Cisco Networking Academy Chapter 9: Single-Area OSPF PDF

Chapter 9: Single-Area OSPF Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 1  Chapter 9 9.1 Characteristics of OSPF 9.2 Configuring Single-area OSPFv2 9.3 Configure Single-area OSPFv3 Presentation_ID...

Chapter 9: Single-Area OSPF Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 1  Chapter 9 9.1 Characteristics of OSPF 9.2 Configuring Single-area OSPFv2 9.3 Configure Single-area OSPFv3 Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 2  Chapter 9: Objectives Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 3  Open Shortest Path First Evolution of OSPF Interior Gateway Protocols 1989 1988 updated in 2008 Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 4  Open Shortest Path First Features of OSPF Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 5  Open Shortest Path First Components of OSPF Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 6  Open Shortest Path First Components of OSPF (cont.) OSPF Routers Exchange Packets - These packets are used to discover neighboring routers and also to exchange routing information to maintain accurate information about the network. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 7  Open Shortest Path First Link-State Operation If a neighbor is present, the OSPF-enabled router attempts to establish a neighbor adjacency with that neighbor Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 8  Open Shortest Path First Link-State Operation (cont.)  LSAs contain the state and cost of each directly connected link.  Routers flood their LSAs to adjacent neighbors.  Adjacent neighbors receiving the LSA immediately flood the LSA to other directly connected neighbors, until all routers in the area have all LSAs. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 9  Open Shortest Path First Link-State Operation  Build the topology table based on the received LSAs.  This database eventually holds all the information about the topology of the network.  Execute the SPF Algorithm. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 10  Open Shortest Path First Link-State Operation (cont.) From the SPF tree, the best paths are inserted into the routing table. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 11  Open Shortest Path First Single-area and Multiarea OSPF Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 12  Open Shortest Path First Single-area and Multiarea OSPF (cont.) Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 13  OSPF Messages Types of OSPF Packets Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 14  OSPF Messages Hello Packet OSPF Type 1 packet = Hello packet:  Discover OSPF neighbors and establish neighbor adjacencies.  Advertise parameters on which two routers must agree to become neighbors.  Elect the Designated Router (DR) and Backup Designated Router (BDR) on multiaccess networks like Ethernet and Frame Relay. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 15  OSPF Messages Hello Packet (cont.) Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 16  OSPF Messages Hello Packet Intervals OSPF Hello packets are transmitted:  To 224.0.0.5 in IPv4 (all OSPF routers)  Every 10 seconds (default on multiaccess and point-to- point networks)  Every 30 seconds (default on non-broadcast multiaccess [NBMA] networks)  Dead interval is the period that the router waits to receive a Hello packet before declaring the neighbor down  Router floods the LSDB with information about down neighbors out all OSPF enabled interfaces  Cisco’s default is 4 times the Hello interval Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 17  OSPF Operation OSPF Operational States When an OSPF router is initially connected to a network, it attempts to:  Create adjacencies with neighbors  Exchange routing information  Calculate the best routes  Reach convergence  OSPF progresses through several states while attempting to reach convergence. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 18  OSPF Operation OSPF DR and BDR Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 19  OSPF Router ID OSPF Network Topology Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 20  OSPF Router ID OSPF Network Topology Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 21  OSPF Router ID Router IDs Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 22  Configure Single-area OSPFv2 The network Command Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 23  Wildcard Masks Calculating the Wildcard Mask Calculating wildcard masks can be challenging. One shortcut method is to subtract the subnet mask from 255.255.255.255. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 24  Configure Single-Area OSPFv2 Passive Interface  By default, OSPF messages are forwarded out all OSPF-enabled interfaces. However, these messages really only need to be sent out interfaces connecting to other OSPF-enabled routers.  Sending out unneeded messages on a LAN affects the network in three ways:  Inefficient Use of Bandwidth  Inefficient Use of Resources  Increased Security Risk  The Passive Interface feature helps limiting the scope of routing updates advertisements. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 25  Configure Single-area OSPFv2 Configuring Passive Interfaces Use the passive-interface router configuration mode command to prevent the transmission of routing messages through a router interface, but still allow that network to be advertised to other routers. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 26  OSPF Cost OSPF Accumulates Costs Cost of an OSPF route is the accumulated value from one router to the destination network. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 27  Verify OSPF Verify OSPF Neighbors Verify that the router has formed an adjacency with its neighboring routers. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 28  Verify OSPF Verify OSPF Protocol Settings Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 29  Verify OSPF Verify OSPF Process Information Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 30  Verify OSPF Verify OSPF Interface Settings Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 31 Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 32