OSPF technology serial 6: OSPF multi-area, nearly 7000 words, very detailed!

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This article brings you the sixth article of OSPF technology serialization: OSPF multi-area. Ruige will introduce the theoretical knowledge points related to multi-area at the beginning, including principles, advantages, etc., and will focus on the configuration of multi-area, the old rules, The command examples of Huawei, Cisco, and Junifer are arranged.

Table of contents:

Article directory

OSPF (Open Shortest Path First) is an interior gateway protocol (IGP) that is widely used in wide area networks. It calculates the shortest path based on the Dijkstra algorithm to select the best path for data transmission. The multi-area function of OSPF enables network administrators to divide the network into multiple areas to improve scalability and management performance.

Basic Principles of OSPF Protocol

The OSPF protocol uses link state information to construct network topology, and calculates the shortest path by exchanging link state updates. Each OSPF router maintains a Link State Database (LSDB), which stores topology information for the entire network.

The concept and role of multi-region

Multi-area refers to the process of dividing the entire OSPF domain into multiple logical areas. Each area runs an independent SPF calculation, and the area border routers (ABRs) exchange routing information between areas.

The division of multiple regions has the following advantages:

Scalability : Dividing the network into multiple areas can reduce the size of the link state database and reduce the complexity of SPF calculation, thereby improving the scalability of the network.
Management performance : Through area division, network administrators can better organize and manage the network, and assign the responsibilities of area border routers and area internal routers.
Control routing information propagation : Routing information exchange between areas is carried out through area border routers, which can control the propagation range of routing information and improve network security and isolation.

OSPF area division

In OSPF multi-area configuration, dividing the network into multiple areas is a key step. The purpose of zone division is to improve the scalability and management performance of the network.

Regional division strategy

When dividing OSPF areas, we need to consider the network topology and performance requirements.

Here are some common zoning strategies:

Based on physical location : Divides the network into areas based on the physical location of routers. This division strategy is suitable for routers distributed in different geographical locations, such as in different offices, branch offices or data centers.
Function-based : Divide the network into areas based on different functional requirements. For example, the network can be divided into core areas, aggregation areas, and edge areas to meet the functional and performance requirements of different areas.
Service Provider Based : For Internet Providers (ISPs), the network can be divided into regions based on different customers or service types. Each zone can provide specific services, such as Internet access, virtual private network (VPN), and so on.

Example topology diagram and area division scheme

Let us use a sample topology diagram to demonstrate how to divide OSPF areas.

Based on the above sample topology diagram, we can divide the following areas:

Area 0 : As the backbone area, the core area connecting all other areas. Both R1 and R2 are Area Border Routers (ABRs) for this area.
Area 1 : includes R1 and the networks directly connected to it.
Area 2 : includes R1 and the network directly connected to it.
Area 3 : includes R2 and the network directly connected to it.

Through such area division, we can better manage and control the routing information in each area, and at the same time reduce the size of the link state database (LSDB) of the entire network.

Inter-area routing

In OSPF, each area has an area ID (Area ID), which is used to uniquely identify the area. Inter-area routing refers to the process of exchanging and transferring routing information between different areas. When a router wants to send data to another area, it will forward the data packet to the ABR in the area, and the ABR is responsible for delivering the data packet to the target router in the target area.

Area Border Router (ABR)

ABRs are routers that connect two or more areas. It has interfaces of two or more areas and is responsible for exchanging and forwarding routing information between these areas. ABR has the following main functions:

Inter-area routing information exchange : ABRs are responsible for exchanging routing information with ABRs in adjacent areas to ensure that routing information between different areas is transmitted.
Route aggregation : ABR aggregates routing information from different areas to reduce the amount of routing information in the entire domain. This improves the scalability of the network.
Application of routing policies : ABR can adjust and filter routing information according to the needs, so as to control the transmission range of routing information between different areas.
Maintenance of area boundaries : ABR is responsible for maintaining the connectivity of area boundaries to ensure normal communication between different areas.

Practical Case: Inter-area Routing Configuration

In the preceding example topology, R1 is the ABR of Area 0 and Area 1, and R2 is the ABR of Area 0 and Area 3. Now, let's look at how to configure inter-area routing.

