Routing Protocols and Concepts – Chapter 11 potx

OSPF: Hello Protocolneighbors a Backup Designated Router Type: OSPF Packet Type: Hello 1,DD 2, LS Request 3, LS Update 4, LS ACK 5 Router ID: ID of the originating router Area ID:

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OSPF (Single Area OSPF)

Routing Protocols and Concepts – Chapter 11

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Introduction to OSPF

OSPF Message Encapsulation

– There exist 5 types (next slide)

– Contains - Router ID an area ID

and Type code for OSPF packet

type

– Contains - Source IP address,

Destination IP address, & Protocol es a o add ess, & o oco

field set to 89 the destination

address is set to one of two

multicast addresses : 224.0.0.5 or

224 0 0 6

224.0.0.6.

– Contains - destination MAC address is

also a multicast address:

01-00-5E-00-00-05 or 01-00-5E-00-00-06.

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I t d ti t OSPF

5 OSPF Packet Types:

1 Hello - Hello packets are used to establish and

maintain adjacency with other OSPF routers

2 DBD - The Database Description (DBD) packet

contains an abbre iated list of the sending ro ter's

contains an abbreviated list of the sending router's

link-state database and is used by receiving

routers to check against the local link-state

database

3 LSR - Receiving routers can then request more

information about any entry in the DBD by sending

a Link-State Request (LSR)

4 LSU - Link-State Update (LSU) packets are

used to reply to LSRs as well as to announce new

5 LSAck - When an LSU is received, the router

sends a Link-State Acknowledgement (LSAck) to

confirm receipt of the LSU.

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OSPF: Hello Protocol

neighbors

a Backup Designated Router

Type: OSPF Packet Type: Hello (1),DD (2), LS Request (3), LS Update (4), LS ACK (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

l t )

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)

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Establish adjacencies:

– They must agree on three values: Hello Why 10 second hello interval

Also need to have the same Area ID.

interval , Dead interval , and network type

OSPF Hello Intervals

– Hello interval indicates how often an OSPF

router transmits its Hello packets

Why 10 second hello interval communications consider better than the 30 second routing update for RIP? router transmits its Hello packets

– Usually multicast (224.0.0.5) for

ALLSPFRouters

– sent every 10 seconds on multiaccess and y

point-to-point segments

– Sent every 30 seconds for NBMA segments

OSPF Dead Intervals

– This is the time that must transpire before the

neighbor is considered down

– Default time is 4 times the hello interval

– For multiaccess and point-to-point segments For multiaccess and point to point segments,

this period is 40 seconds

– For NBMA networks, the Dead interval is 120

seconds

If the Dead interval expires before the routers

– If the Dead interval expires before the routers

receive a Hello packet, OSPF will remove that

neighbor from its link-state database

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To reduce the amount of OSPF traffic onTo reduce the amount of OSPF traffic on

multiaccess networks, OSPF elects a

Designated Router (DR) and Backup

Designated Router (BDR) g ( )

Hello protocol packets contain information that

is used in electing DR and BDR

The DR is responsible for updating all other

– The DR is responsible for updating all other

OSPF routers (called DROthers) when a change

occurs in the multiaccess network

– The BDR monitors the DR and takes over as The BDR monitors the DR and takes over as

DR if the current DR fails

In the figure, R1, R2, and R3 are connected

through point to point links Therefore no More detail discussion

through point-to-point links Therefore, no

DR/BDR election occurs

– The DR/BDR election and processes will be

discussed in a later topic and the topology will

More detail discussion

on the DR, BDR, DROther later You need to know this for

discussed in a later topic and the topology will

be changed to a multiaccess network. need to know this for

CCNA exam

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OSPF Link-state Updates

–Used to deliver link state advertisements

–Contains information about neighbors & path costs

–An LSU packet can contain 11 different types of LSAs,

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received from other

2 SPF algorithm used to g

create SPF tree

3 SPF tree used to populate

routing tablerouting table

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Administrative Distance

Default Administrative Distance for OSPF is 110

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–It is good practice to authenticate transmitted It is good practice to authenticate transmitted routing information

– This is an interface specific configuration p g

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

configured with the same password or authentication information

MD5 authentication

k ID th t

Note: Authentication

does not encrypt the

router's routing table

uses a key ID that allows the router to reference multiple passwords, making

?

password migration easier and more secure

?

