Network setup - How to - Routers - Cisco - Linux

Once you have the knowledge to configure the network using VLAN, it is necessary for users of different logical subnets to communicate with each other. This requires use routing between VLANs.

There are many ways to perform inter-VLAN routing, such as:

• Router on a Stick
• Traditional Routing
• Routing between VLANs with Linux Server

This time will be described a bit about how to configure routing between VLANs with Layer 3 switches, using for this a Catalyst 3750G-24PS switch, one of many Cisco switches that support Layer 3 routing.


The icon that represents the Layer 3 switch is different from the icons to represent a Layer 2 switch. In order to understand how the Layer 3 switch routing process does need to know a little about Switches Virtual Interfaces or SVI, which is what really allows routing between VLANs.

SVI

SVI is a logical interface configured for a specific VLAN. You need to configure an SVI for a VLAN if you want to route between VLANs. By default, an SVI is created for the default VLAN (VLAN 1) to permit remote switch administration.

Layer 3 Forwarding

A Layer 3 switch is capable of routing packets between VLANs. The procedure is the same as is used with a router, except that SVI act as router interfaces to route data between VLANs.


Configuring Interfaces (SVI)

The interface configuration is very simple, is similar to the configuration of the interface vlan 1, except that you must specify the vlan ID. A sample configuration for configuring the SVI interface Vlan 10.

SW1# configure terminal
Sw1(config)# interface vlan 10
Sw1(config-subif)# ip address 172.16.10.1 255.255.255.0
Sw1(config-subif)# no shutdown

Similarly for each VLAN is desired route, you must perform routing between VLANs using Layer 3 switches, improves the performance of traditional approaches, because these devices have better performance routers.

The only thing not to forget is, that the IP address of the SVI interfaces is the default gateway of the devices within each VLAN. In the example, the default gateway of Computers belonging to VLAN 10, will be the 172.16.10.1 (virtual interface on the switch).

The VLAN routing scheme, using Layer 3 switches can be as simple as the example shown here, or as complex as making distribution of SVI interfaces between multiple switches, or implementing a routing protocol such as RIP , BGP or EIGRP, for sharing routes.

To build a LAN, it is necessary to take into account many aspects to the network meets the needs of small and medium enterprises, ie designing the network is necessary to use techniques to get closer to success.

The hierarchical model has many benefits, as it allows the network more predictable. This, because the campus network is divided into layers and each layer defined in the services they provide. Compared to other network designs, a hierarchical network is managed and expanded more easily and resolve problems faster.

Among the advantages that exist to separate the networks in three levels, is that it is easier to design, implement, maintain and scale the network, plus it makes it more reliable, with better cost / benefit ratio. Each layer has specific functions assigned and not necessarily refer to a physical separation, but logic, so that various devices can have a single layer or a device by the features of more than one of the layers.

The layers and their typical functions are:

• The access layer, and controls the users access workgroup (workgroup access) or network resources. This layer performs the Ethernet switching. The access layer interfaces with end devices such as PCs, printers and IP phones, to provide access to the rest of the network. The access layer can include routers, switches, bridges, hubs, and wireless access points (AP). The main purpose of the access layer is to provide a means of connection to the network devices and control devices which can communicate over the network.

• The distribution layer is the means of communication between the access layer and the core. The functions of this layer are to provide routing, packet filtering, access to the WAN and determine which packets must arrive at Core. It also determines what is the fastest way to meet network requirements, for example, how to bring a file from a server.
  Here also are deployed network policies, eg routing, access-list, packet filtering, queuing, security is implemented and network policies, routing between VLANs and other workgroup functions are defined domains broadcast and multicast.

• The core layer is literally the core of the network, its only function is to switch traffic as fast as possible and is responsible for carrying large amounts of traffic in a reliable and fast, so the latency and speed are important factors in this layer.
  
  In case of failure, it affects all users, so that fault tolerance is important. Moreover, given the importance of speed, does not function as may increase latency, and access-list, interVLAN routing, packet filtering.
  
  The core layer is essential for interconnectivity among devices of the distribution layer, therefore, it is important that the core is highly available and redundant.
  
  The selection of switches or network equipment should be considered according to the requirements of each layer based on the hierarchical model.

In the access layer equipment can be found the following characteristics:

• 10/100/1000 Ethernet connectivity.
• PoE
• vlan
• QoS

In the distribution layer, it is advisable to purchase equipment which complies with the following features.

• Redundant links.
• Connections to 1G or 10G
• Capacity Routing (L2 and L3 Switches)
• QoS
• High switching capacity, like forwarding high rates.

While in the core layer, the switches have the following characteristics:

• Connections to 1G or 10G
• QoS
• High switching capacity, like forwarding high rates.
• Redundant links.

