Each broadcast and Non-Broadcast Multi-Access (NBMA) network has a designated router. This router is the central point for collecting and distributing LSAs, so it must have enough CPU and memory capability to handle the workload. Configurations can be used to control the DR/BDR election process.
The designated router decreases routing protocol traffic by enabling a decrease in the number of adjacencies required on a broadcast or non-broadcast multi-access (NBMA) network.
Two routers connected to a multi-access network attempt to become the designated router. The router with the highest OSPF priority becomes the designated router. Suppose the interface priorities are the same on all routers. In that case, the router with the highest router ID is elected the DR. We can manually configure the router ID to manipulate the DR/BDR election.
Another way to manipulate the DR/BDR election process is the configuration of interface priorities. It is an interface-specific value that provides better control on a multiaccess network. This also allows a router to be the DR in one network and a DROTHER in others. The OSPF priority values are between 0 – 255. When the priority value is 0, the interface cannot become a DR or BDR. The router interface with a higher OSPF priority value becomes the DR and BDR. To manipulate the default priorities, use the following commands:
ip ospf priority <value>– OSPFv2 Command
ipv6 ospf priority <value>– OSPFv3 command
In Figure 1, all routers have the default OSPF priority. Therefore, the router ID decides the DR (R1) and BDR (R2). We can change the priority value on an interface from 1 to a higher value, which could change a specific router to become a DR or BDR during the next election. It is important to know that when OSPF is running and enabled, the administrator must shut down the OSPF process on all routers and then re-enable the OSPF process to force a new DR and BDR election.
Changing the OSPF Priority
In the topology in Figure 1, R1 is the DR, and R2 is the BDR. It has been decided that:
R1 should never be a DR or BDR and will be configured with a priority of 0.
R2 should be on the default priority value.
R3 should be the DR and will be configured with a priority of 200.
R4 should be the BDR and will be configured with the priority of 199.
Configuration on R1
R1>enable
R1#config terminal
R1(config)#interface gig 0/0
R1(config-if)#ip ospf priority 0
R1(config-if)#exit
R1(config)#exit
R1#clear ip ospf process
Configuration on R3
R3>enable
R3#config terminal
R3(config)#interface gig 0/0
R3(config-if)#ip ospf priority 200
R3(config-if)#exit
R3(config)#exit
R3#clear ip ospf process
Configuration on R4
R4>enable
R4#config terminal
R4(config)#interface gig 0/0
R4(config-if)#ip ospf priority 199
R4(config-if)#exit
R4(config)#exit
R4#clear ip ospf process
Also, clear the OSPF process on Router R2 and then check the output of the “show ip ospf interface” command. Figures 2 and 3 illustrate the output of this command for R3 (DR) and R4(BDR), even with small router IDs.
In the previous lesson, we discussed the challenges of the OSPF Multi-access network. The two challenges are unwanted adjacencies and the flooding of LSAs. The solution is the Designated Router (DR). DRs are designated to coordinate topology updates.
On multiaccess networks, at initialization, a Designated Router (DR) and a backup designated router (BDR) are elected. The DR is the collection and distribution point for LSAs sent and received.
The BDR listens passively to this exchange and maintains a relationship with all the routers. If the DR stops sending Hello packets, the BDR promotes itself and assumes the Designated Router (DR) role.
The update is sent to the Designated Router (DR) when a topology change occurs. Then, the DR will update all other routers in the area. Each router must update its neighbor if the Designated Router (DR) does not exist.
If the routers are interconnected, this can lead to multiple unnecessary updates. Having a Designated Router (DR) will help eliminate this problem.
In the OSPF network, a non-DR or non-BDR router becomes a DROTHER. DROTHERs only form full adjacencies with the DR and BDR in the network. The DROTHERs send their LSAs to the DR and BDR using the multicast address 224.0.0.6, which addresses all Designated Router (DR) routers.
Now look at Figures 1, R1 has been elected as the designated router for the LAN. The number of adjacencies has been reduced to 3, which is 6 without a Designated Router (DR).
