Shulou(Shulou.com)06/02 Report--

Overview of OSPF routing protocols:

OSPF-- Open shortest path first selection Protocol (OSPF) is an interior gateway protocol (Interior Gateway Protocol, referred to as IGP), which is used to make routing decisions within a single autonomous system (autonomous system,AS). It is an implementation of the link-state routing protocol, which belongs to the Internal Gateway Protocol (IGP), so it operates within the autonomous system. Autonomous system (AS) AS refers to routers and network groups that are under the control of a regulatory body. It can be a router directly connected to a LAN, but also connected to an Internet; it can be a number of Lans interconnected by an enterprise backbone. All routers in an autonomous system must be connected to each other, run the same routing protocol, and assign the same autonomous system number. An AS can run only one routing protocol. Interior Gateway Protocol (IGP) IGP is a protocol for exchanging routing information between gateways (hosts and routers) within an autonomous network. Routing information can be used in internetwork protocols (IP) or other network protocols to explain how routing transmission is carried out. Internet nets are divided into multiple domains or autonomous systems. A domain is a set of hosts and a collection of routers that use the same routing protocol and is managed by a single organization. IGP protocols such as RIP, OSPF, IS-IS, etc. External Gateway Protocol (EGP) EGP is a protocol that exchanges routing information between two neighboring gateway hosts (each with its own route) in an autonomous system network. EGP is often used to exchange routing table information between two hosts on the Internet. The routing table includes a list of known routers, addresses they can reach, and cost metrics associated with the path of each route in order to select the best available path. Each router sends information to its neighboring routes at regular intervals, usually between 120 and 480 seconds, and then the neighboring routes send back their complete routing table. EGP protocols such as BGP and so on. The working process of OSPF

Establishment of adjacency →→ link-state database →→ shortest path number →→ routing table learning link-state information characteristics of Dijkstra algorithm OSPF can adapt to large-scale network route change convergence speed no routing ring support variable length subnet mask VLSM support area partition support multicast address transmission protocol OSPF area OSPF divides multiple areas in the AS. Each OSPF router maintains only complete link-state information for its area. Area0-- backbone area (core) (default unique) all other Area only connect to Area0

The area ID area ID can be expressed as a decimal number or as a copy of the IP address that uniquely identifies the router in the IPRouter IDOSPF area. Router ID selection rules select the highest IP address on the router's lookback interface. If there is no lookback interface, select the highest IP address in the physical port (not recommended There is no guarantee that the port will not fail) you can also use the router-id command to specify the Router ID router role designated router DR (boss): responsible for updating information on all other OSPF routers. Backup designated router BDR (second): responsible for monitoring the status of the backup DR and taking over its role in the event of a current DR failure. Other routers DRothers: only form adjacencies with DR and BDR.

DR and BDR election methods automatically elect the router with the largest Router ID on the segment will be selected as DR, and the second will be BDR to manually select the priority range of DR and BDR. The higher the value, the higher the priority, and the default is 1. If the priority is the same, you need to compare the size of Router ID. If the priority of the router is set to 0, it will not participate in the election of DR and BDR for DR and BDR.

The priority of the router can directly affect the election process, but it cannot be forced to change the identity of the existing DR and BDR routers.

OSPF Multicast address 224.0.0.5-distribute information to other routers 224.0.0.6-receive feedback from other routers-OSPF metric is COSTCOST=108/BW (bandwidth), the greater the bandwidth, the lower the cost. The shortest path is calculated based on the cost specified by the interface (cost).

OSPF packets OSPF packets are carried within IP packets, using a packet type Hello packet with the protocol number 89OSPF: used to discover and maintain neighbor relationships, and to elect DR and BDR. Database description package (DBD): used to send summary information to neighbors to synchronize the link-state database. Link-state request packet (LSR): sent after the router receives a DBD containing new information to request more detailed information. Link-state update packet (LSU): a link-state advertisement (LSA) is sent when a LSR is received. A LSU packet may contain multiple LSA. Link-state acknowledgement packet (LSAck): acknowledges that LSU has been received, and each LSA protocol is acknowledged separately. Seven states of OSPF

Down status: did not receive Hello message Init status: received Hello message activate router 2-Way status: elect the two largest RouterID, but did not confirm the identity of DR and BDR