The inter-VLAN routing is needed once you have a vlan network infrastructure implemented because users need to exchange information from one network to another.

It is important to remember that each VLAN is a single broadcast domain. Therefore, by default, computers in separate VLANs can not communicate.

There is a way to enable end stations to communicate with them, this way is called inter-VLAN routing.


Inter-VLAN routing is a process that allows you to forward network traffic from one VLAN to another using a router. VLANs are associated with unique IP subnets in the network. This subnet configuration facilitates the process of routing in a multi-VLAN.

Traditionally, the routing of the LAN using routers with multiple physical interfaces. You need to connect each to a separate network interface and configure it to a different subnet.

In a traditional network that uses multiple VLANs to segment network traffic into logical broadcast domains, routing is done by connecting different physical interfaces on the router to different physical ports on the switch. The switch ports are connected to the router in access mode, in this way, various static VLANs are assigned to each interface port. Each switch interface would be assigned to a different static VLAN. Each router interface can then accept traffic from the VLAN associated with the switch interface that is connected and traffic can be routed to other VLANs connected to other interfaces.

The traditional inter-VLAN routing requires multiple physical interfaces on the router and the switch. However, not all inter-VLAN routing configurations require multiple physical interfaces.

Some router software allows to configure the router interfaces as trunks. This opens new possibilities for routing between VLANs. "router-on-a-stick" is a type of router configuration in which a single physical interface routes traffic between multiple VLANs on a network.


The router interface is configured to function as trunk and is connected to a switch port configured in trunking mode. The router performs routing between VLANs to accept traffic from VLAN tagging on the interface from the adjacent switch trunk and internally route between VLANs, using subinterfaces. The router then forwards the VLAN routing traffic for VLAN tagged target by the same physical interface.

The subinterfaces are multiple virtual interfaces associated with a physical interface. These interfaces are configured in software on a router configured independently with an IP address and VLAN assignment to work on a specific VLAN. The subinterfaces are configured for different subnets that correspond to the VLAN assignment, to facilitate routing logic before the VLAN tag data frames and forward for the physical interface. Learn more about interfaces and subinterfaces to the next topic.

Some switches can perform Layer 3 functions, which replaces the need for dedicated routers for basic routing in a network. Multilayer switches can perform routing between VLANs.

To enable a multi-layer switch for routing functions, you must configure the VLAN on the switch interfaces with corresponding IP addresses that match the subnet to which the VLAN is associated to the network. Multilayer switch must also have IP routing enabled.

Interfaces and subinterfaces

Traditional routing requires routers that have multiple physical interfaces to facilitate inter vlan routing. The router performs routing by connecting each of its physical interfaces to a single VLAN. In addition, each interface is configured with an IP address for the subnet associated with the VLAN connected to it. When you configure IP addresses on physical interfaces, network devices connected to each VLAN can communicate with the router using the physical interface connected to the same VLAN. In this configuration, network devices can use the router as a gateway to access devices in other VLANs.

Configure the subinterface

The configuration of the router subinterfaces is similar to the configuration of physical interfaces, except that it is necessary to create the subinterface and assign a VLAN.

The syntax for the subinterface is always the physical interface, in this case f0 / 0, followed by a dot and a number of subinterface. The subinterface number is configurable, but is usually associated to reflect the number of VLANs.

Before assigning an IP address to a subinterface, the subinterface must be configured to operate in a specific VLAN using the command "encapsulation dot1q VLAN ID". In the example, the subinterface is assigned to Fa0/0.10 VLAN10. Once assigned to the VLAN, the command "ip address 172.16.10.1 255.255.255.0" assign the proper IP for the VLAN subinterface.

Router#configure terminal
Router(config)# interface f0/0.10
Router(config-subif)# encapsulation dot1q 10
Router(config-subif)# ip address 172.16.10.1 255.255.255.0
Router(config-subif)# no shutdown

Unlike a typical physical interface, subinterfaces are not enabled with the command "no shutdown" in the level so the subinterface configuration of Cisco IOS software. However, when the physical interface is enabled with the command "no shutdown", all configured subinterfaces are enabled. Similarly, if the physical interface is disabled, all subinterfaces are disabled.

One advantage of using a trunk is that it reduces the number of switch ports and router. Not only does this save money but also reduces the complexity of the configuration. As a result, the focus of the subinterface on the router can be extended to a much higher number of VLANs that a configuration with a physical interface VLAN design.

Port Limits

The physical interfaces are configured for a VLAN interface on the network. In networks with many VLANs can not use a single router for inter VLAN routing on. The routers are physically limited to avoid containing a large number of physical interfaces.

Subinterfaces allow you to extend the router to accommodate more VLAN than allowed by the physical interfaces. The Inter-VLAN routing in large environments with many VLANs can be accommodated better if you use a single physical interface with many subinterfaces.

