Routing Protocols and Concepts: Chapter 11 ppt

Hello Protocol  Type: OSPF packet type: Hello Type 1, DBD Type 2, LS Request Type 3, LS Update Type 4, LS ACK Type 5  Router ID: ID of the originating router  Area ID: Area from whi

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Chapter 11

OSPF

quangkien@gmail.com

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 For further explanation

and details, please read

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 The router ospf command

 The network command

 DR/BDR Election Process

 OSPF Interface Priority

 More OSPF Configuration

 Redistributing an OSPF

Default Route

 Fine-tuning OSPF

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Introduction to OSPF

 Background of OSPF

 OSPF Message Encapsulation

 OSPF Packet Types

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Introduction to OSPF

 OSPF is:

 Classless

 Link-state routing protocol

 Uses the concept of areas for scalability

 RFC 2328 defines the OSPF metric as an arbitrary value called cost

 Cisco IOS software uses bandwidth to calculate the OSPF cost metric.

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Background of OSPF

 1987 - Initial development by IETF OSPF Working Group.

 1989 - OSPFv1 was published in RFC 1131.

 1991 - OSPFv2 was introduced in RFC 1247 by John Moy

 ISO was working IS-IS

 IETF chose OSPF as its recommended IGP (interior gateway

protocol).

 In 1998 - OSPFv2 specification was updated in RFC 2328 and is the

current RFC for OSPF.

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OSPF Message

Encapsulation

 This data field can include one of five OSPF packet types

 In the IP packet header:

 Protocol field is set to 89 (OSPF)

 Destination address is typically set to one of two multicast addresses:

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OSPF Packet

Types

 Five types of OSPF LSPs (link-state packets).

 Hello: Used to establish and maintain adjacency.

 DBD (Database Description): Abbreviated list of the sending router’s

link-state database

 LSR (Link-State Request) : Used by routers to request more information

about any entry in the DBD

 LSU: (Link-State Update): Link-state information.

 LSAck (LSA Acknowledgment): Router sends a link-state (LSAck) to

confirm receipt of the LSU

Figure includes CCNP information

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 Hello Interval, Dead Interval, Network Type

 Elect the Designated Router and Backup Designated Router on

multiaccess networks such as Ethernet and Frame Relay

Hello packets :

More in later

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Hello

Protocol

 Type: OSPF packet type: Hello (Type 1), DBD (Type 2), LS Request (Type

3), LS Update (Type 4), LS ACK (Type 5)

 Router ID: ID of the originating router

 Area ID: Area from which the packet originated

 Network Mask: Subnet mask associated with the sending interface

 Hello Interval: Number of seconds between the sending router’s Hellos

 Router Priority: Used in DR/BDR election (discussed later)

 Designated Router (DR): Router ID of the DR, if any

 Backup Designated Router (BDR): Router ID of the BDR, if any

 List of Neighbors: Lists the OSPF Router ID of the neighboring router(s)

These will be discussed

throughout this chapter.

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 Before an OSPF router can flood its link states, must discover neighbors

 Includes the OSPF Router ID (later)

 Receipt confirms there is another OSPF router on this link

 Adjacency is now established

 Routers are not considered fully adjacent, at this point each router is aware of the other OSPF router on the link

Neighbor

Establishment

Note: Full adjacency

happens after both

routers have exchanged

any necessary LSUs

and have identical

link-state databases

(CCNP)

More later

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Hello Intervals

 By default, OSPF Hello packets are sent:

 10 seconds on multiaccess and point-to-point segments

 30 seconds on nonbroadcast multiaccess (NBMA) segments (Frame

Relay, X.25, ATM)

 In most cases, OSPF Hello packets are sent as multicast to an address

reserved for ALLSPFRouters at 224.0.0.5.

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Dead Intervals

 Dead interval - Period, expressed in seconds, that the router will wait to

receive a Hello packet before declaring the neighbor “down.”

