routing with a link-state protocol

© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE I Chapter 6 1

Routing With a Link-State Protocol

Introducing Routing and Switching in the Enterprise – Chapter 6

© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 2

Objectives

Describe and plan a network using OSPF.

Design and configure a network using single-area OSPF.

Work with multi-protocol environments.


Describe and Plan a Network Using OSPF

Link-state routing protocol

Open standard

Classless interior gateway protocol

Divides the network into areas for scalability



DOES NOT send frequent periodic updates of the entire routing table

Sends an update only when a change in the topology occurs, such as a link going down.

In addition, OSPF performs a full update every 30 minutes.



Compared with distance vector protocols, link-state routing protocols:

• Requires more complex network planning and configuration

• Requires increased router resources

• Requires more memory for storing multiple tables

• Requires more CPU and processing power for the complex routing calculations



RIP

receive updates from their immediate neighbors, but with no details about the network as a whole.

OSPF

generate a complete map of the network from their own viewpoint

OSPF does not automatically summarize at major network boundaries



Bandwidth determines the cost of a link

Higher bandwidth = lower cost = desirable route

Cost = 100,000,000 / bandwidth of link in bps

Manual cost configuration at an interface: ip ospf cost



SPF algorithm (Diijkstra’s Algorithm)

Each router has a topological tree (map of network)

SPF tree information stored in topology database



OSPF • Establish and maintain neighbor relationships, or adjacencies,

with other connected OSPF routers (advanced form of neighborship between routers that are willing to exchange routing information)

• When routers initiate an adjacency with neighbors, an exchange of link-state updates begins.

• Routers reach a FULL state of adjacency when they have synchronized views on their link-state database



OSPF • Hello protocol is used to initially establish and maintain

adjacencies – sends hello packets to directly connected OSPF routers using the address of 224.0.0.5

• Packets are sent every 10 seconds on Ethernet and broadcast links and every 30 seconds for non-broadcast links

• Settings shared include the hello interval, dead interval, and network type, as well as the authentication type and authentication data if configured



Designated router (DR)

Backup designated router (BDR)

DROther



DR and BDR reduces the number of updates sent, unnecessary traffic flow, and processing overhead on all routers

All routers accept updates from the DR only

On broadcast network segments there is only one DR and BDR.

All other routers must have a connection to the DR and BDR.

When a link fails, the router with information about the link sends the information to the DR, using the multicast address 224.0.0.6.

The DR is responsible for distributing the change to all other OSPF routers, using multicast 224.0.0.5..



The router ID is an IP address that is determined by:

1. The value configured with the router-id command

2. If no value is set with the router-id command, the highest configured IP address on any loopback interface

3. If no loopback interface is configured, the highest IP address on any active physical interface

The router ID can be viewed using the following show commands:

show ip protocols, show ip ospf, or show ip ospf interface


Within a local network, the router with the highest router ID is elected the DR. The second highest is elected as the BDR.

An administrator can force the DR and BDR election by configuring a priority using the interface configuration command: ip ospf priority number

By default, OSPF routers have a priority value of 1. Highest priority setting will win the election for DR, regardless of highest router ID.

The highest value that can be set for router priority is 255.

A value of 0 signifies that the router is ineligible to be DR or BDR.


EXERCISE 6.1.3.5


Area 0 – Backbone AreaOther areas up to 65,635 – must connect to Area 0

Roles of routers in an OSPF Autonomous System:Area Border Router (ABR) - connects an area to the backbone

Autonomous System Border Router (ASBR) - router that connects an area to a different routing protocol

Activity 6.1.4.2


Design and Configure a Network Using Single-Area OSPF

OSPF configuration steps:

Enable OSPF + process ID

Advertise networks + wildcard mask and area ID

Activity 6.2.1.3

Lab 6.2.1.4



LSPs do not need to be sent periodically.

An LSP only needs to be sent:•During initial startup of the router or of the routing protocol process•Whenever there is a change in the topology, including a link going down or coming up, or a neighbor adjacency being established or broken

Other information included in the LSP •sequence numbers and aging information - used by each router to determine if it has already received the LSP from another router - allows a router to keep only the most current information in its link-state database.



• Before two routers can form an OSPF neighbor adjacency, they must agree on three values: Hello interval, Dead interval, and network type

• Every 10 seconds on multiaccess and point-to-point segments• OSPF Hello packets are sent as multicast to an address reserved for

ALLSPFRouters at 224.0.0.5



The router selects the DR based on the highest value of any one of the following parameters, in the sequence listed:

1. Interface Priority: The interface priority is set with the priority command.

2. Router ID: The router ID is set with the OSPF router-id configuration command.

3. Highest Loopback Address: The loopback interface with the highest IP address is used as the router ID by default. OSPF favors loopback interfaces since they are logical interfaces and not physical interfaces. Logical interfaces are always up.

4. Highest Physical Interface Address: The router uses the highest active IP address from one of its interfaces as the router ID. This option poses a problem if interfaces go down or are reconfigured.

After changing the ID of a router or interface priority, reset neighbor adjacencies. Use the clear ip ospf process command. This command ensures that the new values take effect.



Tune OSPF parameters:

Interface priority

Router ID

Loopback and interface addresses

Bandwidth (cost )

Use the clear ip ospf process command to ensure that the new values take effect



When the DR is elected, it remains the DR until one of the following conditions occurs:

The DR fails.

The OSPF process on the DR fails.

The multiaccess interface on the DR fails.

If the DR fails, the BDR assumes the role of DR and an election is held to choose a new BDR.

If a new router enters the network after the DR and BDR have been elected, it will not become the DR or the BDR even if it has a higher OSPF interface priority or router ID than the current DR or BDR.



Packet Tracer Lab – DR & BDR Exploration 3 – 11.4.3.2

Packet Tracer Lab – Fine Tuning OSPF

Exploration 3 – 11.5.2.3



Verification and troubleshooting commands:

show ip ospf neighbor

show ip protocols, show ip route

show ip ospf, show ip ospf interface


The default configuration of OSPF exchanges information between neighbors in plain text.

A hacker on a network could use packet sniffing software to capture and read OSPF updates and determine network information.

When authentication is enabled in an area, routers will only share information if the authentication information matches.

Authentication protects integrity of routing information

Type 1 - Simple password authentication

Type 2 - Message Digest 5 (MD5)


Type 2 - Message Digest 5 (MD5)

Requires a key (password) and a key ID on each router.

Lab – 6.2.2.2


Work with Multi-Protocol Environments



Default route configured on ASBR

ASBR distributes the route into the OSPF network



Benefits of OSPF summarization:

Reduces number of networks advertised

Reduces memory requirements

Reduces number of entries in router updates

Isolates flapping and other problems to their location



To configure an OSPF ABR router to summarize these networks to another OSPF area, issue the following command in router configuration mode:

area area-id range ip-address ip-address-mask


Work with Multi-Protocol Environments OSPF limitations:

Increased router memory and processing demands

Strict design requirements

Knowledgable administrator required

Initial discovery process takes up network bandwidth



Administrative distance (AD) determines routes that appear in routing table


Summary OSPF is a classless interior link-state routing protocol

used in enterprise networks

OSPF uses bandwidth to generate the cost metric

OSPF routers elect a DR and BDR on multi-access networks

Router ID or router priority can be used to dictate the selection of DR and BDR

The OSPF network command uses a wildcard mask

Default route distribution and inter-area route summarization are used in OSPF networks

routing with a link-state protocol

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