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Basic OSPF Configuration

– Since OSPF is a classless

routing protocol the subnet mask

is will be configured as part of our

OSPF configuration.

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The router ospf command

To enable OSPF on a router use the following

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OSPF network command

– Requires entering: q g

•network address

•wildcard mask - the inverse of the subnet mask

•area-id - area-id refers to the OSPF area OSPF area

is a group of routers that share link state information

Router(config-router)#networknetwork-address wildcard-askareaarea-id

Router(config router)#networknetwork address wildcard ask areaarea id

255.255.255.255

- 255.255.255.240 -

Subtract the subnet mask

-0 -0 -0 03 Wildcard mask

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Cisco IOS now properly handles overlapping network area configuration commands

Consider the following example: g p

ht lnetwork 0.0.0.0 255.255.255.255 area 0 process-are.html

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ospf network definition for adding all interfaces / default route

What’s the difference?

router ospf 1 network 0.0.0.0 0.0.0.0 area 0

vs

vs.

router ospf 1 network 0.0.0.0 255.255.255.255 area 0

Both add all existing interfaces into area 0 and all later added interfaces also Both statements are valid

http://blog.sazza.de/?p=427

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B i OSPF C fi ti

– 3 criteria for deriving the router ID

-Takes precedence over loopback and physical interface addresses

2 If router-id command not used then router chooses highest

IP address of any loopback interfaces

The interface does not need to be enabled for OSPF, meaning that it does not need to be included in one of the

meaning that it does not need to be included in one of the OSPF network commands

However, the interface must be active - it must be in the

up state

However!!!!!!

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kingdom is built, you are the

kingdom is built, you are the KING for life

is the ID for the router, unless

…………

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OSPF Router ID

Router ID (not configured) & Loopback addresses

(configured)

– Highest loopback address will be used as router ID

– Advantage of using loopback address the loopback

interface cannot fail Æ OSPF stability

The OSPF router-id command

– Introduced in IOS 12.0

– OSPF router-id command, which is a fairly recent

addition to IOS, it is more common to find loopback , p

addresses used for configuring OSPF router IDs.

– Command syntax

Router(config)#router ospf process-id

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

Modifying the Router ID

– Use the command Router#clear ip ospf process Use the command Router#clear ip ospf process

This command does not work in PT

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Modifying the Router ID y g

The router ID is selected when OSPF is

configured with its first OSPF network command

– If the OSPF router-id command or the loopback If the OSPF router id command or the loopback

address is configured after the OSPF network command, the router ID will be derived from the interface with the highest active IP address.

f

Modifying the Router ID

The router ID can be modified with

1 the IP address from a subsequent OSPF router-id

command b reloading the ro ter or command by reloading the router or

2 by using the following command:

Router#clear ip ospf process

3 Modifying a router ID with a new loopback or

physical interface IP address may require reloading the router

reloading the router

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Duplicate Router IDs

an OSPF domain, routing may not function

properly

– If the router ID is the same on two

neighboring routers, the neighbor establishment may not occur

will display a message similar to:

– %OSPF-4-DUP_RTRID1: Detected router

ith d li t t ID with duplicate router ID

so that they have unique OSPF router IDs

¾ Because some IOS versions do not support

the router-id command, we will use the

loopback address method for assigning

router IDs

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– 1 IP address is elected per router,

• Highest physical address (or)

• Highest logical address (loopback)

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Verifying OSPF

Use the show ip ospf command to verify &

trouble shoot OSPF networks:

Neighbor adjacency

Adjacency indicated by

The OSPF state of the interface is

“full state” full state

adjacency-•Neighbor ID - The router ID of the neighboring router.

•Pri - The OSPF priority of the interface

St t Th OSPF t t f th i t f FULL t t Consequence of no adjacency

No link state information exchanged

Inaccurate SPF trees & routing tables

•State - The OSPF state of the interface FULL state

means that the router and its neighbor have identical OSPF link-state databases.