 Cisco uses a default of four times the Hello interval

 40 seconds - Multiaccess and point-to-point segments

 120 seconds - NBMA networks.

 Dead interval expires

 OSPF removes that neighbor from its link-state database

 Floods the link-state information about the “down” neighbor out all OSPF-enabled interfaces

 Network types are discussed later in the chapter.

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 Used to reduce the amount of OSPF traffic on multiaccess networks

 DR is responsible for updating all other OSPF routers.

 BDR is the backup if the current DR fails.

 R1, R2, and R3 are connected through point-to-point links

 No DR/BDR election occurs

 Much more later

More later

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OSPF Algorithm

 Each OSPF router maintains a link-state database containing the

LSAs received from all other routers

 When a router has received all the LSAs and built its local link-state

database, OSPF uses Dijkstra’s shortest path first (SPF)

algorithm to create an SPF tree

 The SPF tree is then used to populate the IP routing table with the

best paths to each network.

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Administrative Distance

 Administrative distance (AD) is the

trustworthiness (or preference) of

the route source

 OSPF has a default AD of 110.

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Authentication

 OSPF can be configured for authentication

 This practice ensures that routers will only accept routing information from other routers that have been configured with the same password or

authentication information

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Basic OSPF Configuration

 Lab Topology

 The router ospf command

 The network command

 OSPF Router ID

 Verifying OSPF

 Examining the Routing Table

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 Notice that the addressing scheme is discontiguous.

 OSPF is a classless routing protocol

 There are three serial links of various bandwidths and that each router

has multiple paths to each remote network

Topology

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 Does not have to match other OSPF routers

 This differs from EIGRP

 We are using the same process ID simply for consistency

R1(config)# router ospf 1

R1(config-router)#

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The network Command

 The network command (same function as when used with other IGP

routing protocols):

 Any interfaces on a router that match the network address in the

network command will be enabled to send and receive OSPF packets.

 This network (or subnet) will be included in OSPF routing updates

 Requires the wildcard mask

 Used to specify the interface or range of interfaces that will be enabled for

OSPF

Router(config-router)# network network-address wildcard-mask area area-id

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The network Command

 The wildcard mask can be configured as the inverse of a subnet mask

 R1’s FastEthernet 0/0 interface is on the 172.16.1.16/28 network

 The subnet mask for this interface is /28 or 255.255.255.240

 The wildcard mask would be 0.0.0.15

 Note:

 Like EIGRP, some Cisco IOS software versions allow you to simply

enter the subnet mask instead of the wildcard mask

 The Cisco IOS software then converts the subnet mask to the wildcard

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The network Command

 The area area-id refers to the OSPF area

 A group of OSPF routers that share link-state information

 All OSPF routers in the same area must have the same

link-state information in their link-link-state databases

 This is accomplished by routers flooding their individual link

states to all other routers in the area

 In this chapter, we configure all the OSPF routers within a single

area

 This is known as single-area OSPF.

 The network commands must be configured with the same area ID

on all routers

 Although any area ID can be used, it is good practice to use an area

ID of 0 with single-area OSPF

 This convention makes it easier if the network is later configured as

multiple OSPF areas where area 0 becomes the backbone area.

 Mult-Area OSPF is discussed in CCNP.

Router(config-router)# network network-address wildcard-mask area area-id

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The network Command

 network commands for all three routers, enabling OSPF on all interfaces

 At this point, all routers should be able to ping all networks

R1(config-router)# network 172.16.1.16 0.0.0.15 area 0

Area-ID must be the same on all routers

Router-ID does NOT have to

be the same on all routers

Wildcard mask must

be used

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 OSPF Router ID is an IP address used to uniquely identify an OSPF router.

 Also used in the DR and BDR process (later)

OSPF Router ID

Router ID?

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 Cisco routers derive the router ID based on three criteria and with the

following precedence:

1 Use the IP address configured with the OSPF router-id command.

2 If the router ID is not configured, the router chooses the highest IP

address of any of its loopback interfaces.

