrix
The LSDB generates a shortest spanning tree with itself as the root, and the path to each node is a route
In this example, no OSPF route is generated because all links are directly connected. The default route has a higher priority than OSPF