OSPF Network Types | Broadcast and Point To Point Networks

Summary

This script delves into OSPF (Open Shortest Path First) networking, focusing on OSPF neighbors and network types. It explains the distinction between broadcast and point-to-point networks, the default election of Designated Routers (DR) and Backup Designated Routers (BDR) to optimize OSPF message traffic, and the importance of matching OSPF network types for stable neighbor adjacencies. The script also covers how to force an OSPF election and the potential inefficiencies of using broadcast network configurations in point-to-point scenarios.

Takeaways

• 🌐 OSPF (Open Shortest Path First) is a protocol used for routing and can operate on various network types, including point-to-point and broadcast networks.
• 🔗 In a point-to-point OSPF network, two routers are directly connected, commonly seen in WAN connections, and it's a simple model with only one recipient for each message.
• 📡 Broadcast networks in OSPF are typically connected via a switch and can include multiple routers and devices, such as when a router connects to a VLAN on a switch.
• 📶 Ethernet is a common protocol for broadcast networks, characterized by its use of broadcasts, which OSPF routers can leverage to communicate.
• 📣 OSPF does not use traditional broadcast messages but instead utilizes multicast to send Hello messages to a specific multicast address (224.0.0.5) for neighbor discovery.
• 🤖 To manage the OSPF communication efficiently in broadcast networks, OSPF elects a Designated Router (DR) and a Backup Designated Router (BDR) to minimize the number of messages exchanged.
• 🔄 When a router introduces a new network, it sends Link State Advertisements (LSAs) only to the DR and BDR, which then distribute the information to other routers, reducing network traffic.
• 🆔 Each OSPF router has a priority, which can be adjusted to influence the election of DR and BDR. The router with the highest priority becomes the DR, followed by the one with the next highest.
• 🔄 OSPF elections are not preemptive, meaning a change in priority does not automatically trigger a new election. An election can be forced by clearing the OSPF process on the current DR or BDR.
• 🔄 If the DR fails, the BDR is promoted to DR, and a new BDR is elected from the remaining routers. If the new BDR is needed, another election is held.
• 🛠 The OSPF network type can be manually changed using the 'ip ospf network' command, but it must match on all routers in the segment to maintain neighbor adjacency.

Q & A

What are the two main OSPF network types discussed in the script?

The two main OSPF network types discussed in the script are broadcast and point-to-point.

What is the significance of the OSPF multicast address 224.0.0.5?

224.0.0.5 is OSPF's special multicast address used for sending Hello messages, allowing OSPF routers to discover each other on a network.

Why does OSPF use multicast instead of broadcast messages?

OSPF uses multicast to send Hello messages to 224.0.0.5, which allows OSPF routers to communicate without sending messages to all devices on the network, thus reducing network traffic.

What is the role of the Designated Router (DR) in an OSPF network?

The Designated Router (DR) is responsible for managing the OSPF communication on a broadcast network. It receives Link State Advertisements (LSAs) from routers adding new networks and then distributes this information to other routers.

What is the Backup Designated Router (BDR) and what happens when the DR fails?

The Backup Designated Router (BDR) is a secondary router that steps in if the DR fails. When the DR fails, the BDR is promoted to DR, and one of the DR others is promoted to BDR.

How does OSPF handle the election of DR and BDR?

OSPF elects a DR and BDR based on router priority. The router with the highest priority becomes the DR, and the next highest becomes the BDR. If routers have the same priority, the router ID is used to break the tie.

What is the purpose of the 'show ip ospf neighbor' command?

The 'show ip ospf neighbor' command is used to display information about OSPF neighbors, including their roles as DR, BDR, or DR other.

How can you change the OSPF priority on a router?

You can change the OSPF priority on a router by entering interface configuration mode and using the command 'ip ospf priority' followed by the desired priority value.

Why might changing the OSPF priority not immediately affect the DR and BDR election?

OSPF elections are not preemptive, meaning changing the priority does not trigger an immediate election. To force an election, you need to clear the OSPF process on the current DR and BDR routers.

What happens if a router changes its OSPF network type to point-to-point in a broadcast network?

If a router changes its OSPF network type to point-to-point in a broadcast network, it will cause the neighbors to drop the adjacency as the network types must match for OSPF communication to function correctly.