For R1, it will configure the connection between the two regions. On R1, we need to configure the interfaces of area 0 and area 1 as ABR, and enable inter-area routing transmission. In this way, R1 can transfer routing information between area 0 and area 1.

On R2, you also need to configure the interfaces in area 0 and area 3 as ABRs, and enable inter-area route transmission. In this way, R2 can transfer routing information between area 0 and area 3.

By configuring ABR and enabling inter-area routing transmission, the routing information between different areas can be transmitted and forwarded correctly, realizing the connectivity of the entire OSPF multi-area network.

OSPF multi-area design

In OSPF multi-area configuration, designing an appropriate area structure is a key step. A well-designed multi-region network can provide high availability, redundancy, and flexibility.

considerations

When designing OSPF multi-area, the following factors need to be considered:

Network scale : According to the scale and complexity of the network, divide an appropriate number of regions. Reasonable area division can improve the scalability and management performance of the network.
High Availability : Design a multi-region structure with high availability to ensure that if a region or link fails, the network can still function normally.
Redundancy : Ensure network redundancy and fault tolerance by creating redundant paths between multiple regions. In this way, when an area or link fails, the network can automatically switch to an alternate path.
Location of area border routers (ABRs) : The locations of ABRs are chosen reasonably to minimize the exchange of routing information and delays between areas.
Security : When designing a multi-zone network, consider security measures such as inter-zone access control lists (ACLs), inter-zone encryption, etc. to protect the network from unauthorized access and attacks.

Practical Case: Multi-Region Design

Consider the following example topology:

In the example topology above, we could have the following design to satisfy the above considerations:

Area division : Divide the network into areas based on physical location and functional requirements. For example, divide an office building into one zone and a data center into another zone.
High availability and redundancy : multiple intra-region routers (IRs) in each region to provide redundant paths, and multiple ABRs between regions to provide redundant inter-region paths.
ABR location : Select the location of the ABR to make the exchange and delivery of routing information as efficient as possible. In the example topology, R1 and R2 are set up as ABRs, which connect multiple areas.
Security : Implement appropriate security measures such as inter-zone access control lists (ACLs) and encryption techniques to protect the security of inter-zone communications.

Through the above design, we can build an OSPF multi-area network with high availability, redundancy and flexibility.

Tuning an OSPF Multi-Area Network

When configuring and managing an OSPF multi-area network, tuning the network to improve performance is an important task. By optimizing the OSPF multi-area network, we can improve data transmission efficiency, reduce latency, and ensure network reliability and stability.

Adjust interregional connections

Inter-area connections are a key part of an OSPF multi-area network. By adjusting the parameters and attributes of inter-area connections, we can optimize the routing information transmission and convergence performance between areas.

Here are some ways to tune connections between regions:

Adjusting the link cost : By properly adjusting the cost (Cost) of the inter-area link, it can affect the preference of the router when calculating the shortest path. Allocating a lower cost to the preferred path can achieve load balancing and optimization of traffic.
Use area summary : summarize the routing information of multiple areas to reduce the amount of inter-area routing information. This reduces the size of the Link State Database (LSDB) and speeds up SPF calculations.
Use virtual link : In some cases, by establishing a virtual link (Virtual Link) between areas, the inter-area connection problem caused by the fact that the physical link is not directly connected can be solved.

Adjust Area Border Router (ABR)

Area Border Routers (ABRs) play a key role in OSPF multi-area networks. By adjusting the parameters and configuration of ABR, we can optimize the routing information exchange and forwarding performance between areas. Here are some ways to tune ABR:

Adjust the bandwidth and priority of the ABR interface : according to the bandwidth and importance of the inter-area connection, adjust the bandwidth and priority of the ABR interface appropriately. This can affect the priority and weight of the ABR in routing information forwarding.
Inter-area filtering and policy adjustment : By implementing inter-area filtering and routing policies on the ABR, you can control the transmission range and route selection of inter-area routing information.

Practical Cases and Tuning Suggestions

Here are some practical examples and tuning suggestions that can be used to tune OSPF multi-area networks:

Optimize inter-area link bandwidth : For inter-area links with high traffic, ensure that the link bandwidth is sufficient to meet traffic demands to avoid performance bottlenecks.
Regularly monitor the link state database (LSDB) : Regularly check and clean up the LSDB, and delete unnecessary, obsolete or invalid routing information to reduce the size of the LSDB.
Regularly perform link state update optimization : adjust the time interval and frequency of link state updates to reduce link state update overhead and improve network response performance.
Use Fast Hello Timer : Set the Hello Timer to a shorter interval to detect neighbor router failures and status changes more quickly.

