OSPF basic concept
1.OSPF (Open Shortest Path First, Open Shortest Path First)
is the IETF (Internet Engineering Task Force, Internet Engineering Task Force) developed by a link-state interior gateway protocol. Currently used for IPv4 OSPF Version 2.
Since the link-state routing algorithm to calculate the shortest path using the SPF, the scale becomes larger when the site has better scalability than the distance vector algorithm, therefore, OSPF also have better performance.
OSPF support load balancing (Loading Balancing) and the type of service based routing (TOS Routing), also supports multiple routing forms, such as host-specific route and sub-routes and the like. If there are equal to the best path cost of a route of a plurality of answer network, OSPF traffic can be evenly distributed on to these routes.
OSPF support internal hierarchical routing, the network domain is divided into several regions (Area). Self-contained area (Self-Contained), the topology of the hidden area to other areas. Each region can be cooperatively performed by using OSPF routing, while retaining the ability to make independent areas changes its internal topology. This makes it easier to expand the OSPF protocol and finishing can be used in large-scale networks.
OSPF routing calculation process
l Each OSPF router generated based on the surrounding network topology LSA (Link State Advertisement, LSA), and by updating the LSA packets sent to other OSPF routers in the network.
l Each OSPF router collects other router advertisement of LSA, LSA put all together to compose a LSDB (Link State Database, the link state database). LSA is a description of the network topology around router, LSDB is a description of the entire autonomous system network topology.
l OSPF router transforms the LSDB to a weighted directed graph, which actually reflects the entire network topology. All the routers are identical to FIG.
l OSPF router transforms the LSDB to a weighted directed graph, which actually reflects the entire network topology. All the routers are identical to FIG.
Router ID
router if you want to run OSPF protocol, then there must be RID (Router ID, the router ID). RID is a 32-bit unsigned integer that can uniquely identifies a router in one autonomous system.
RID can be configured manually or automatically generated; If no RID command, automatically generates a RID in the following order:
L If the current device is configured Loopback interface, to select all Loopback interface highest IP address as the RID;
L If the current device is not configured Loopback interface, it will select all of the configured IP address on the link and the maximum effective value of the IP address of the interface as an RID.
AS (Autonomous System) a group of routers using the same routing protocol to exchange routing information, abbreviated as AS.
OSPF protocol packets
Hello packet: Periodically sent to find and maintain OSPF neighbor relationships. It contains the values of some timers, DR (Designated Router, designated router), BDR (Backup Designated Router, Backup Designated Router) and known neighbors.
Database Description (DD) packets: request required LSA to each other. After two routers exchanging the DD packets, that the peer routers which the local LSDB LSA missing, then the LSR LSA needs to send a request message to the other required. Including a summary LSA needed.
LSU (LSU) packet: LSA need to send to the other party
Link State Acknowledgment (the LSAck of) packets: used to acknowledge the received LSA. We need to make sure the content is the LSA Header (a packet can confirm multiple LSA).
Type of the LSA
OSPF link state information is described in the package are released out in the LSA, LSA commonly used types are the following:
L Router LSA (Type1): produced by each router, and the router link-state description overhead, propagation in a single area.
l Network LSA (Type2): is produced by DR, the link state of the local description of all routers in the propagation area only.
l Network Summary LSA (Type3): is produced by ABR (Area Border Router, ABR), a route segment in the region descriptor, and flooded to other areas.
l ASBR Summary LSA (Type4): is produced by the ABR, describes the route to ASBR (Autonomous System Boundary Router, ASBR) is advertised to the relevant region.
l AS External LSA (Type5): from an ASBR, external route description to AS (Autonomous System, AS), advertised in all regions (except Stub and NSSA area).
l NSSA External LSA (Type7): in the region generated by an ASBR NSSA (Not-So-Stubby Area ), describe routes to AS outside spread only in the NSSA.
l Opaque LSA: A proposed type of LSA, the LSA from the back of the head followed standard and application specific information may be used by the OSPF protocol, or by some application distributed throughout the OSPF routing domain. Opaque LSA divided into Type 9, Type10, Type11 three types, flooding regions; wherein, Type Opaque LSA 9 only in flooding the local link, Opaque LSA Type 10 only in the local area flooding scope, type LSA 11 may be flooded range in an autonomous system.
