This page was exported from Free Cisco Training & Resources - Certification Exam Preparation [ https://www.ciscobibles.com ] Export date:Fri Mar 29 5:42:24 2024 / +0000 GMT ___________________________________________________ Title: CCNP BSCI Notes - Understanding Simple Single-Area OSPF --------------------------------------------------- OSPF Fundamentals Link-state routing protocols utilize more internal resources in favor of reducing bandwidth consumption. All OSPF routers in an area share the same Link State Database (LSDB). Link State Advertisements (LSAs) are flooded to all neighboring routers. OSPF tables:        Neighbor table        Topology database        Routing table Forming adjacencies Routers multicast hellos to 224.0.0.5 every 10 seconds on a broadcast link and every 30 seconds on a nonbroadcast link. Once hellos are exchanged, neighboring routers add one another to their neighbor tables. Contents of a hello packet:          Router ID - 32-bit unique number (IP address)          Hello/dead intervals - Timers          Neighbor list - List of neighboring router IDs          Area ID          Priority - Used in electing the DR and BDR          DR and BDR          Authentication (if enabled)          Stub Area Flag - On if this is a stub area Neighbor states:          Down          Attempt - Used for manually configured neighbors on an NBMA link; unicast hellos sent to neighbor from which hellos have stopped being received          Init - Hello packet received from neighbor, but without the recipient's router ID          2-Way - Bi-directional communication has been established          Exstart - The DR and BDR have been elected, link-state exchange starting          Exchange - Exchange of database descriptor (DBD) packets          Loading - Exchange of link-state information          Full - Full adjacency established Example OSPF packet capture Cisco OSPF will load balance over up to four equal-cost links; configurable up to six. Designated Routers Neighbors on a broadcast segment elect a designated router (DR) and backup designated router(BDR), which peer with all other routers on the segment. All non-designated routers peer only with the DR and BDR. Multicast destinations:          224.0.0.5 - All OSPF routers          224.0.0.6 - All designated OSPF routers (DR and BDR only) DRs are chosen based on priority (0 - 255). 1 is default; routers with 0 priority will never be elected. Priority ties are broken by choosing the higher router ID. DRs are elected on point-to-point Ethernet links even though this is unnecessary (Ethernet is always seen as a broadcast medium). Interfaces can be configured to operate in point-to-point mode to prevent this. (B)DRs are not preempted. New election will take place only when a current (B)DR goes offline or its OSPF process is administratively restarted. Areas All routers in an area maintain an identical topological database. Areas are defined to logically segment a network and reduce routing table size and complexity. All areas connect to area 0 (the backbone area). Router types:         Backbone routers - Routers in area 0         Area Border Routers (ABRs) - Routers in multiple areas         Autonomous System Boundary Routers (ASBR) - Routers which redistribute information from another AS         Internal - Routers which have all interfaces in a single area Routers can fill multiple roles. Packet Types OSPF is IP protocol 89.          Hello - Used to establish communication with directly connected neighbors          Database Descriptor (DBD) - Lists router IDs from which the router has an LSA and its current sequence number          Link State Request (LSR) - Request for an LSA          Link State Update (LSU) - Reply to an LSR with the requested information          Link State Acknowledgment (LSAck) - Used to confirm receipt of link-state information Packet Fields          Version - Version of OSPF being run          Type          Length          Router ID          Area ID          Checksum          Authentication type (none/plaintext/md5)          Authentication data          Data Configuring OSPF in a Single Area Necessary information:         OSPF process ID (locally significant)         Participating interfaces         Area ID         Router ID Enable OSPF        Configure Included Networks         A single interface can be specified by supplying its IP address and a null wildcard mask: network 192.168.0.1 0.0.0.0 area 0 Router ID If no router ID has been administratively declared, a router will choose the highest loopback IP address. If no loopback addresses are present, the highest IP address of the first active interface will be used. A router ID can be manually specified:        Best practice dictates the creation of a loopback address to be used as the router ID for stability and continuity:         Default Cost Link cost is a 16-bit value (0-65535); default cost is calculated as 100Mbps/interface bandwidth.(Interfaces 100Mbps and faster are assigned a cost of 1.) OSPF cost can be manually specified per interface:         An alternative to defining static costs per interface is to change the numerator bandwidth (default100Mbps):         Reference speed is a 32-bit value (1 - 4294967). If reference speed is modified, the same modification should be performed on all routers within the area. Router Priority Default DR election priority is 1, and a router with a priority of 0 will not become a DR. Priority range is 0 - 255.         Verifying OSPF Configuration         show ip ospf - OSPF process details         show ip ospf database - Contents of the topology database         show ip ospf interface - Interfaces participating in OSPF         show ip ospf neighbor - Neighbor information         show ip protocols - Displays all active routing protocols         show ip route         debug ip ospf events         debug ip packet --------------------------------------------------- Images: http://www.ciscobibles.com/wp-content/uploads/2009/11/clip_image002_thumb9.gif http://www.ciscobibles.com/wp-content/uploads/2009/11/clip_image004_thumb8.gif http://www.ciscobibles.com/wp-content/uploads/2009/11/clip_image006_thumb8.gif http://www.ciscobibles.com/wp-content/uploads/2009/11/clip_image008_thumb8.gif http://www.ciscobibles.com/wp-content/uploads/2009/11/clip_image010_thumb7.gif http://www.ciscobibles.com/wp-content/uploads/2009/11/clip_image012_thumb5.gif http://www.ciscobibles.com/wp-content/uploads/2009/11/clip_image014_thumb5.gif --------------------------------------------------- --------------------------------------------------- Post date: 2009-11-19 15:39:38 Post date GMT: 2009-11-19 07:39:38 Post modified date: 2010-07-23 00:37:43 Post modified date GMT: 2010-07-22 16:37:43 ____________________________________________________________________________________________ Export of Post and Page as text file has been powered by [ Universal Post Manager ] plugin from www.gconverters.com