Outlines

00:00

🌐 OSPF Neighbors and Network Types Overview

This paragraph discusses the concept of OSPF neighbors and the different types of networks they can be part of. It explains that OSPF operates on a small scale, connecting just two routers, or a larger scale involving multiple routers and devices on a switch. The four OSPF network types are introduced: broadcast, point-to-point, point-to-multipoint, and non-broadcast multi-access (NBMA). For the CCNA exam level, the focus is on broadcast and point-to-point networks. The paragraph emphasizes the simplicity of point-to-point networks, commonly used in WAN connections, and the use of Ethernet as the most common protocol, which relies on broadcasts. It clarifies that OSPF uses multicast instead of broadcast for communication, specifically addressing 224.0.0.5 for OSPF Hello messages. The designated router (DR) and backup designated router (BDR) election process is introduced to optimize OSPF message traffic on broadcast networks, with the DR and BDR handling the flood of LSA and LSR messages to prevent network performance issues.

05:02

🛠️ Manipulating OSPF Designated Router Elections

This paragraph delves into the process of influencing the OSPF designated router (DR) and backup designated router (BDR) elections. It explains that routers can be configured with a priority to affect the election, with the default priority being one and the ability to set it to zero to prevent a router from becoming a DR or BDR. The paragraph clarifies that OSPF elections are not preemptive, meaning a change in priority does not automatically trigger an election. To force an election, the OSPF process must be cleared on the current DR and BDR routers, not the newly configured one. The paragraph also discusses the implications of changing the OSPF network type on a router's interface, emphasizing that all neighbors must have the same network type configured. Mismatches in network types can cause adjacency problems, as demonstrated by the logs showing repeated formation and dropping of neighbor relationships. The importance of matching network types for OSPF operation is highlighted, along with a brief mention of a lab exercise at the end of the video for further exploration of changing OSPF network types.

Keywords

💡OSPF Neighbors

OSPF Neighbors refer to routers that are part of the same OSPF process and exchange routing information. In the video, it is discussed how OSPF neighbors can be part of a small network connecting just two routers or a larger network involving multiple routers and devices. The concept is central to understanding OSPF operations as it forms the basis for OSPF's ability to build a routing table.

💡Network Types

OSPF categorizes networks into types to determine how routing information is exchanged. The script mentions four types: broadcast, point-to-point, point-to-multipoint, and non-broadcast multi-access (NBMA). For the CCNA exam level, the focus is on broadcast and point-to-point networks. Understanding network types is crucial for OSPF configuration and operation, as it impacts the way routers communicate and establish adjacencies.

💡Point-to-Point

A point-to-point network is a direct connection between two routers, commonly used in WAN connections. The script explains that in a point-to-point network, a message sent by one router is intended for only one other router, making it a straightforward model. This network type is significant in OSPF as it simplifies the routing process by eliminating the need for multicast addressing.

💡Ethernet

Ethernet is a common network protocol used in local area networks, which the script mentions as the most common protocol for OSPF networks. It is characterized by its use of broadcasts, which OSPF routers leverage by sending multicast Hello messages to a specific multicast address to discover and communicate with OSPF neighbors.

💡Multicast

Multicast is a method of sending messages to a group of destinations simultaneously at the data link layer. In the context of OSPF, multicast is used instead of broadcast to communicate Hello messages to the special multicast address 224.0.0.5, allowing OSPF routers to identify each other without flooding the network with broadcasts.

💡Designated Router (DR)

In OSPF, the Designated Router is a router elected on a broadcast network to manage the exchange of OSPF information and reduce the number of messages required. The script explains that the DR and the Backup Designated Router (BDR) are responsible for collecting and disseminating OSPF Link State Advertisements (LSAs), which optimizes network performance by minimizing OSPF traffic.

💡Backup Designated Router (BDR)

The Backup Designated Router is a secondary router elected in OSPF to take over the DR's responsibilities if the DR fails. The script illustrates the importance of the BDR in maintaining OSPF operations by providing a seamless transition in case of a DR failure.

💡LSA (Link State Advertisement)

LSAs are packets of information that describe the state of a router's links to other routers. The script describes how a router floods an LSA to its neighbors when a new network is added, which is a fundamental part of OSPF's method for route advertisement and propagation.

