Hello everyone,
Today, I'm going to introduce some basic concepts and working principles of OSPF.
Basic Concepts of OSPF
Router ID
A router ID is a 32-bit unsigned integer, which identifies a router in an autonomous system (AS). A router ID must exist before the router runs OSPF.
A router ID can be manually configured or automatically obtained.
If no router ID has been configured, the router automatically obtains a router ID using the following methods in descending order of priority.
The router preferentially selects the largest IP address from its loopback interface addresses as the router ID
If no loopback interface has been configured, the router selects the largest IP address from its interface IP addresses as the router ID.
A router can obtain a router ID again only after a router ID is reconfigured for the router or an OSPF router ID is reconfigured and the OSPF process restarts.
Area
Area (area) refers to the logical division of routers into different groups, and each group is identified by an area number (Area ID). Each area has its own LSDB, and different areas have different LSDBs. The router maintains a separate LSDB for each area to which it is connected. Since detailed link status information will not be published outside the area, the size of the LSDB has been greatly reduced. The backbone area is responsible for publishing the routing information summarized by the area border routers between non-backbone areas to realize inter-area intercommunication.
Summarize the reasons for the area division:
Reduce the scale of LSDB and reduce the consumption of router memory and CPU.
The scope of routing information notification is reduced, which improves the efficiency of information transmission and accelerates convergence.
Improve the stability of the network. When the network topology changes, reduce the scope of influence and avoid routing shock.
Figure 1 OSPF areas
Router Type
Routers are classified as internal routers, ABRs, backbone routers, or ASBRs by location in an AS
Figure 2 Router type layout
Figure 3 Router type descriptions
In NBMA networks and broadcast networks, there are also the following router role classifications:
Figure 4 The router type in NBMA networks and broadcast networks
In NBMA and broadcast networks, the following two concepts need to be distinguished:
Figure 5 Neighbor relationship and Adjacency relationship
LSA
OSPF encapsulates routing information into LSAs for transmission.
Figure 6 LSAs and their functions
Figure 7 Support status of LSAs in different types of areas
Packet Type
OSPF packets are classified as Hello, Database Description (DD), Link State Request (LSR), Link State Update (LSU), or Link State Acknowledgment (LSAck) packets.
Figure 8 OSPF packets and their functions
Route Type
Route types are classified as intra-area, inter-area, Type 1 external, or Type 2 external routes. Intra-area and inter-area routes describe the network structure of an AS. Type 1 or Type 2 AS external routes describe how to select routes to destinations outside an AS.
Figure 9 OSPF routes
OSPF Network Type
OSPF supports four network types: point-to-point network, broadcast network, NBMA network, and point-to-multipoint network.
Figure 10 OSPF Network Type
Basic Principles of OSPF
OSPF route calculation involves the following processes:
1. Adjacency establishment
The adjacency establishment process is as follows:
a. The local and remote routers use OSPF interfaces to exchange Hello packets to establish a Neighbor relationship.
b. The local and remote routers negotiate a master/slave relationship and exchange Database Description (DD) packets.
c. The local and remote routers exchange link state advertisements (LSAs) to synchronize their link-state databases (LSDBs).
2. Route calculation
OSPF uses the shortest path first (SPF) algorithm to calculate routes to implement fast route convergence.
OSPF Neighbor States
To exchange routing information on an OSPF network, neighbor routers must establish adjacencies. The differences between neighbor relationships and adjacencies are described as follows:
Neighbor relationship: After the local router starts, it uses an OSPF interface to send a Hello packet to the remote router. After the remote router receives the packet, it checks whether the parameters carried in the packet are consistent with its own parameters. If the parameters carried in the packet are consistent with its own parameters, the remote router establishes a neighbor relationship with the local router.
Adjacency: After the local and remote routers establish a neighbor relationship, they exchange DD packets and LSAs to establish an adjacency.
OSPF has eight neighbor states: Down, Attempt, Init, 2-way, Exstart, Exchange, Loading, and Full. Down, 2-way, and Full are stable states. Attempt, Init, Exstart, Exchange, and Loading are unstable states, which last only several minutes.
Figure 11 OSPF neighbor states
Figure 12 OSPF neighbor states and their meanings
Adjacency Establishment
Adjacencies can be established in either of the following situations:
Two routers have established a neighbor relationship and communicate for the first time.
The designated router (DR) or backup designated router (BDR) on a network segment changes.
The adjacency establishment process is different on different networks.
Adjacency establishment on a broadcast network
On a broadcast network, the DR and BDR establish adjacencies with each router on the same network segment, but DR others establish only neighbor relationships.
Figure 13 Adjacency establishment process on a broadcast network
The adjacency establishment process on a broadcast network is as follows:
1. Neighbor relationship establishment
a. Router A uses the multicast address 224.0.0.5 to send a Hello packet to Router B through the OSPF interface connected to a broadcast network. The packet carries the DR field of 1.1.1.1 (ID of Router A) and the Neighbors Seen field of 0. A neighbor router has not been discovered, and Router A regards itself as a DR.
b. After Router B receives the packet, it returns a Hello packet to Router A. The returned packet carries the DR field of 2.2.2.2 (ID of Router B) and the Neighbors Seen field of 1.1.1.1 (Router A's router ID). Router A has been discovered but its router ID is less than that of Router B, and therefore Router B regards itself as a DR. Then Router B's state changes to Init.
c. After Router A receives the packet, Router A's state changes to 2-way.
2. Master/Slave negotiation and DD packet exchange
a. Router A sends a DD packet to Router B. The packet carries the following fields:
Seq field: The value x indicates the sequence number is x.
I field: The value 1 indicates that the packet is the first DD packet, which is used to negotiate a master/slave relationship and does not carry LSA summaries.
