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Single-Area OSPFv2 Concepts, imagen, imagen, imagen, imagen, imagen,…
Single-Area OSPFv2 Concepts
1.1 OSPF Features and Characteristics 📦
1.1.4 Single-Area and Multiarea OSPF.
Single-Area OSFP
All routers are in one area.
Multiarea OSPF.
sing multiple areas, in a hierarchical fashion.
1.1.2 Components of OSPF
Routers running OSPF exchange messages to convey routing information using five types of packets.
The router builds the topology table using results of calculations based on the Dijkstra shortest-path first (SPF) algorithm. The SPF algorithm is based on the cumulative cost to reach a destination.
OSPF messages are used to create and maintain three OSPF databases, as follows:
Adjacency database - This creates the neighbor table.
Link-state database (LSDB) - This creates the topology table.
Forwarding database - This creates the routing table.
1.1.5 Multiarea OSPF
Multiarea OSPF.
Hierarchical division of a large routing domain into smaller areas.
Interarea routing still occurs, but CPU-intensive operations, like recalculating the database, are confined to each area.
SPF (Shortest Path First) Process
1.1.1 Introduction to OSPF
OSPF is a link-state routing protocol that was developed as an alternative for the distance vector Routing Information Protocol (RIP).
1.1.3 Link-State Operation
Build the Link State Database
Execute the SPF Algorithm
Exchange Link-State Advertisements
Choose the Best Route
Establish Neighbor Adjacencies
OSPFv3.
Is the equivalent of OSPFv2 for IPv6
Support for Address Families
OSPFv2 and OSPFv3 have separate processes with independent adjacency tables, topology tables, and routing tables
The configuration and verification commands in OSPFv3 are similar to those used in OSPFv2.
1.2 OSPF Packets :package:
1.2.3 Link-state Updates
Routers initially exchange Type 2 DBD packets, which is an abbreviated list of the LSDB of the sending router.
It is used by receiving routers to check against the local LSDB.
Type 4 LSU packet is used to reply to an LSR packet.
LSAs
Type 3 or 4: Summary LSAs
Type 5: Autonomous System External LSAs
Type 6: Multicast OSPF LSAs
Type 2: Network LSAs
Type 7: Defined for Not-So-Stubby Areas
Type 1: Router LSAs
Type 8: External Attributes LSA for Border Gateway Patrol
Type 5 packet is used to acknowledge the receipt of a Type 4 LSU.
Type 3 LSR packet is used by the receiving routers to request more information about an entry in the DBD.
LSUs contain LSAs
1.2.4 Hello Packet
Elect the Designated Router (DR) and Backup Designated Router (BDR) on multiaccess networks like Ethernet.
Point-to-point links do not require DR or BDR.
Discover OSPF neighbors and establish neighbor adjacencies.
Advertise parameters on which two routers must agree to become neighbors.
1.2.2 Types
Type 1 Hello
Discovers neighbors and builds adjacencies between them.
Establishes and mantains adjacency with other OSPF routers. :check:
Type 2 Database Description
Checks for Database synchronization between routers.
Contains an abbreviated list of the LSDB of the sending router and is used by receiving routers to check against the local LSDB.
Type 3 Link-state Request
Request specific link-state records from router to router.
Receiving routers can then request more information about any entry in the DBD by sending an LSR. :
Type 4 Link-state Update
Sends specifically requested link-state records.
This is used to reply to LSRs and to announce new information. LSUs contain several different types of LSAs.
Type 5 Link-state Acknowledgment
Aknowledges the other packet types.
When an LSU is received, the router sends an LSAck to confirm receipt of the LSU. The LSAck data field is empty.
1.3 OSPF Operation
1.3.6 LSA Flooding With a DR
Flooding LSAs
This event triggers every other router to also send out an LSA. If every router in a multiaccess network had to flood and acknowledge all received LSAs to all other routers on that same multiaccess network, the network traffic would become quite chaotic.
LSAs and DR
The solution to managing the number of adjacencies and the flooding of LSAs on a multiaccess network is the DR. On multiaccess networks, OSPF elects a DR to be the collection and distribution point for LSAs sent and received. A BDR is also elected in case the DR fails. All other routers become DROTHERs. A DROTHER is a router that is neither the DR nor the BDR.
The DR is only used for the dissemination of LSAs. The router will still use the best next-hop router indicated in the routing table for the forwarding of all other packets.
1.3.5 The Need for a DR
Multiaccess networks can create two challenges for OSPF regarding the flooding of LSAs, as follows:
Creation of multiple adjacencies - Ethernet networks could potentially interconnect many OSPF routers over a common link. Creating adjacencies with every router is unnecessary and undesirable. It would lead to an excessive number of LSAs exchanged between routers on the same network.
Extensive flooding of LSAs - Link-state routers flood their LSAs any time OSPF is initialized, or when there is a change in the topology. This flooding can become excessive.
1.3.4 Synchronizing OSPF Databases
The other four types of OSPF packets are used during the process of exchanging and synchronizing LSDBs.
1.3.3 Establish Neighbor Adjacencies
When OSPF is enabled on an interface, the router must determine if there is another OSPF neighbor on the link.
The router sends a Hello packet that contains its router ID ou all OSPF-enabled interfaces, Only OSPFv2 routers will process these packets.
The OSPF router ID is used by the OSPF process to uniquely identify each router in the OSPF area. A router ID is a 32-bit number formatted like an IPv4 address and assigned to uniquely identify a router among OSPF peers.
When a neighboring OSPF-enabled router receives a Hello packet with a router ID that is not within its neighbor list, the receiving router attempts to establish an adjacency with the initiating router.
1.3.2 OSPF Operational States
ExStart State
On point-to-point networks, the two routers decide which router will initiatie the DBD packet exchange and decidte upon the initial DBD packet sequence number.
Exchange State
Routers exchange DBD packets, if any aditional information is required the transition to loading to the full state.
Two-Way State
Comunnication between the two routers is bidirectional, On multiaccess links, the routers elect a DR and a BDR.
Init State
Hello packets are received from neihbor, contain the router ID of the sending router and transition to Two-Way state.
Down State
No Hello packets received = Down , Router sends Hello packets, Transition to init sta
Loading State
LSRs and LSUs used to gain extra route information. Routers processed by the SPF algorithm. Transition to Full state.
Full State
The link-state database of the router is fully synchronized