•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

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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Note:

two routers that are adjacent may have their

– This will be discussed in a DR and BDR

section.

adjacency if:

– The subnet masks do not match, causing

the routers to be on separate networks. •Neighbor ID - The router ID of the neighboring router. •Pri - The OSPF priority of the interface

St t Th OSPF t t f th i t f FULL t t – OSPF Hello or Dead Timers do not match.

– OSPF Network Types do not match.

Th i i i i t OSPF

•State - The OSPF state of the interface FULL state

means that the router and its neighbor have identical OSPF link-state databases.

•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 – There is a missing or incorrect OSPF

network command.

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

Show ip protocols

– OSPF process ID, p ,

– the router ID,

– networks the router is advertising,

– the default administrative distance, 110 for OSPF.

Show ip ospf

– OSPF process ID

– router ID router ID

– OSPF area information

– the last time the SPF algorithm was calculated

•R1 has participated in during the past 11 and a half hours is to send small Hello packets to its neighbors

Show ip ospf interface

– The quickest way to verify Hello and Dead intervals

• for OSPF routers to become neighbors, their OSPF Hello and Dead intervals must be identical

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Configuring OSPF loopback address and router priority

The command show ip ospf interface will display the

interface priority value as well as other key information p y y

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Examining the routing table

Use the show ip route command to display the routing table

-An “O’ at the beginning of a route indicates that the router source is OSPF

-OSPF does not automatically summarize at major network

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– The lower the cost, the more likely the interface is

to be used to forward data traffic

The Cisco IOS uses the cumulative

b d idth f th t i i t f f

bandwidths of the outgoing interfaces from

the router to the destination network as the

cost value.

-Cost is based on bandwidth of an interface

Cost is calculated using the formula

10 8 / bandwidth -Reference bandwidth

The 100Mbps (FastEthernet) and higher will have the same OSPF cost of 1.

This reference bandwidth can be modified using

auto-cost reference-bandwidth command

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OSPF Metric

COST of an OSPF route is the accumulated value from one router to the destination network

•For example, in the figure,

the routing table on R1 shows

64 + 1 65

the routing table on R1 shows

a cost of 65 to reach the

R2 assigns the value 1 as

the cost for 10.10.10.0/24

•R1 then adds the

additional cost value of 64

to send data across the

default T1 link between

R1 and R2

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OSPF Metric

Sometimes the actual speed of a link is different than

the default bandwidth

Thi k it i ti th t th b d idth l fl t

– This makes it imperative that the bandwidth value reflects

link’s actual speed

Reason: so routing table has best path information

bandwidth

– Most serial link default to 1.544Mbps

– However, some serial interfaces may default to 128 kbps However, some serial interfaces may default to 128 kbps

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M dif i OSPF t t i

Modifying OSPF cost metric

OSPF uses cost as the metric for determining the

best route

Cost is calculated using the formula 10 8 /bandwidth ,

where bandwidth is expressed in bps (Cost =

100,000,000/Bandwidth)

The Cisco IOS automatically determines cost based The Cisco IOS automatically determines cost based

on the bandwidth of the interface

It is essential for proper OSPF operation that the

correct interface bandwidth is set

Router(config)#interface serial 0/0

Router(config-if)#bandwidth 64

The default bandwidth for Cisco serial interfaces is

1 544 Mbps or 1544 kbps 1.544 Mbps, or 1544 kbps

COD has these 2 types

of serial cards in the lab

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OSPF Metric: Bandwidth

Remember, this bandwidth value

does not actually affect the speed of

the link; it is used by some routing

protocols to compute the routing

metric

– It is important that the bandwidth

l fl t th t l d f th

value reflect the actual speed of the

link so that the routing table has

accurate best path information.

The figure displays the routing table g p y g

for R1

– R1 believes that both of its serial

interfaces are connected to T1 links,

f th li k i 64 kb li k

• one of the links is a 64 kbps link

• the other one is a 256 kbps link

– This results in R1's routing table

having two equal-cost paths to the

having two equal cost paths to the

192.168.8.0/30 network, when Serial

0/0/1 is actually the better path

How to modify the cost of all the links?

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