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2025-01-19 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > Network Security >
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OSPF (Open Shortest Path First)
OSPF is a link-state routing protocol that is an IGP (Interior Gateway Protocol) that runs inside an AS.
Version of OSPF
OSPFv2: Network for IPv4
OSPFv3: Network for IPv6
Three Tables of OSPF
1. Routing table: contains routes in this area and even AS area (except special areas)
2. Topology table: record information of all network segments (including LSDB and LSA)
3. Neighbor table: Record neighbor relations, neighbor records table connected to this router.
4 Types of OSPF Routers
Zone internal router: A router that is contained between zones and is not an edge router.
Backbone area routers: routers that belong to the interior of Area 0, non-border routers.
Area border router (ABR): A router that connects backbone areas to non-backbone areas.
AS Area Border Router (ASBR): A router between homebrew systems. (It can be simply understood that as long as there is a route into the router can be called ASBR router).
5 Types of OSPF Packets
HELLO: Used to establish neighbor relationships
DD: Information describing the local database.
LSR: Requests the required LSA information from the other party. (established after DD negotiation)
LSU: Update LSA
LSAck: Confirm that the updated LSA is complete.
Address of OSPF update message
224.0.0.5 Under the OSPF protocol, all routers establish connections and exchange data using the address 224.0.0.5. 224.0.0.6: Used to transfer information between DR other routers and DR, BDR. Data cannot be transferred between DR and other.
The role of a router in an OSPF domain
DR: Root router
BDR: Backup Root Router
DR other: Other routers
DR/BDR elections
Elected according to priority. (router priority, loopback, mac, etc.)
DR/BDR/DR other cannot be preempted.
You can set the priority of a router to 0, which does not mean that the router does not participate in the preemption election DR.
OSPF-defined LSA types
LSA Class 1: This route is generated and describes the link state and link cost of the router. (only transmitted within the region)
LSA Class 2: Describes the link state of the segment and is transmitted within the region.
LSA Class 3: Information describing a network segment that propagates through regional supports.(ABR)
LSA Class 4: Describes the path to the ASBR and propagates in non-ASBR areas.
LSA Class 5: Describes routing information outside an AS that can be propagated throughout the domain.
LSA Class 6: Group Membership LSA is an extension of standard OSPF to support multicast routing functionality.
LSA Class 7: Generated by ASBRs within the NSSA area, describes networks outside the AS. NSSA external LSAs flood only within the NSSA area from which they originated.
LSA Type 8: Link-local LSA Type 8, used primarily for OSPFv3.
Network types supported by OSPF
1. Point-to-point: PPP HDLC
2. Broadcast type:Ethernet
3. Point to point
The process of establishing adjacency relationship in OSPF
1. Down: This is the initial state of a neighbor session, indicating that no information has been received recently from the neighbor. On NBMA networks, Hello packets may still be sent to neighbors in the Down state at a lower frequency.
2. Attempt: This state applies only to neighbors connected on the NBMA network. This status indicates that no information has been received recently from the neighbor, but further attempts are needed to contact the neighbor. Hello packets are sent to neighbors at certain intervals.
3. Init: In this state, a Hello packet was recently received from a neighbor. However, there is still no bidirectional communication with neighbors, for example, the router itself is not present in the Hello packet sent by the neighbor.
4. Two-way communication state (2-Way): This state means that two-way communication has been established between the two routers. DR and BDR elections will also take place in this state (only routers in the 2-Way state are eligible for DR and BDR)
5. ExStart: This state is the first step in creating adjacency relationships. The goal of this state is to determine the master-slave relationship of the router when information is exchanged and to determine the sequence number of the initial database description (DD) packet. The router with the highest router ID becomes the primary router.
6. ExChange: A router in this state describes its complete link-state database by sending DD packets to its neighbors. Each DD packet has a sequence number and needs to be explicitly acknowledged. Only one DD packet can be sent at a time. In this state, the router can also send link-state request packets to ask neighbors for the latest LSA. In fact, these state adjacencies are perfectly capable of sending and receiving all types of OSPF protocol packets.
7. Loading state: In this state, the router sends a Link State Request packet to neighbor routers requesting the latest LSA for information exchange state discovery.
8. Full adjacency state: In this state, neighbor routers form a full adjacency relationship. These adjacency relationships are described in router LSAs and network LSAs.
9. Opaque LSA: Flooding scope is local link only.
10. Opaque LSA: Flooding extent is the area from which the LSA originated.
11. Opaque LSA: The flooding extent is the entire OSPF domain.
RFC2370 defines (Extended Properties of LSA) LSA, called Opaque LSA These three new LSAs provide a common mechanism for OSPF scalability. They can carry information for OSPF or they can carry information directly for applications.
The biggest difference between neighbor and adjacency states is whether or not data is exchanged.
OSPF Area
Backbone Area: The core area of the entire OSPF domain. All other non-backbone areas must be directly connected to backbone areas. Backbone regions can distribute routing information between different non-backbone regions.
Terminal area: the origin and termination area of traffic. An area within which AS External Route Advertisements (AS External LSAs) are not allowed to flood. The ABR in the stub area advertises a default route to all routers in the area, and any traffic destined for networks outside the AS will be forwarded according to the default route.
Non-complete stub area: routes outside the AS are allowed to be introduced and spread to other areas, but NSSA still does not allow routes outside the AS advertised by other areas to enter the area. To enable NSSA to introduce AS external routing, OSPF defines a new LSA type, NSSA External LSA (Type 7 LSA). The NSSA ASBR will generate Type 7 LSAs and flood within the NSSA area. The NSSA ABR converts Type 7 LSAs into Type 5 LSAs and floods them to other areas.
Totally non-pure peripheral zone: Totally non-pure peripheral zone (NSSA) is also a Cisco proprietary zone type that, in addition to having the same characteristics as NSSA, does not allow Type 3 LSA flooding within the zone. ABR in a completely non-stub area also advertises a default route to all routers in the area. Setting the zone to be completely non-stub also significantly reduces the number of routing table entries.
Totally Stubby Areas: Totally Stubby Areas are a type of area proprietary to Cisco. Fully peripheral areas not only do not allow AS external route advertisements to flood within them, they also do not allow inter-area summary routes (Class 3 LSAs) to flood within them. As with the stub area, the full stub area ABR advertises a default route to all routers in the area, and any traffic destined for other areas and networks outside the AS will be forwarded according to the default route. Setting the zone to a full periphery significantly reduces the number of routing table entries.
Name of device in OSPF domain
Virtual links for OSPF
A logical channel established in a non-backbone area between two ABRs.
Points for attention in virtual link configuration
1. Add the Router ID of the ABRs connected to each other to the Vlink-peer of the peer. (Huawei router experiment)
Detailed steps omitted
OSPF supports authentication
1. regional certification
2. interface certification
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