OSPF special areas
Command: area area-id stub
Features:
LSA (LSA i.e. category 5) filtering external routes, the AS does not accept external, 3 LSA normal passage
may be provided on the ABR default measure: area area-id default-metric metric default value 1.
only one outlet, not through a virtual link, not the trunk area, no ASBR (except that the ABRs may also be ASBRs), preferably only one ABR, a plurality of ABR may lead to suboptimal route.
ABR will be automatically sent to the stub area within a point of their own default routes.
Advantages:
reducing the size of the topology table (database-table) to reduce the route (route-table) Table Size
Command: internal router area area-id stub, area area -id stub no-summary on the ABR.
Features:
Isolation class 3 and class 5 LSA, namely: an outer route and does not accept the AS does not accept the other within the same routing area AS summary.
ABR will be automatically sent to the area of a point to their default route.
As with the stub area, only one exit, no virtual link through, no ASBR (except that the ABRs may also be ASBRs), not the trunk area. Preferably only one ABR.
Benefits:
ratio of stub to further reduce the topology table (database-table) and routing table (route-table) size. In addition in this region completely stub and a default route entry point to route entries generated ABR, the other routing entry is no longer needed.
Can be found, the distal region completely stub area except that: the stub area may allow other OSPF routing area (Inter-Area Route) enter, can not completely stubby area. (That is, whether it isolated 3 LSA) as saying totally stubby area just stub area and more isolated than three categories LSA, like any other.
Command: all routers within the area area area-id nssa
characteristics:
to allow acceptance of external routing information to LSA type transmission 7, and ABR responsible for converting into LSA LSA Type 7 Type 5.
Imported from other AS or other routing protocols on the definition of the class is not 7 LSA, let this other routers in the OSPF AS recognizable in the NSSA ABR will into seven categories Category 5 LSA. Again: All stub area ABR always filter out Category 5 LSA.
AS contact with other routers called ASBR.
Because of its external network may be routed to the OSPF redistribution process, it does not automatically send an ABR default route to point to their NSSA area, but can manually send default route (R1 (config-router) within the NSSA # default- information originate [always]), and only the default channel may be sent on the ABR.
The biggest difference is that the stub area and NSSA, NSSA region may allow their external route redistribution into OSPF, and the stub area can not.
Benefits:
In addition to reducing the same with the stub area topology table to reduce the size of the routing table, but also have the flexibility to add other AS (or other routing protocol weight distribution) by an ASBR
command: in the ABR R4 (config-router) #area 1 nssa no-summary, other area 1 nssa can be.
Features: In Totally Not-so-Stubby Area ( Totally NSSA), ABR will be filtered out to route all external routing and other OSPF area (Inter-Area Route) enters Totally NSSA region, but router can be external route redistribution into OSPF process, that is, within the routers Totally NSSA area can become ASBR, due to the OSPF routing to other regions did not go, so ABR automatically send a default route to point to its own internal Totally NSSA,
the difference between the NSSA Totally NSSA that, NSSA area can Enable route (Inter-area route) access to other OSPF area, and can not Totally NSSA region, but ABR Totally NSSA area will automatically send a default route to point to its inner Totally NSSA region.
3,4,5-based filter lsa, ABR will generate a default class 3 lsa, external routes can be introduced in the region
7 or 7-LSA LSA is here, this area, it becomes a class 5 propagates the
benefits:
automatic default route found, further reducing the table size than NSSA 
summary:
in short, that
stub class filtration 4,5 lsa, ABR will generate a default class 3 LSA, the area can not redistribute routes
Total filtration 3,4,5 stub class lsa, ABR will generate a default class 3 LSA, not introduced into the region of the outer routes
nssa 4,5-based filtering lsa, ABR will generate a default LSA class 7, can be introduced in the region external routes
Total nssa 3,4 filtering, class 5 lsa, ABR will generate a default class 3 lsa, external routes can be introduced in the region
