CCNP RoutingSemester 5

Chapter 4OSPF

Using OSPF in a Single Area

OSPF is more complex than RIP. To harness its power, it is important to spend time in the analysis and design of the network. Sometimes, it is necessary to redesign the addressing scheme to support the hierarchical structure that OSPF requires.

Hello Packet FieldsField

Router ID

Hello/Dead Intervals

Neighbor

Area ID

Function

Identifies the router within the AS

Hello maintains presence of router in its neighbor databases; works like a keepalive

Another router with whom updates will be exchanged to synchronize database

Hello packet must come from a router within same area to be valid

Hello Packet FieldsField

Router Priority

DR IP Address

BDR IP Address

AuthenticationPassword

Stub Area Flag

Function

Used to manually select DR and BDR

Used to allow the router to create unicast traffic to DR

Allows router to create unicast traffic to the BDR

Used for security

Identifies which type of LSA will be transmitted and accepted

OSPF Packets

HelloProtocol

DatabaseDescriptor

Used to find neighbors and to determine designated router and backup designated router – continued propagation of Hello protocol maintains the transmitting router in the topology database of those that hear the message

Used to send summary info to neighbors to synchronize topology databases

OSPF Packets

Link StateRequest

Link StateUpdate

Link StateAcknowledge

Works as a request for more detailed info which is sent when router receives a database descriptor that contains new info

Works as LSA packet issues in response to request for database info in LSA request packet

Acknowledges link-state update

OSPF DatabasesAdjacencyDatabase

Link-StateDatabase(topological database)

ForwardingDatabase(routing table)

List of all neighbor routers to which a router has established bidirectional communication. Unique for each router

List of info about all other routers in the network. This database shows the network topology. All routers within an area have identical link-state databases

List of routes generated when an algorithm is run on the link-state database. Each router’s routing table is unique and holds info on how / where to send packets

Types of OSPF Networks

Type

Broadcast Multiaccess

NonbroadcastMultiaccess

Point-to-Point

Point-to-Multipoint

Determining Characteristic

Ethernet, Token Ring, or FDDI

Frame Relay, X.25, SMDS

PPP, HDLC

Configured by an administrator

DRElection?

Yes

No

Yes

No

Designated Router (DR)

For every multiaccess IP network, one router will be elected the DR. This DR has two main functions: first, to become adjacent to all other routers on the network, and second, to act as a “spokesperson” for the network. As spokesperson, the DR sends other IP networks network LSAs that list all local routers. Because the DR becomes adjacent to all other routers on the IP network, it is the focal point for collecting routing information (LSAs).

Backup Designated Router (BDR)

The DR could represent a single point of failure, so a second router is elected as the BDR to provide fault tolerance. Thus, the BDR must also become adjacent to all routers on the network and must serve as a second focal point for LSAs. But, unlike the DR, the BDR is not responsible for updating the other routers or sending network LSAs. Instead, the BDR keeps a timer on the DR’s update activity to ensure that it is operational. If the BDR does not detect activity from the DR before the timer expires, the BDR takes over the role of DR and a new BDR is elected.

Steps in the Operation of OSPF

OSPF routers progress through five distinct steps of operation:

1. Establish router adjacencies2. Elect a DR and BDR (if necessary)3. Discover routes4. Select the appropriate routers to use5. Maintain routing information

Message-Digest AuthenticationRouter(config)#ip ospf message-digest-key key-id md5 [encryption-type] password

key_id An identifier in the range 1 to 255, which allows for multiple keys. The key ID configuration on each router must match to authentication

md5 A required value that specifies the MD5 algorithm will be used

encryption-type Optional value that specifies the type of encryption to use (0 to 7). Type 7 indicates Cisco proprietary encryption. Type 0 is the default

password An alphanumeric password to be used as the message-digest key

Learning a New Route(2500 Series)

• Router takes first entry from update – first network with info about state of its link

• Router verifies that type of LSA is one that can be accepted by this router

• Router issues a lookup to its topological database when the LSA is valid

• Entries NOT in topological database will be flooded immediately out all OSPF interfaces except receiving interface

• Further questions are required when LSA entry is in topological database

Learning a New Route(2500 Series)

• If sequence numbers are the same, router calculates checksum for LSAs and uses the LSA with the higher checksum

• When checksum numbers are the same, router checks MaxAge field to ascertain which is more recent update

• Router determines whether the LSU has arrived outside the wait period before another computation is allowed

• When new LSA entry passes the tests, it is flooded out all OSPF interfaces except for the receiving interface

Learning a New Route(2500 Series)

• Current copy replaces old LSA entry. If there was no entry, current copy is placed in database

• Received LSA is acknowledged• If LSA entry was in database, but LSA just received

has an older sequence number, the process asks whether info in database is the same

• If info is different and new LSA has an older sequence number, receiving router discards the LSA update and issues its own LSA

• After initial flood, updates are sent only when there are changes in the area or when the 30 min. timer goes off

GlossaryAdjacency

Area

Autonomous System

Formed when two neighboring routers have exchanged info and share the same topology table. Database are synchronized and see same networks

A group of routers that share the same area ID. Each router in the area has the same topology table. Each router is an internal router. The area is defined on an interface basis in the OSPF configuration

Routers that share the same routing protocol within the same organization

GlossaryBackup DesignatedRouter(BDR)

Cost

Backup to the designated router in case the DR fails. BDR performs none of the DR functions while the DR is operating correctly

A metric for OSPF. Not defined in standard with a value. Cisco uses default of inverse of bandwidth – higher the speed, lower the cost. Can be overridden with manual configuration – do only with full knowledge of network

GlossaryDatabase Descriptor(DDP)

Designated Router (DR)

Referred to as DBD – database descriptor packet – packets exchanged between neighbors during the exchange state. DDPs contain LSAs which describe the links of every router in the neighbor’s topology table

Responsible for making adjacencies with all neighbors on a multiaccess network such as Ethernet or FDDI. DR represents the multiaccess network in that it ensures every router on the link has the same topology database

GlossaryExchangeState

ExStart

init

Method by which two neighboring routers discover the map of the network. When these routers become adjacent, they must first exchange DDPs to ensure they have the same topology table

State in which the neighboring routers determine the sequence number of the DDPs and establish the master/slave relationship

State in which hello packet has been sent from the router, which is waiting for reply to establish two-way communication

Glossary

InternalRouter

Link-State Adv LSA

Link-StateDatabase

Router that has all its interfaces in the same area

Packet describing router’s links and state of those links

Topology map. Map of every router, its links, state of the links. Also has a map of every network and every path to each network

GlossaryLink-StateRequest LSR

Link-StateUpdate LSU

Neighbor

NeighborTable

Router requests additional info when LSA entry is not present or is older than the DDP when comparing the topology database

Update sent in response to the LSR – it is the LSA that was requested

Router on the same link with whom routing info is exchanged

Table built from hello received from neighbor

GlossaryPriority

SPF Tree

TopologyTable

Cisco tool by which designated router can be manually elected or prevented from taking part in a DR/BDR election

Tree of topological network. Drawn after SPF algorithm has been run. Algorithm prunes database of alternative paths and creates loop-free shortest path to all networks

Same as link-state database

More GlossaryFlood

Fully Adjacent

Loading State

Loopback Interface

Setup Script

Two-wayState

Refers to network info – sent to every device in domain

When routing tables of two neighbors are fully synchronized

State where router will request more detail using LSR

Virtual interface that does not exist physically. If it doesn’t exist, it can’t go down

Question and answer dialogue offered by Cisco router

State during process in which two routers are creating an adjacency. This is stage before routing info is exchanged