3 If no loopback interfaces are configured, the router chooses the highest

active IP address of any of its physical interfaces.

 The interface does not need to be enabled for OSPF, i.e it does not

need to be included in one of the OSPF network commands.

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 R1: 192.168.10.5, which is higher than either 172.16.1.17 or 192.168.10.1

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Verifying the Router ID

R1# show ip protocols

Routing Protocol is “ospf 1”

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Router ID 192.168.10.5

Router ID 192.168.10.9

Router ID 192.168.10.10

show ip ospf can also be used (later)

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Loopback Address

 The advantage of using a loopback interface is that, unlike physical

interfaces, it cannot fail

 Because the OSPF router-id command, which is discussed next, is a fairly recent addition to Cisco IOS software, it is more common to find loopback addresses used for configuring OSPF router IDs

Router(config)# interface loopback number

Router(config-if)# ip address ip-address subnet-mask

R1(config)# interface loopback 0

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1 Use the IP address configured with the OSPF router-id command.

2 Highest IP address of any of its loopback interfaces.

3 Highest active IP address of any of its physical interfaces.

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OSPF router-id Command

 The OSPF router-id command was introduced in Cisco IOS Software

Release 12.0(T) and takes precedence over loopback and physical

interface IP addresses for determining the router ID

1 Use the IP address configured with the OSPF router-id command

2 Highest IP address of any of its loopback interfaces

3 Highest active IP address of any of its physical interfaces

Router(config)# router ospf process-id

Router(config-router)# router-id ip-address

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Modifying the Router ID (Extra)

 The router ID is selected when OSPF is configured with its first OSPF

network command.

 If the OSPF router-id command or the loopback address is configured

after the OSPF network command, the router ID is derived from the

interface with the highest active IP address

 The router ID can be modified with the IP address from a subsequent OSPF

router-id command by reloading the router or by using the following

command:

Router# clear ip ospf process

 Modifying a router ID with a new loopback or physical interface IP address may require reloading the router

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Duplicate Router IDs

 When two routers have the same router ID in an OSPF domain, routing might not function properly

 If the router ID is the same on two neighboring routers, the neighbor establishment might not occur

 When duplicate OSPF router IDs occur, Cisco IOS software displays a message above

%OSPF-4-DUP_RTRID1: Detected router with duplicate router ID

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Verifying New Router IDs (Loopbacks)

Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Router ID 10.1.1.1

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Verifying OSPF

 The show ip ospf neighbor command enables you to verify and

troubleshoot OSPF neighbor relationships

R1# show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface 10.3.3.3 1 FULL/ - 00:00:30 192.168.10.6 Serial0/0/1 10.2.2.2 1 FULL/ - 00:00:33 192.168.10.2 Serial0/0/0

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Verifying OSPF

 Neighbor ID: The router ID of the neighboring router.

 Pri: The OSPF priority of the interface (later)

 State: The OSPF state of the interface

 FULL state means that the router’s interface is fully adjacent with its neighbor and they have identical OSPF link-state databases

 OSPF states are discussed in CCNP.

 Dead Time: The amount of time remaining that the router will wait to receive an

OSPF Hello packet from the neighbor before declaring the neighbor down

 This value is reset when the interface receives a Hello packet.

 Address: The IP address of the neighbor’s interface to which this router is

directly connected

 Interface: The interface on which this router has formed adjacency with the

neighbor

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Verifying OSPF

 Excellent command to begin troubleshooting

 Routers must first form an adjacency before link-state information can be exchanged

 Then routes will be added to the routing table

 Note: On multiaccess networks such as Ethernet, two routers that are adjacent

may have their states displayed as 2WAY

 This is discussed in a later section.

Tiêu đề	Routing Protocols and Concepts: Chapter 11 ppt
Tác giả	Rick Graziani, Allan Johnson
Thể loại	bài giảng

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