DR/BDR elections only occur in multiaccess networks, not point-to-point ones.
Default DR/BDR Election Process
There are some criteria for a Designated Router (DR) and Backup Designated Router (BDR) election:
The first criteria for the Designated Router (DR) and backup designated router (BDR) election are the interface priorities. All OSPF-enabled routers in the network elect the router with the highest interface priority as the DR, and the router with the second-highest interface priority is elected as the BDR. The priority range is between 0 – 255. If the interface priority is set to 0, the router cannot become the DR. Serial interfaces have default priorities set to 0; therefore, they do not elect Designated Router (DR)and BDRs. The default interface priority of multiaccess broadcast interfaces is 1. Therefore, all routers have an equal priority value without configuring the interface priority, so this method must be finished on a tie.
So, if the first method is finished on tie due to the same interface priority value, the router with the highest router ID is elected the DR, and the router with the second-highest router ID is the BDR. We can configure the router ID manually. If the router ID is not configured, the heist loopback IP address becomes the router ID. If no loopback interfaces are configured, the highest active IPv4 address determines the router ID.
If we have a running IPv6 network and the router does not have IPv4 addresses configured, the router ID must be manually configured; otherwise, OSPFv3 does not start. The command for router ID configuration is “router-id rid.”
The DR and BDR election process starts when the first router with an OSPF-enabled interface is active on the multiaccess network. When the OSPF “network” command for the interface command is configured, the DR and BDR election process takes place. If the OSPF-enabled router is powered on, it also starts the election process, which only takes a few seconds.
Now, look at Figure 2. All Ethernet router interfaces have a default priority of 1. The OSPF router ID is used to elect the DR and BDR based on the above selection criteria. R1, with the highest router ID, becomes the DR, and R2, with the second-highest router ID, becomes the BDR.
DR/BDR Election Process
When a new router with a higher priority or higher router ID is added to the network after the DR and BDR election, this router does not take over the DR or the BDR role because those roles have already been assigned, and the new router does not initiate a new election process. After the DR is elected, it remains the DR until it fails or the OSPF process on the DR fails or is stopped, and if the multiaccess interface on the DR fails or is shut down.
When DR fails, the BDR automatically gets to the role of DR even if another DROTHER with a higher priority or router ID is added to the network after the initial DR/BDR election. So, when BDR is promoted to DR, a new BDR election process occurs, and the DROTHER with the higher priority or router ID is elected as the new BDR. In Figure 2, the R3 with the higher RID is selected as the DR, and the router with the second-highest RID is selected as the BDR.
In Figure 3 above, the current DR (R1) fails; therefore, the pre-elected BDR (R2) assumes the role of DR. Then, an election is held to choose a new BDR. R3 and R4 are the DROTHER, but R3’s RID is higher than R4’s, so R3 is elected as the BDR.
In Figure 4, R1 and R2 are both going to fail. In this situation, the existing BDR takes the role of DR, and only BROTHER, R4, is selected as the BDR.
In Figure 5, R1 rejoins the network but does not take over either the DR or BDR role; instead, it becomes a DROTHER.DR (R3) and BDR (R4) retain the DR and BDR roles. R1 automatically becomes a DROTHER.
Verifying DR/BDR Roles
When routers are connected over a common multiaccess broadcast network, OSPF automatically elects a DR and BDR. In the above example in Figure 1, R1 is elected as DR, and R2 is elected as BDR. Figures 6 to 9 illustrate the “show ip ospf interface” command output for all routers.
In the example, R1 was elected as the DR because the router ID of the R1 was the highest in this network. RW is the BDR because it has the second-highest router ID in the topology. The show ip ospf interface command can verify the OSPF router role.
Verifying DR/BDR Adjacencies
We can verify the OSPF neighbour adjacencies using the “show ip ospf neighbor” command. The state for neighbours adjacencies in the multi-access network are:
FULL/DROTHER- This is a DR or BDR router that is fully adjacent with a non-DR or BDR router. Both neighbours can replace Hello packets, updates, queries, replies, and acknowledgements.
