OSPF fully annotated

OSPF routing protocol

Interior Gateway Protocol and Exterior Gateway Protocol

Autonomous system (AS): the area that uses the same protocol.
Interior Gateway Protocol (IGP): the gateway protocol that works in the autonomous system.
Exterior Gateway Protocol (EGP): works between ASs, allowing two independent ASs to connect , Enable communication between them.
AS refers to a collection of routers that are managed by the same technical management organization and use a unified routing strategy.
OSPF is a link state routing protocol

OSPF working process

·Neighbor list
·Link state database
·Routing table
Establish adjacency relationship———>Link state database——>Shortest path tree———>Routing table Use
yourself as the root to calculate the optimal path.

OSPF area

In order to adapt to large-scale networks, OSPF divides multiple areas within the AS.
Each OSPF router only maintains the complete link state information of the area in which it is located.

Area ID

The area ID can be expressed as a decimal number
or as an IP

Backbone area

Responsible for routing information dissemination between
areas Non-backbone areas

Router ID

The IP address that uniquely identifies the router in the OSPF area

Router ID selection rules

Select the IP address with the highest value on the router loopback interface (the loopback network port is equivalent to inside the router).
If there is no loopback interface, select the highest IP address among the physical ports.
You can also use the router-id command to specify the Router ID (the highest priority)

DR and BDR

Establish adjacency relationships in the broadcast network to
form n(n-2)/2 adjacency relationships

Designated Router (DR)

It is to solve the problem of excessive link connections. BDR is equivalent to the meaning of a deputy captain. After DR hangs up, BDR will be used. Other routers can only form adjacency with DR and BDR.

DR and BDR election methods

Automatic election of DR and BDR: The router with the largest router ID on the network segment will be elected as the DR, and the second one will be elected as the BDR.
Manually select DR and BDR: the priority range is 0~255, the larger the value, the higher the priority, the default is 1.
If the priority is the same, you need to compare the Router ID, if the priority of the router is 0, it will not participate DR and BDR elections.
In a live environment, generally speaking, the DR is turned on first, and the BDR is turned on second. Because the election can be completed in about ten seconds, the election may be successful before the second router is turned on. The priority of the router can affect an election process, but it cannot force the DR and BDR that are already in charge to be in the lower position. Only when they are not working can the DR and BDR be replaced.

OSPF multicast address

224.0.0.5 Send Hello packets to each other for election. DR and BDR send messages to DRother
224.0.0.6 DRother sends DR and BDR
OSTF metric value cost
cost=10^8/BW
shortest path is based on the cost specified by the interface (cost) computational

Interface Type	Cost (10^8/BW)
Fast Ethernet	1
Ethernet	10
56K	1785

The smaller the cost value, the better.
OSPF packets are
carried in IP packets, using protocol number 89

OSPF packet type

OSPF packet type	description
Hello bag	Used to discover and maintain neighbor relationships, 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)	After the router receives the DBD containing new information, it is sent to request more detailed information
Link State Update Package (LSU)	Send link state advertisement (LSA) after receiving LSR, one LSU packet may contain several LSAs
Link status acknowledgement packet (LSACK)	Confirm that DBD/LSU has been received, each LSA needs to be confirmed separately

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OSPF start

Including Down state, Lnit state, 2-Way state, Exstart state, Exchange state, Loading state, Full state.

OSPF network type

OSPF divides the network into four types

Point-to-point network (Point-to-point)
-automatically discover neighbors without DR/BDR, multicast 224.0.0.5
Broadcast MultiAccess (BMA)-automatically
discover neighbors, select DR/BDR, multicast 224.0.0.5, 224.0.0.6None
Broadcast MultiAccess NBMA
-Manually specify neighbors, select DR/BDR, unicast
Point-to-Multipoint network (Point-to-Multipoint)-automatically
discover neighbors , No need for DR/BDR, multicast 224.0.0.5