Through the above tuning methods and suggestions, the performance and reliability of the OSPF multi-area network can be improved, and the efficiency and response time of data transmission can be improved.

Comparison of OSPF Multi-Area and Other Routing Protocols

Choosing the right routing protocol is critical when building and configuring a network. Different routing protocols have different characteristics and applicability in a multi-area environment.

Advantages of OSPF Multi-Area

First, let's review the advantages of OSPF multi-area:

Scalability : By dividing the network into multiple areas, OSPF can reduce the size of the link state database (LSDB) and improve the scalability of the network.
Management performance : Zone division makes network management more flexible and controllable. Administrators can better organize and manage routers and links in different areas.
Fast convergence : OSPF uses link state information to calculate the shortest path, enabling the network to quickly converge and adapt to topology changes.
Flexible policy control : OSPF supports rich routing policy control, administrators can adjust inter-area filtering and routing selection according to requirements.

Comparing Different Routing Protocols

Now, let's compare OSPF Multi-Area with other common routing protocols:

RIP : RIP is a distance vector routing protocol that measures the pros and cons of a path by the number of hops. Compared with OSPF, RIP performs poorly in terms of scalability and convergence, and is not suitable for large networks or complex topologies.
EIGRP : EIGRP is an enhanced interior gateway routing protocol that combines the properties of distance vector and link state. EIGRP performs well in terms of scalability and fast convergence, but it is a Cisco proprietary protocol and has poor interoperability with other vendors' equipment.
IS-IS : IS-IS is a link-state routing protocol, similar to OSPF. It scales well in large networks and has strong support for multiple regions. IS-IS is widely used in ISP and large enterprise networks.
BGP : BGP is a path vector routing protocol primarily used for Internet border routing. It is highly scalable and flexible, but more complex to configure and manage than Interior Gateway Protocol (IGP).

When selecting a routing protocol, it is necessary to comprehensively consider factors such as network scale, complexity, performance requirements, and supplier constraints.

OSPF multi-area configuration

Ruige will start from Huawei, Cisco, and Junifer. For H3C commands, please refer to Huawei’s commands. For Ruijie, please refer to Cisco’s commands.

Huawei

configuration preparation

Before starting the configuration, make sure you have the following preparations:

A Huawei router (for example, AR1200 series) and a switch (for example, S5700 series).
Understand the basic principles and multi-area functions of OSPF protocol.
Make sure you have the management rights of Huawei devices and basic command line configuration capabilities.

topology design

In this topology, we have a core area (Area 0) and three sub-areas (Area 1, Area 2, Area 3). Each sub-region is connected to the core region and each has some internal networks.

configuration steps

Step 1: Basic Configuration

First, we need to do basic device configuration.

Step 2: Configure Interregional Connections

In an OSPF multi-area network, inter-area connections are very important. We need to configure appropriate connections for each zone. The following is an example of configuring inter-area connectivity on a router:

Configure inter-area connections on R1

Create a virtual interface on R1

[R1] interface vlanif 10
[R1-Vlanif10] ip address 10.0.0.1 255.255.255.0
[R1-Vlanif10] ospf network-type p2p

Add virtual interface to zone 0

[R1] ospf 1
[R1-ospf-1] area 0
[R1-ospf-1-area-0] network 10.0.0.0 0.0.0.255

Add the physical interface of R1 to the corresponding zone

[R1] interface GigabitEthernet 0/0/0
[R1-GigabitEthernet0/0/0] ip address 192.168.1.1 255.255.255.0
[R1-GigabitEthernet0/0/0] ospf network-type p2p
[R1-GigabitEthernet0/0/0] ospf 1 area 1

Configure the inter-area connection between R1 and R2

[R1] ospf 1
[R1-ospf-1] area 1
[R1-ospf-1-area-1] network 192.168.1.0 0.0.0.255
[R1-ospf-1-area-1] network 192.168.2.0 0.0.0.255