FULL/DR- The router is fully adjacent with the indicated DR neighbour. The router can exchange Hello packets, updates, queries, replies, and acknowledgements with the DR.
FULL/BDR- The router is fully adjacent with the indicated BDR router. Both can exchange Hello packets, updates, queries, replies, and acknowledgements.
2-WAY/DROTHER- The non-DR or BDR router has a neighbour adjacency with another non-DR or BDR router. Both can exchange Hello packets.
Usually, FULL and 2-way are the normal states of an OSPF router. If a router displays a state other than FULL and 2-way, it means that there are some problems forming neighbours’ adjacencies.
Only DROTHERs form FULL adjacencies in multi-access networks with the DR and BDR. It forms a 2-WAY neighbour adjacency with other DROTHERs joining the network. When two DROTHER routers form a neighbour adjacency, the state displays as 2-WAY/DROTHER. Figure 10 illustrates the output of the “show ip ospf neighbours” command on all four routers.
OSPF defines five types of network. The types are point-to-point, Broadcast Multi-access, Non-broadcast Multi-access (NBMA), point-to-multi-point and virtual networks. The Multiaccess networks can create two challenges for OSPF regarding the flooding of LSAs:
Multiple adjacencies– Open shortest path first (OSPF) try to find the best path between the source and the destination router. There are multiple OSPF enabled routers may interconnect over a common link. The OSPF router tries to create adjacency with each OSPF enabled router. Creating adjacencies with each router is unnecessary and unwanted. This would lead to an unnecessary number of LSAs exchanged between routers on the same network.
Extensive LSA flooding– OSPF enabled the router to flood their link-state packets (LSP) when OSPF is initialized, or when there is a change in the network topology. This flooding can become excessive.
We can calculate the number of required adjacencies using the following formula.
n (n – 1) / 2
n is the number of OSPF enabled router in the network. The figure below illustrates the simple topology of three routers, all routers are attached to the same multiaccess Ethernet network. These routers will form six adjacencies:
n(n-1) /2
4(4-1)/2 =6
If we add a router to the multiaccess network, the adjacencies will increase dramatically. To decrease the adjacencies we required some special technique which will be discussed the incoming article.
We have already discussed the frame control field format in the previous lesson that the Frame Control field contains multiple subfields. The figure below illustrates the subfields of the frame control field.
Protocol Version– It is a 2-bit field that indicates the current version of the protocol. The receiving devices use this value to determine if the version of the protocol of the received frame is supported.
Frame TypeandFrame Subtype– The frame type is a 2-bit frame that determines the function of the frame i.e management(00), control(01), or data(10). The value 11 is reserved. The subtype is a 4-bit long field showing the sub-type of the frame like 0000 for association request, 1000 for the beacon. There are multiple subtype fields for each frame type. Each subtype determines the specific function to perform for its associated frame type.
ToDSandFromDS– Both frame field size is 1 bit long. The ToDS fieldset indicates that the destination frame is for DS(distribution system) and From DS field indicates the frame coming from DS. Both frames are only used in data frames of wireless clients associated with an AP.
More Fragments– It is a 1-bit long field Indicates whether more fragments of the frame, either data or management type, are to follow. If set to 1 means the frame is followed by other fragments.
Retry– The retry field is also 1 bit in the size. If the frame is set to 1, which indicates that the current frame is a retransmission of an earlier frame.
Power Management– The 1-bit long field Indicates whether the sending device is in active mode or power-saver mode. If the frame value is set to 1, it is mean that the station goes into power-save mode. If the field is set to 0, the station stays active.
More Data– The 1-bit long field indicates to a device in a power-save mode that the AP has more frames to send. It is also used for APs to indicate that additional broadcast/multicast frames are to follow.
Security– The 1-bit long field indicates whether encryption and authentication are used in the frame. It can be set for all data frames and management frames, which have the subtype set to authentication.