💡LSR (Link State Request)

An LSR message is sent by OSPF routers to request more information about a specific LSA. The script mentions that after neighbors learn about a new route from an LSA, they may use LSR to get detailed information, which is part of the OSPF process to ensure all routers have the most up-to-date routing information.

💡OSPF Priority

OSPF priority is a configurable value that influences the election of the DR and BDR on a broadcast network. The script explains that routers with higher priority are more likely to be elected as DR or BDR, and changing this priority can influence the election process, although it does not trigger an immediate election.

💡Interface Configuration

Interface configuration in OSPF involves setting specific parameters for an interface to influence OSPF operations. The script discusses how changing the OSPF priority or network type on an interface can affect OSPF neighbor relationships and elections, emphasizing the importance of consistent interface configurations for proper OSPF operation.

Highlights

OSPF neighbors can be on a small network connecting just two routers or a larger network with multiple routers and devices.

There are four OSPF network types: broadcast, point-to-point, point-to-multipoint, and non-broadcast multi-access (NBMA).

For CCNA, focus on broadcast and point-to-point OSPF network types.

Point-to-point OSPF is common in WAN connections and is simple as only one router can receive a message from another.

Ethernet is the most common protocol for OSPF broadcast networks, which use broadcasts for communication.

OSPF routers use multicast to send and receive Hello messages on the special multicast address 224.005.

OSPF can experience performance issues with a large number of routers due to the volume of messages in broadcast networks.

OSPF elects a Designated Router (DR) and a Backup Designated Router (BDR) to manage communication on broadcast networks.

Routers with the highest priority become DR and BDR, with priority defaulting to one but can be adjusted.

DR and BDR routers use multicast address 224.006 to receive LSAs from other routers, reducing OSPF message traffic.

Use 'show IP ospf neighbor' to identify the roles of OSPF neighbors in a network.

Changing a router's OSPF priority does not trigger an immediate election for DR or BDR.

OSPF elections are not preemptive; you must force an election by clearing the OSPF process on the current DR and BDR routers.

Even if a router has a higher priority, it may not become the DR if the current DR and BDR are stable.

A point-to-point OSPF network may not necessarily be two directly connected routers; it could be on the same VLAN.

Changing the OSPF network type to point-to-point can improve efficiency in certain topologies.

Mismatched OSPF network types between neighbors will cause adjacency to fail and require correction.

Review of designated and backup designated routers in OSPF concludes the discussion.

