Network basics Lecture 14: OSPF routing protocol preliminary

Open Shortest Path First (Open Shortest Path First) is a very active link status Internal Gateway Protocol (IGP). When people find RIP (route information Protocol) it cannot be applied to larger networks, and OSPF is adopted when the convergence speed is too slow. This article will give a general introduction to the OSPF protocol, and the next lecture on basic network knowledge will further study the Protocol itself and the setting methods of the OSPF region.

OSPF is the most widely used Internal Gateway Protocol (IGP ). When discussing IGP, we talk about a routing domain or autonomous system (). Imagine a medium-sized enterprise with multiple office buildings and multiple departments. All devices are connected over the network and share two redundant Internet connections, all the online devices in these office buildings belong to the same. However, if the OSPF protocol is used, we also need to understand the concept of a region, which allows further division of autonomous domains, for example, further subdivision by each department in each building.

To understand the design requirements of OSPF, let's discuss the working principle of OSPF. The following describes some related terms:

* Vroid ID: Each vro in OSPF is assigned a unique 32-bit unsigned integer. This is the vroid ID, which is the highest IP address on the vro, you can increase the value by setting a higher address for the loopback interface of the selected router.

* Adjacent routers: two routers that can be connected through a common link.

* Adjacent Relationship: two-way relationship between two adjacent routers. A neighbor is not necessarily adjacent.

* LSA: The Link Status broadcast packet (LSA) is sent to the adjacent router. It describes the routing information within the specified link.

* Hello protocol: the router on the network uses this Protocol to determine its adjacent router and generate an LSA.

* Region: a hierarchical system. Each region has a group of routers that exchange LSA with other routers. Each region limits the LSA in its own region, and summarizes routes are encouraged.

As we have already said, OSPF is a link-State routing protocol. You can think of it as a distributed map of the network. To obtain the distributed information, OSPF requires three steps.

First, when a router running the OSPF protocol is added to the network, it sends a Hello packet to locate its adjacent router and select a specified router (DR ). The Hello data package contains the link status information and the list of adjacent routers. The function of sending a Hello packet to a neighboring router is to make an ACK (correct command response) and prove that the communication between routers is bidirectional. OSPF quickly judges the 2nd-layer topology. If it is a point-to-point link, it knows enough and considers the link to be "ready". If it is on a broadcast link, the router must wait for the selection and then determine whether the link is feasible.

If you have a Priority ID during DR election, you can select the strongest router as the DR. Otherwise, the router with the highest IP address wins. The key to DR and BDR roles is that they are the routers that generate the LSA, and they must exchange databases with other routers in the subnet. Therefore, non-specified routers are adjacent to the DR. The role of DR/DBR is to ensure the scalability of protocol operations. The only way to ensure that all routers have the same information is to synchronize their databases. For example, if you already have 21 vrouters and want to add one, you must establish 21 new adjacent relationships without DR and BDR. And synchronize with each vro. If you put the database on the DR (in case you need to create a backup), it is easy to add more vrouters, each router only needs to synchronize with the DR.

Database Exchange after Hello Packet Exchange is one of the steps to establish the adjacent relationship, and this step is very important. If the database is not synchronized, it may cause routing loops, black holes, and other dangers. The third step for establishing an adjacent relationship is Reliable Flooding (a Reliable LSA transfer, database update, and network stability), or LSA exchange.

We will introduce the details of LSA in the next basic network knowledge and further discuss the concept of a region. Now you only need to know that region zero is a unique region, and if it is multi-region OSPF, each region must be connected to a region zero. Region 0 is also called the main region (BA ). The OSPF network has multiple regions. If you want to add virtual links (VL) to the two regions without having a dialogue between the regions, the Network may become very messy. In addition, there are several different types of routers in OSPF.

ABR

Regional Border Router. Located in Zone zero and other regions.

DR, BDR

Vro. Save the subnet database, and send and receive update data to other routers in the same network (through multi-point transmission ).

ASBR

VBR of the autonomous system. A router that is unique and confusing. ASBR connects one or more AS and exchanges routing information between. The role of ASBR is to re-allocate routes from another AS to its own.

The concept of reallocation has finally emerged: Suppose we have a router (internal router, rather than the backbone router BR) and want to connect it to a new network that we don't control. After the connection, we have several options: First, we use non-IGP (Internal Gateway Protocol) routing protocols, such as BGP (Border Gateway Protocol) to exchange routing information. The second is that the summary route is good and is added to the static route of the new network in this router. Any data packet that uses this route to reach this router will be sent to the new network, but OSPF does not know its existence. To this end, although we "re-allocate" These routing information to OSPF. We do not want to import BGP routers over kb into OSPF. However, if Static Routing is selected, We need to publish the newly added Static Routing Information to the entire intranet, so that every vro in our autonomous system can understand the routes of the new network. Once the internal router is notified that it wants to re-publish Static Routing Information to OSPF, it becomes an ASBR and the entire network can be connected to the new network.

In the next basic network knowledge, we will discuss regions and LSA, so as to consolidate your understanding of the entire OSPF concept and understand what is a high-quality OSPF design.

Summary

OSPF is an Internal Gateway Protocol (IGP) for fast convergence of links, with millions of users.

OSPF allows neighboring routers to establish adjacent relationships and transmit LSA information through DR and BDR.

The region in OSPF is used to restrict the LSA and summarize the route information. Each router is connected to the region zero (main region.

Network basics lecture 13: routing information protocol (RIP)
Network basics lecture 12: Basic knowledge of routing protocols
Lecture 11 on basic network knowledge: Internet management architecture
Lecture 10 on basic network knowledge: TCP protocol understanding (advanced tutorial)
Lecture 9 on basic network knowledge: a preliminary understanding of the TCP protocol
Lecture 8 on basic network knowledge: a preliminary understanding of the IP protocol
Lecture 7: understanding and using ICMP protocol
Lecture 6 on basic network knowledge: Implementing subnets, including IPv6)
Lecture 5: Learning Spanning Tree Protocol
Lecture 4: understanding the data link layer
Network basics Lecture 3: Understanding OSI network layering
Network basics Lecture 2: Understanding subnet and CIDR
One of the basic network knowledge lectures: understanding the meaning of IPv4 addresses