Reserved – It is a 1-bit long field that indicates that all received data frames must be processed in order. If the field value is set to 1 the received frames must be processed in strict order.
All Layer 2 frames contain a header, payload, and FCS section. The 802.11 frame format is similar to the Ethernet frame format, with the exception that it contains more fields. The figure below illustrates the wireless 802.11 frame formats and fields.
Frame Control– It is 2 bytes long field which determines the type of wireless frames. it is an important field of wireless 802.11 frame containing subfields including control information. The subfields are protocol version, frame type, address type, power management, and security settings, etc.
Duration– The 4-byte long field usually used to indicate the remaining duration needed to receive the next frame transmission.
Address 1 to 4– Each address field is 6 bytes long. The fields contain standard IEEE 802 MAC addresses (48 bit each). The meaning of each address depends on the DS bits in the frame control field. Address 1 is usually the MAC address of the receiving wireless device or AP.
Address 2 usually contains the MAC address of the transmitting wireless device or AP. The address 3 sometimes contains the MAC address of the destination, such as the router interface to which the AP is attached. Address 4 is usually missing because it is used only in ad hoc mode.
Sequence Control– The 16 bits long field contains the Sequence Number (12 bits) and the Fragment Number (4 bits) subfields. The Sequence Number indicates the sequence number of all frames. The Fragment Number indicates the number of each frame sent of a fragmented frame.
OSPF is a dynamic link-state routing protocol replacing the distance vector routing protocol, RIP. However, OSPF has important advantages over RIP. OSPF offers faster convergence and scales to much larger network implementations. The OSPF defines five types of OSPF networks:
Point-to-point—A point–to–point network is when two routers are interconnected over a common link without any other router. The figure below illustrates a point-to-point link.
Broadcast multiaccess—When multiple routers are interconnected over an Ethernet network, it is known as an OSPF Broadcast multi-access network. It contains multiple devices on the same shared media sharing data.
Ethernet LANs are the most common example of broadcast multi-access networks. Different hosts, printers, routers, and other devices are all members of the same network. The figure below illustrates the broadcast multi-access network.
Nonbroadcast multiaccess (NBMA)– When Multiple routers are interconnected in a nonbroadcast network known as Nonbroadcast multi-access (NBMA), such as Frame Relay, which does not allow broadcast. The figure below illustrates the NBMA.
Point-to-multipoint– This network contains a router connected with multiple sites. Usually, this is a hub-and-spoke topology over an NBMA network. The figure below illustrates the point-to-multipoint network.
Virtual links – A special OSPF network interconnects remote OSPF areas to the backbone area. The figure below illustrates the special virtual links. In the figure, area 10 cannot connect directly to the area0. A special OSPF area must be configured to connect area 10 to area 0. The router R1 and R2 area1 is a special OSPF virtual link for interconnecting area 10 and area 0
Wireless antennas are elements of a radio communications system that radiate and/or collect radiofrequency energy. They are usually connected using a low-loss coaxial cable to a power amplifier, splitter, and filter or directly to wireless devices. For outdoor usage, wireless antennas are often attached via mounting clamps to a mast or the side of a building via mounting brackets. Wireless Antennas used indoors are typically ceiling-mounted or wall-mounted.
A wireless antenna is also an important element of APs. Business-class APs use external wireless antennas for better results. Cisco uses different types of antennas with 802.11 APs. The type of antenna depends on deployment conditions, layout, and distance coverage. APs can be used. The figure below illustrates the general type of Wireless Antennas.
Omni Directional Wi-Fi Antennas
Omnidirectional antennas cover 360-degree areas and are better for open areas such as hallways, conference rooms, and outdoor areas. This type of antenna sends out and receives signals from all directions.
Omnidirectional antennas are commonly used on Wi-Fi routers and mobile adapters that support connections from several directions. Wi-Fi devices and APs usually use basic dipole antennas of the rubber duck design. These antennas usually have again between 2 and 9 dBi.