M field: The value 1 indicates that the packet is not the last DD packet.
MS field: The value 1 indicates that Router A declares itself a master.
To improve transmission efficiency, Router A and Router B determine which LSAs in each other's LSDB need to be updated. If one party determines that an LSA of the other party is already in its own LSDB, it does not send an LSR packet for updating the LSA to the other party. To achieve the preceding purpose, Router A and Router B first send DD packets, which carry summaries of LSAs in their own LSDBs. Each summary identifies an LSA. To ensure packet transmission reliability, a master/slave relationship must be determined during DD packet exchange. One party serving as a master uses the Seq field to define a sequence number. The master increases the sequence number by one each time it sends a DD packet. When the other party serving as a slave sends a DD packet, it adds the sequence number carried in the last DD packet received from the master to the Seq field of the packet.
b. After Router B receives the DD packet, Router B's state changes to Exstart, and Router B returns a DD packet to Router A. The returned packet does not carry LSA summaries. Because Router B's router ID is greater than Router A's router ID, Router B declares itself a master and sets the Seq field to y.
c. After Router A receives the DD packet, it agrees that Router B is a master and Router A's state changes to Exchange. Then Router A sends a DD packet to Router B to transmit LSA summaries. The packet carries the Seq field of y and the MS field of 0. The value 0 indicates that Router A declares itself a slave.
d. After Router B receives the packet, Router B's state changes to Exchange, and Router B sends a new DD packet containing its own LSA summaries to Router A. The value of the Seq field carried in the new DD packet is changed to y + 1.
Router A uses the same sequence number as Router B to confirm that it has received DD packets from Router B. Router B uses the sequence number plus one to confirm that it has received DD packets from Router A. When Router B sends the last DD packet, it sets the M field of the packet to 0.
3. LSDB synchronization
a. After Router A receives the last DD packet, it finds that many LSAs in Router B's LSDB do not exist in its own LSDB, so Router A's state changes to Loading. After Router B receives the last DD packet from Router A, Router B's state directly changes to Full, because Router B's LSDB already contains all LSAs of Router A.
b. Router A sends an LSR packet for updating LSAs to Router B. Router B returns an LSU packet to Router A. After Router A receives the packet, it sends an LSAck packet for acknowledgment.
The preceding procedures continue until the LSAs in Router A's LSDB are the same as those in Router B's LSDB. Router A's state changes to Full. After Router A and Router B exchange DD packets and update all LSAs, they establish an adjacency.
Adjacency establishment on an NBMA network
The adjacency establishment process on an NBMA network is similar to that on a broadcast network. The blue part shown in Figure 14 highlights the differences from a broadcast network.
On an NBMA network, all routers establish adjacencies only with the DR and BDR.
Figure 14 Adjacency establishment process on an NBMA network
The adjacency establishment process on an NBMA network is as follows:
1. Neighbor relationship establishment
a. After Router B sends a Hello packet to a down interface of Router A, Router B's state changes to Attempt. The packet carries the DR field of 2.2.2.2 (ID of Router B) and the Neighbors Seen field of 0. A neighbor router has not been discovered, and Router B regards itself as a DR.
b. After Router A receives the packet, Router A's state changes to Init and Router A returns a Hello packet. The returned packet carries the DR and Neighbors Seen fields of 2.2.2.2. Router B has been discovered but its router ID is greater than that of Router A, and therefore Router A agrees that Router B is a DR.
2. Master/Slave relationship negotiation and DD packet exchange
The procedures for negotiating a master/slave relationship and exchanging DD packets on an NBMA network are the same as those on a broadcast network.
3. LSDB synchronization
The procedure for synchronizing LSDBs on an NBMA network is the same as that on a broadcast network.
Adjacency establishment on a point-to-point (P2P)/Point-to-multipoint (P2MP) network
The adjacency establishment process on a P2P/P2MP network is similar to that on a broadcast network. On a P2P/P2MP network, however, no DR or BDR needs to be elected and DD packets are transmitted in multicast mode.
Route Calculation
OSPF uses the shortest path first (SPF) algorithm to calculate routes, implementing fast route convergence.
OSPF uses an LSA to describe the network topology. A Type 1 LSA describes the attributes of a link between routers. A router transforms its LSDB into a weighted, directed graph, which reflects the topology of the entire AS. All routers in the same area have the same graph.
Figure 15 Weighted, directed graph
Based on the graph, each router uses the SPF algorithm to calculate an SPT with itself as the root. The SPT shows routes to nodes in the AS.
Figure 16 SPT
When a router's LSDB changes, the router recalculates the shortest path. Frequent SPF calculations consume a large number of resources and affect router efficiency. Changing the interval between SPF calculations can prevent resource consumption caused by frequent LSDB changes. The default interval between SPF calculations is 5 seconds.
The route calculation process is as follows:
1. A router calculates intra-area routes.
The router uses an SPF algorithm to calculate the shortest paths to other routers in an area. Type 1 and Type 2 LSAs accurately describe the network topology in an area. Based on the network topology described by a Type 1 LSA, the router calculates paths to other routers in the area.
2. The router calculates inter-area routes.
For the routers in an area, the network segment of the routes in an adjacent area is directly connected to the area border router (ABR). Because the shortest path to the ABR has been calculated in the preceding step, the routers can directly check a Type 3 LSA to obtain the shortest path to the network segment. The autonomous system boundary router (ASBR) can also be considered connected to the ABR. Therefore, the shortest path to the ASBR can also be calculated in this phase.
3. The router calculates AS external routes.
AS external routes can be considered to be directly connected to the ASBR. Because the shortest path to the ASBR has been calculated in the preceding phase, the router can check Type 5 LSAs to obtain the shortest paths to other ASs.
That is all I want to share with you! Thank you!