Configure inter-area connections on R2

Create a virtual interface on R2

[R2] interface vlanif 10
[R2-Vlanif10] ip address 10.0.0.2 255.255.255.0
[R2-Vlanif10] ospf network-type p2p

Add virtual interface to zone 0

[R2] ospf 1
[R2-ospf-1] area 0
[R2-ospf-1-area-0] network 10.0.0.0 0.0.0.255

Add the physical interface of R2 to the corresponding zone

[R2] interface GigabitEthernet 0/0/0
[R2-GigabitEthernet0/0/0] ip address 192.168.2.1 255.255.255.0
[R2-GigabitEthernet0/0/0] ospf network-type p2p
[R2-GigabitEthernet0/0/0] ospf 1 area 2

Configure inter-area connections between R2 and R1 and R3

[R2] ospf 1
[R2-ospf-1] area 2
[R2-ospf-1-area-2] network 192.168.2.0 0.0.0.255
[R2-ospf-1-area-2] network 192.168.3.0 0.0.0.255

Step 3: Configure intra-area routing

Next, we need to configure intra-area routing within each area. The following is an example of configuring intra-area routing on a router:

Configure intra-area routing in Area 1

[R1] ospf 1
[R1-ospf-1] area 1
[R1-ospf-1-area-1] network 192.168.1.0 0.0.0.255
[R1-ospf-1-area-1] network 10.1.1.0 0.0.0.255

Configure intra-area routing in Area 2

[R2] ospf 1
[R2-ospf-1] area 2
[R2-ospf-1-area-2] network 192.168.2.0 0.0.0.255
[R2-ospf-1-area-2] network 10.2.2.0 0.0.0.255

Configure intra-area routing in Area 3

[R2] ospf 1
[R2-ospf-1] area 3
[R2-ospf-1-area-3] network 192.168.3.0 0.0.0.255
[R2-ospf-1-area-3] network 10.3.3.0 0.0.0.255

Step 4: Verify configuration

Finally, we need to verify that the configuration is working correctly. You can verify OSPF neighbor relationships and routing information with the following commands:

[R1] display ospf peer
[R1] display ospf routing-table
[R2] display ospf peer
[R2] display ospf routing-table

Make sure that the neighbor relationship is successfully established and that correct routing information appears in the routing table.

cisco

configuration preparation

Before starting the configuration, make sure you have the following preparations:

A Cisco router (for example, Cisco ISR series) and a switch (for example, Cisco Catalyst series).
Understand the basic principles and multi-area functions of OSPF protocol.
Make sure you have administrative privileges on Cisco devices and basic command-line configuration capabilities.

topology design

In this topology, we have a core area (Area 0) and three sub-areas (Area 1, Area 2, Area 3). Each sub-region is connected to the core region and each has some internal networks.

configuration steps

Step 1: Basic Configuration

First, we need to do basic device configuration.

Step 2: Configure Interregional Connections

Configure inter-area connections on R1

Add the physical interface of R1 to the corresponding zone

R1(config)# interface GigabitEthernet0/0
R1(config-if)# ip address 192.168.1.1 255.255.255.0
R1(config-if)# ip ospf 1 area 1

Configure the inter-area connection between R1 and R2

R1(config)# router ospf 1
R1(config-router)# network 192.168.1.0 0.0.0.255 area 1
R1(config-router)# network 192.168.2.0 0.0.0.255 area 1

Configure inter-area connections on R2

Add the physical interface of R2 to the corresponding zone

R2(config)# interface GigabitEthernet0/0
R2(config-if)# ip address 192.168.2.1 255.255.255.0
R2(config-if)# ip ospf 1 area 2

Configure inter-area connections between R2 and R1 and R3

R2(config)# router ospf 1
R2(config-router)# network 192.168.2.0 0.0.0.255 area 2
R2(config-router)# network 192.168.3.0 0.0.0.255 area 2

Step 3: Configure intra-area routing

Next, we need to configure intra-area routing within each area. The following is an example of configuring intra-area routing on a router:

Configure intra-area routing in Area 1

R1(config)# router ospf 1
R1(config-router)# network 192.168.1.0 0.0.0.255 area 1
R1(config-router)# network 10.1.1.0 0.0.0.255 area 1