Directional Wi-Fi Antennas
Directional antennas spot the lobs in a specific direction and provide better gain. A directional Wi-Fi Antenna enhances the signal strength while receiving and transmitting.
The drawback of the directional antenna is that it sends and receives strong signals only in one direction and less signal strength in all other directions. Directional antennas are usually used to extend the range of a Wi-Fi network into hard-to-reach areas of buildings or in situations where 360-degree coverage is not required.
Yagi antennas
Yagi antenna is used for long-distance communication. A Yagi antenna is another directional radio antenna used for long-distance Wi-Fi networking in a specific direction. These antennas are extremely high gain used with outdoor APs to extend the gain up to 12 dBi or higher.
MIMO technology increases wireless bandwidth. The IEEE 802.11n/ac/ad standard uses MIMO technology, which uses several antennas to send and receive more data than possible using a single antenna.
Four antennas can increase throughput. All wireless routers are not the same. For example, 802.11n routers have a capacity of 150 Mbps using one Wi-Fi radio with one antenna. We can get more data using more than one radio and antenna in parallel. We can add more radios to the network in parallel. Each radio will increase the 150 Mbps data speed into the router.
Wireless Deployment is depending on the requirement of an organization. Some organizations want coverage for small are while other needed coverage for a large area. Cisco offers deployment both for small and large organizations.
Small Wireless Deployment
For small wireless deployment requirements, Cisco offers autonomous AP solutions. The different types of autonomous APs are the following:
Cisco WAP4410N AP– This AP is ideal for a small organization supporting a small group of users. The AP can be powered up from POE or AC directly using AD to DC adopter. The configuration interface is GUI.
Cisco WAP121 and WAP321 APs– WAP 121 and WAP 321 APs are perfect for small organizations want to install multiple APs on their wireless deployment. The APs are used as medium level business APs. These APs are also supporting clustering with a single point of setup. We can power up AP either from AC or from POE.
Cisco AP541N AP– AP541N is suitable for small- to mid-sized networks. The APs are a strong cluster and can be easily managed. It can be configured using its GUI interface. It is a controller-less clustering technology. The AC and PoE both option are available for power input.
The WAP121, WAP321, and AP541N APs support the clustering to provide a single point of administration without a controller. The administrator can easily view and manage the deployment of APs as a single wireless network. The clustering facility makes configuration and setup of these devices very easy. The easy administration help to grow the wireless network. A single configuration can be put into multiple APs within the cluster.
The management of the network is easy just like a single system without worrying about interference between APs, and without the configuration of each AP. The WAP121 and WAP321 have the feature Single Point Setup (SPS). The SPS AP wireless deployment very easy and the administrator can configure and deploy the network very fast
We can scale the using the SPS. It supports up to four WAP121 and up to eight WAP321. It provides broader coverage and we can add additional users according to the organization requirement. The Cisco AP541N AP can bunch up to 10 APs together and can support multiple clusters. The setting for clustering between two APs is the following:
Enabled cluster mode on the APs.
The APs radio mode must be the same.
The same Cluster Name must be used for all devices want to join the cluster.
All APs must be are connected to the same network segment.
Large Wireless Deployment Solutions
For larger organizations, Cisco provides control-based APs solutions. The controller-based APs are a better, reliable, and scalable option for a larger network. It supports many APs, including the Cisco Meraki Cloud Managed Architecture and the Cisco Unified Wireless Network Architecture.
Cisco Meraki Cloud Managed Architecture
The Cisco Meraki cloud architecture simplifies the wireless deployment. In the system, all APs are managed from a centralized controller in the cloud which provides control and visibility without the cost and difficulty. It also reduces management costs. We can centrally update firmware; configure security settings, wireless network, and SSIDs of all APs from a central location
The Meraki cloud infrastructure only allows flowing management data through the Meraki’s infrastructure and never allow user traffic. The Cisco Meraki cloud architecture requires the following:
Cisco MR Cloud Managed Wireless APs
Meraki Cloud Controller (MCC)
Web-based Dashboard
Different types of APs are available for wireless deployment which manageable from a centralized location. That is possible using MCC. The MCC is a cloud-based service that frequently monitors, optimizes, and reports the performance of the network. All configuration and diagnostics are done from the web-based dashboard.