Transcripts

let's think about ospf neighbors again

as we've seen there is a network between

Neighbors

this could be a very small Network only

enough to connect two routers together

and nothing more

or it could be a bigger Network there

could be several routers connecting to a

switch

there might even be other devices on the

same switch

from ospf's perspectives

that's only two of the possible Network

types

in total there are four Network types

there's broadcast like when the routers

are connected to a switch

there's point to point when two routers

directly connect to each other

there's also point to multi-point

and non-broadcast multi-access or nbma

for a CCNA exam level we're only going

to be worried about the first two

broadcast and point to point so you can

ignore the other two for this series

point-to-point is the simplest one to

understand

one router directly connects to another

this is more common with a Wan

connection that is a connection from one

site to another

when one router sends a message there's

only one other router that could

possibly receive it

this is a nice and uncomplicated model

on the downside we can't add more

routers to the network we can only

create more networks and add routers

there

the most common Network protocol would

have to be ethernet

one key feature of ethernet is that it

uses broadcasts

when an ospf router connects with an

Ethernet interface it will be on a

broadcast network

this includes connecting a router to a

VLAN on a switch

the key points for an Ethernet Network

are

they can be more than one device on the

network

when a router sends a message other

devices may see it

and it scales well as it's easy to add

more routers

there can be other devices on the same

VLAN not only routers

so how does a router know which of these

devices are ospf routers

it would be fair to think that the

router would send a broadcast message to

everything it is a broadcast network

after all

that's not right though ospf doesn't

actually use broadcast messages

it uses multicast

this enables ospf to send a Hello

message to

224.005 which is ospf's special

multicast address

other ospf routers will listen for

messages sent on this address

when they receive the Hello message

they're able to respond to the sender

let's think about a problem that could

occur on a broadcast network

we'll then see how ospf solves this

when a router adds a new network it

floods an LSA to its neighbors this is

how it advertises this network

the neighbors would then request more

information with an LSR message

once they learn the new route they would

tell their neighbors and so on

that's a lot of messages flowing back

and forth

imagine how bad it would be if there

were 20 or 30 routers here this many

messages has the potential to impact the

performance of the network

but ospf is smarter than that

for every broadcast network ospf will

elect one router to be the designated

router or Dr

it will also elect one backup designated

router or bdr

all other routers are called Dr others

each ospf router has a priority which is

set to one by default

of course we can change the priority if

we want to

the router with the highest priority

becomes the Dr and the next highest is

the bdr

if some routers have the same priority

like they do by default the highest

router ID breaks the tie

if a Dr fails the bdr is promoted

then one of the Dr others will be

promoted to bdr

how does that solve the problem

well when a router adds a new network

and it sends out the lsas they aren't

sent to every neighbor

instead they're only sent to the Dr and

bdr

this uses multicast address

224006 which only the Dr and bdrs listen

to

they will then use an LSR to request

more information as normal

the Dr will then distribute this

information out to other routers that's

their Dr others on the network

this cuts down the number of ospf

messages on a broadcast network

it's easy to see what roles our

neighbors have on a router we can look

at the neighbors with show IP ospf

neighbor

this is a topology with five routers so

we will see four neighbors

in the state column we can see that one

of the routers is Dr one is bdr and the

other two are Dr other

that makes this router a Dr other as

well

what if we want to make this route of

the Dr

to do this we enter configuration mode

and then interface configuration mode

remember that a Dr and bdr are elected

for each broadcast segment

that means that these settings can vary

per interface

for example our router could be Dr Rover

on this network and bdr on some other

network

we need to change the priority to

influence the election

the command is ipospf priority

the default priority is one

we'll set this router to 100.

we can also set the priority to zero

this would mean that the router would

never become a Dr or bdr

if we take a look at our neighbors again

we will see that nothing has changed

so what's wrong

nothing it may be surprising to hear

that this is actually what's supposed to

happen

let me explain

ospf elections are not preemptive

that means that changing priority does

not trigger an election

so adding a router with a better

priority will not immediately change the

Dr and bdr

this means we need to force an election

we can do this by clearing the ospf

process

but we don't do it on the router we just

configured

that won't trigger an election

instead we need to do this on the

current Dr and bdr routers

let's head over to R5 which is the

current designated router

here we'll clear the ospf process which

causes the names to drop and reform

back on R1 we can look at the neighbors

again

we see that the router R5 that's 192.168

10.5 is now a Dr other

router R4 has been promoted from BDO to

Dr

so even though there's an election

process R1 hasn't become the Dr

when the designated router drops out the

bdr gets a promotion

the routers hold an election to select

the new bdr

you'll notice that there's no bdr in the

list here that's because R1 the router

we're logged into is now the bdr

for R1 to become the Dr now we would

need to restart the ospf process on R4

let's take a step back and think about

this simple topology

what type of ospf network is this

you might assume that it's point to

point but that's not necessarily true

these two routers could connect to a

switch and be in the same VLAN

that would be an ethernet connection and

therefore a broadcast network

or they could be directly connected

together

but the interface type might still use

ethernet

that would still be a broadcast network

as far as ospf is concerned

even though there are only two routers

they would still need to elect a Dr and

bdr and handle LSUS in the manner we've

been discussing

of course that's not very efficient so

if we want to we can change the ospf

network type

let's go back to R1 and see how that's

done

from before we can see that there are a

few routers here

this is not a very good topology to

change the network type as there are

five routers in the segment

but I want to show you how to make the

change as well as what happens if you

use the wrong network type

once again we configure this under the

interface

we use the ipospf network command

and here we can see the network types we

can choose

we'll set this interface to point to

point

straight away the neighbors drop the key

Point here is that neighbors need to

have the same network type

let's look at the R5 router

the logs on the screen show us that the

neighbor adjacency with R1 repeatedly

forms and drops

we can change the network type on this

router too it's the same command as

before

immediately the other neighbors fail

they keep trying to reconnect but

they'll never be successful as long as

the network types don't match

we'll take a look at a better example of

changing the network type when we get to

the lab at the end of the video

before we move on here's a little review

to help with designated routers and

backup designated routers