ExStart status (quasi-startup state): confirm master-slave router (confirm DR and BDR identity) Exchange status: exchange messages between routers (DBD message and LSAck message) Loading status: run other messages between routers except Hello message and DBD message (most abundant) For example, LSR message, LSU message Full status: routing table generates four types of OSPF: point-to-point network broadcast multiaccess network non-broadcast multiaccess network reasons for OSPF multiarea generation 1, improvement of network scalability 2, fast convergence of OSPF traffic 1, intra-domain traffic between routers in a single area constitutes traffic 2, Traffic volume formed by exchanging packets between routers in different areas of inter-domain traffic 3, traffic volume formed by exchanging packets between routers in external traffic OSPF domain and routers outside OSPF area or in another autonomous system 1, backbone area Area0 is responsible for the propagation of inter-area routing information 2, Non-backbone area-distinguishes the standard area stub area (stub), complete stub area (Totally stubby), non-pure stub area (NSSA) (different LSA running within the triplet area) according to the type of router that can be learned. composition of the link-state database 1, each router creates a database composed of each interface, corresponding neighboring nodes and interface speed, Each entry in the link-state database is called LSA (link-state advertisement). There are six common LSA types, link-state advertisements (LSA), Type 1-Router LSA: issued by all routers in the area.

Type 2Mel-Network LSA: issued by the DR in the area

Type 3Mel-Network Summary LSA: summary link advertisements issued by ABR in other areas (summary and exchange of information)

Type 4--ASBR Summary LSA: issued by ABR and used to communicate ASBR messages (ASBR is equivalent to diplomat)

Type 5--AS external LSA: issued by ASBR to advertise external routes (4, 5 are bundled, all, none)

Type 7--NSSA external LSA: issued by ASBR within the NSSA area and used to advertise external routes for local area connections. Only NSSA has Type 7, and other regions do not have Type 7 (5 and 7 are essentially the same, but there are different regions)

Router selection of routing entries 1. Routers only add optimal routing entries to routing table 2. Routing entries are selected on the basis of (1) administrative distance (2) metric load balancing route entries.

The stub area and the complete stub area meet the following conditions: 1, only one unrouted area serves as the exit of other areas 2, the area cannot be traversed as a virtual link 3, the ASBR4 router without autonomous system boundary in the Stub area, indicates that the backbone area Area0 stub area does not have LSA4, 5, 7 advertises the complete stub area except for an unmanned routing advertisement of LSA3 No LSA3, 4, 5, 7 advertised route redistribution 1, a single IP routing protocol is the preferred scheme for managing IP routing in the network 2, CiscoIOS can execute multiple routing protocols Each routing protocol and the network served by the routing protocol belong to the same autonomous system 3. CiscoIOS uses the route retransmission distribution feature to exchange routing information created by different protocols (exchange different protocol process numbers) to redistribute to the external path type 1 (Type1 externalpath,E1) type 2 external path (Type2 externalpath) of the path type 1 routed in the OSPF domain. E2) Router A has two paths to the external destination 10.1.2.0. The cost of E1 type A-B-D is 25 (20 5) (priority) path A-C-D is 48 (18 million 30) E2 type path A-B-D is 20 path A-C-D cost is 18 (priority)

The NSSA area is an addendum to OSPF RFC that defines a special LSA type 7 that provides features similar to stub area and totally stubby area, including ASBROSPF link state advertisements LSA 7 (NSSA External LSA,NSSA external LSA) NSSA area redistribution route type N1, N2 after passing through the NSSA area ABR and then converted to E1, E2 summary LSA that allows flooding in each area.

OSPF path type priority intra-area path: priority 1 inter-area path: priority 2E1 external path: priority 3E2 external path: priority 4 (1 represents the highest priority 4 for lowest priority) OSPF address summarization also saves resources by reducing the number of LSA flooded, saving resources by shielding some details of network instability, reducing the number of entries in the routing table, virtual link refers to a link to the backbone area through a non-backbone area, the purpose of connecting an area to the backbone area through a non-backbone area through a A non-backbone area connects a segmented backbone area.

Rules and characteristics of configuring virtual links virtual link programming configuration between two ABR routers the transfer area cannot be a stub area the stability of the virtual link depends on the stability of the area it passes through, the virtual link helps to provide logical redundancy

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