Features of OSPF

Adaptable to large-scale networks,
routing changes and convergence are fast,
no routing loops,
variable length subnet masks, VLSM,
region division, and
protocol reports sent by multicast addresses

Principle and configuration of OSPF multi-area

Reasons for generating OSPF multi-area

·Improve the scalability of the network
·Quick convergence

Three kinds of traffic of OSPF

Intra- domain traffic The traffic constituted by the exchange of data packets between routers in a single area.
Inter-domain traffic The traffic
constituted by the exchange of data packets between routers in different areas. The
external traffic between
the routers in the OSPF domain and outside the OSPF area or another autonomous system The traffic constituted by the exchange of data packets between routers within.

OSPF router type

Area Border Router (ABR): used to connect area 0 and other areas (used in the division of different areas of the internal protocol)
Autonomous System Border Router (ASBR): used to connect OSPF AS and other external routes (connect external protocols)

OSPF area type

·Backbone area Area0

·Non-backbone area-distinguished according to the type of routing that can be learned

··Standard area
··Stub
area
·· Totally stubby area ··Non-pure stubby area (NSSN)

Peripheral area and complete peripheral area

Areas that meet the following conditions

There is only one default route as the exit
of its area . The area cannot be used as the traversal area
of virtual links. There is no autonomous system border router ASBR in the stub area. It
is not the backbone area Area0

Peripheral area

· No LSA 4.5.7 notification

Complete peripheral area

· Except for an LSA3 default router advertisement, there is no LSA3.4.5.7 advertisement

Router's selection of routing entries

The router only adds the optimal routing directory to the routing table

The basis for selecting routing entries

·Management distance
·Metric value The main purpose of the
stub area and the complete stub area
is to reduce the LSA entries and routing entries in the area, and reduce the occupation of device cpu and memory; the ABR in the stub area and the complete stub area will automatically generate a default route Publish to the peripheral area or completely peripheral area.
OSPF link state database
· Composition of link state database
· Each router has created a database composed of each interface, corresponding adjacent nodes and interface speed.
·· Each entry in the link state database is called LSA (link state communication). There are six common types.

Type code	description	use
Type1	Router LSA	Sent by routers in the area, describing the link status and cost of the router, and transmitted to the entire area
Type2	Network LSA	Issued by the DR in the area, describing the change information in the area, and transmitted to the entire area
Type3	Network summary LSA	The summary link announcement issued by ABR in other areas, describing the route of a certain network segment in other areas, and transferring between areas
Type4	ASBR summary LSA	Issued by ABR, used to advertise ASBR information, determine the location of ASBR, and will not appear in the area where the ASBR belongs
Type5	AS external LSA	Issued by ASBR, used to advertise external routes, tell routers in the same AS the path to the external AS, and flood the entire AS
Type7	NSSA external LSA	The ASBR in the NSSA area is used to advertise the external routes connected to the area. Similar to Type 5, it is only flooded in the non-pure stub area and will be converted to LSA5 by ABR during delivery.

Route redistribution

A single IP routing protocol is the preferred solution for IP routing in the management network.
Cisco lOS can execute multiple routing protocols. Each routing protocol and the network served by the routing protocol belong to the same autonomous system.
Cisco lOS uses route redistribution features to exchange routes. Routing information created by different protocols

Route redistribution considerations

Measurement value
Management distance

OSPF redistributes routes

Type 1 external path (Type 1 external path, E1) to redistribute to the route in the OSPF domain , mainly considering the overall cost.
Type 2 external path (Type 2 external path, E2) mainly considers external costs.

OSPF virtual link

Virtual link

Refers to a link connected to the backbone area through a non-backbone area

The purpose of the virtual link

Connecting an area to the backbone area
through a non-backbone area Connecting part of the area on both sides of a segmented backbone area through a non-backbone area

Rules and characteristics of configuring virtual links

The virtual link must be configured between two ABR routers. The
transmission area cannot be a peripheral area
. The stability of the virtual link depends on the stability of the area it passes through. The
virtual link helps provide logical redundancy.