Configure intra-area routing in Area 2

R2(config)# router ospf 1
R2(config-router)# network 192.168.2.0 0.0.0.255 area 2
R2(config-router)# network 10.2.2.0 0.0.0.255 area 2

Configure intra-area routing in Area 3

R2(config)# router ospf 1
R2(config-router)# network 192.168.3.0 0.0.0.255 area 3
R2(config-router)# network 10.3.3.0 0.0.0.255 area 3

Step 4: Verify configuration

Finally, we need to verify that the configuration is working correctly. You can verify OSPF neighbor relationships and routing information with the following commands:

R1# show ip ospf neighbor
R1# show ip route ospf
R2# show ip ospf neighbor
R2# show ip route ospf

Make sure that the neighbor relationship is successfully established and that correct routing information appears in the routing table.

Juniper

configuration preparation

Before starting the configuration, make sure you have the following preparations:

A Juniper router (for example, Juniper MX series) and a switch (for example, Juniper EX series).
Understand the basic principles and multi-area functions of OSPF protocol.
Make sure you have administrative privileges and basic command-line configuration capabilities for Juniper devices.

topology design

In this topology, we have a core area (Area 0) and three sub-areas (Area 1, Area 2, Area 3). Each sub-region is connected to the core region and each has some internal networks.

configuration steps

Step 1: Basic Configuration

First, we need to do basic device configuration.

Step 2: Configure Interregional Connections

Configure inter-area connections on R1

set interfaces ge-0/0/0 unit 0 family inet address 192.168.1.1/24
set protocols ospf area 1 interface ge-0/0/0.0

Configure the inter-area connection between R1 and R2

set protocols ospf area 1 interface ge-0/0/0.0
set protocols ospf area 1 interface ge-0/0/1.0

Configure inter-area connections on R2

Add the physical interface of R2 to the corresponding zone

set interfaces ge-0/0/0 unit 0 family inet address 192.168.2.1/24
set protocols ospf area 2 interface ge-0/0/0.0

Configure inter-area connections between R2 and R1 and R3

set protocols ospf area 2 interface ge-0/0/0.0
set protocols ospf area 2 interface ge-0/0/1.0

Step 3: Configure intra-area routing

Next, we need to configure intra-area routing within each area. The following is an example of configuring intra-area routing on a router:

Configure intra-area routing in Area 1

set protocols ospf area 1 interface lo0.0
set protocols ospf area 1 interface ge-0/0/1.0
set protocols ospf area 1 interface ge-0/0/2.0

Configure intra-area routing in Area 2

set protocols ospf area 2 interface lo0.0
set protocols ospf area 2 interface ge-0/0/0.0
set protocols ospf area 2 interface ge-0/0/2.0

Configure intra-area routing in Area 3

set protocols ospf area 3 interface lo0.0
set protocols ospf area 3 interface ge-0/0/1.0
set protocols ospf area 3 interface ge-0/0/2.0

Step 4: Verify configuration

Finally, we need to verify that the configuration is working correctly. You can verify OSPF neighbor relationships and routing information with the following commands:

show ospf neighbor
show ospf route

Make sure that the neighbor relationship is successfully established and that correct routing information appears in the routing table.

Summarize

This document introduces in detail what OSPF multi-area is and how to configure and deploy complex OSPF multi-area networks on devices from different vendors. We take Huawei, Cisco, and Juniper devices as examples to provide detailed configuration steps and topology examples.

During the configuration process, we first performed basic device configuration to ensure that the device can operate normally. We then configured the interzone connectivity, including adding physical interfaces to the appropriate zones and configuring the interzone connectivity. Next, we configured intra-area routing in each area to ensure that routing information in the area propagates normally.

Through the above steps, we have successfully configured a complex OSPF multi-area network. Finally, we use the verify command to check the neighbor relationship and routing information to ensure the correctness of the configuration and the normal operation of the network.

Regardless of whether Huawei, Cisco, or Juniper equipment is used, the basic principles and steps for configuring an OSPF multi-area network are roughly the same. Just make corresponding adjustments according to the command syntax and characteristics of different devices.

Configuring complex OSPF multi-area networks can improve network scalability, management performance, and fast convergence. The design of multi-area network can better organize and manage routers and links in different areas, and improve the reliability and stability of the network.