Cisco Unified Wireless Network Architecture
It s used a split MAC design, controls APs using a WLAN controller also known as WLC. The components of this architecture are lightweight APs and Controllers. The lightweight APs communicate with the WLAN controller using the LWAPP. LWAPP is a special protocol for lightweight APs. The light Weight APs only process the incoming packet. The control has the power of the decision of where to communicate.
Cisco Aironet 1600, 2600, or 3600 wireless APs models provide the best wireless connectivity for hosts and Cisco 2500 Series Wireless Controllers provide small branch or single-site enterprise WLAN deployments with entry-level wireless for data communication. For greater capacity, the Cisco 5760 Wireless Controller and the Cisco 8500 Series Controller are perfect.
In the LAN (Wired Network) each client connects to a switch. The switch is the center point for each client to gain access to the network. The wireless Access Points also connects to the switch. Wireless clients discover nearby APs using their wireless NIC. The wireless Access Points are advertising their SSID for the client. Clients select wireless Access Points and attempt to connect using authentication. After being authenticated, wireless users can access network resources. The wireless Access Points has two types: autonomous APs and controller-based APs.
Autonomous APs
Autonomous APs are useful in situations where only a couple of APs are required in the network. We can configure autonomous APs using the Cisco CLI or a GUI. We can control multiple APs using wireless domain services (WDS) and managed using Cisco Works Wireless LAN Solution Engine (WLSE). The home router is an example of an autonomous AP. In the case of increased wireless demands, more APs would be required. Each AP has its network and operates independently from other APs. The APs can be independently configured. The figure below illustrates the Autonomous APs.
Controller-Based APs
It is server-dependent APs that not need any initial configuration. Controller-based APs are helpful where many APs are required in the network. When a new AP is added to the network, the controller automatically configures and manages the APs. The benefit of the controller is that it can be used to manage many APs. The figure below illustrates the controller-based APs.
A Wireless home router is a device that communicates between the internet and the devices in your home that connect to the internet. It routes traffic between the devices and the internet. Selecting the right kind of router for your home is very important. A wireless home router usually serves as:
Access point– It usually provides connectivity for 802.11a/b/g/n/ac wireless access
Switch—A Wireless home router also contains four full-duplex switch ports with a speed of 10/100/1000 to connect wired devices.
Router– The router server is the default gateway for connecting to other network infrastructures
A wireless router server is a small business or residential wireless access device. It is connected to the Internet Server Provider modem and shares internet services using RF signals. Internal devices wirelessly determine the router by its SSID and attempt to connect and authenticate with it to access the Internet. The SSID is the abbreviation of the Service Set Identifier. The figure below illustrates the workings of the wireless home router.
Most wireless routers provide advanced features, such as high-speed access, video streaming, IPv6 addressing, QoS, configuration utilities, and USB ports to connect printers or portable drives. The quality we should consider when choosing a good router is as follows:
Wi-Fi coverage
Wi-Fi signal availability mostly depends on the home’s size and the barriers preventing signals from reaching their destinations. So, look for a router that has the potential to reach the far-off corners of the home.
Wi-Fi performance
Router technology has changed occasionally, so it selects the latest technology and has updated firmware. The latest technology is the multi-user, multiple-input, multiple-output (MU-MIMO) technology. The MU-MIMO allows the WI-FI router to communicate with multiple devices simultaneously.
Wi-Fi security
Wi-Fi security is an important aspect. Cybercriminals can access your home network and copy your data, install malware and viruses on your devices, and access your personal and financial information. So, having a router providing network-level protection can help protect against cyber-attacks at the port of entry.
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