Performance

Because there is no contention for bandwidth on separate physical interfaces, physical interfaces have better performance when compared with the use of subinterfaces. The traffic for each VLAN connected have access to the full bandwidth of the physical interface of the router connected to that VLAN.

When using subinterfaces for inter-VLAN routing, traffic is being routed competing for bandwidth on the single physical interface. In a busy network, this can cause a bottleneck in communication.

Access ports and trunk ports

The connection of the physical interfaces for inter-VLAN routing requires the switch ports are configured as access ports. The subinterfaces require the switch port is configured as a trunk port so that it can accept traffic tagged in the VLAN trunk. By using subinterfaces, many VLANs can be routed on a single trunk, instead of using a single physical interface for each VLAN.

The use of subinterfaces for inter-VLAN routing, results in a less complex physical configuration using separate physical interfaces, because the number of physical network cables that connect the router to the switch is lower. With fewer cables, less confusion about where the cable is connected to the switch. Because VLANs are interconnected by trunks on a single link, it is easier to solve the problem of physical connections.

On the other hand, the use of subinterfaces to a trunk port, results in a more complex configuration software, which can be difficult to troubleshoot if problems arise. In the model router-on-a-stick is used only accommodate a single interface for all VLANs.

If a routed VLAN have problems with other VLANs, you can not simply trace the cable to see if it is plugged into the correct port. It is necessary to verify the switch port is configured to be a trunk and the VLAN is not being filtered in any of the trunk before you reach the router interface. It is also necessary to check if the router subinterface is configured to use the ID of the VLAN and IP address is correct.

Modes switch port membership

Switch ports

Switch ports are Layer 2 interfaces that are only associated with a physical port. The switch ports are used to handle the physical interface and associated Layer 2 protocols. They do not handle routing or bridging. Switch ports belong to one or more VLANs.

Modes VLAN switch ports

When you configure a VLAN, you must assign an ID number and can give a name if desired. The purpose of the implementations of the standard VLAN is associated with the private VLAN ports. Port is configured to send a frame to a specific VLAN. As mentioned above, the user can configure a VLAN mode to support voice and data traffic from voice coming from a Cisco IP phone. The user can configure a port that belongs to a VLAN by assigning a membership mode that specifies the type of traffic sent by the port and the VLAN to which they may belong. You can configure a port to support the following types of VLAN:

• Static VLAN: ports on a switch are manually assigned to a VLAN. Static VLANs are configured by using the Cisco CLI. This can also be carried out with the GUI management applications, such as the Cisco Network Assistant. However, a convenient feature of the CLI is that if you assign an interface to a VLAN that does not exist, create the new VLAN for the user.


• Dynamic VLAN: This mode is widely used in production networks and is not investigated in this course. However, it is useful to know what a dynamic VLAN. The membership of a dynamic port VLAN is configured using a special server called VLAN Membership Policy Server (VMPS). With the VMPS, the switch ports assigned to VLANs dynamically based on source MAC address of the device connected to the port. The benefit comes when you move a host from a port on a switch in the network to a port on another switch in the network. The switch dynamically assigns the new port to the proper VLAN for that host.


• Voice-VLAN: The port is configured to be in voice mode in order to be able to support an IP phone connected to it. Before you configure a voice VLAN on the port, you must configure a VLAN to a VLAN for voice and data. When first plugged a phone into a switch port that is in voice mode, it sends messages to the phone provides the configuration ID and proper voice VLAN. The IP phone voice tag frames with voice VLAN ID and sends all voice traffic through the voice VLAN.

User productivity and adaptability of the network are key drivers for growth and business success. The implementation of VLAN technology allows network supports a more flexible business goals. The main benefits of using VLANs are as follows:

• Security: the groups that have sensitive data separate from the rest of the network, decreasing the chances of occurrence of violations of confidential information.


• Reducing costs: the cost savings resulting from the low need for expensive network upgrades and more efficient use of links and existing bandwidth.


• Best performance: the division of flat Layer 2 network into multiple logical groupings of work (broadcast domains) reduces unnecessary network traffic and power performance.


• Broadcast storms Mitigation: Dividing a network into several VLANs reduces the number of devices that can participate in a broadcast storm. LAN segmentation prevent a broadcast storm spreading across the network.


• Increased IT staff efficiency: VLANs facilitate the management of the network because users with similar requirements for network share the same VLAN. When you provide a new switch, all policies and procedures that are already configured for the particular VLAN are implemented when assigned ports. It is also easy for IT staff to identify the function of providing a VLAN name.


• Application Management or simpler projects: VLANs add users and network devices to support geographic or business requirements. Have separate functions, makes managing a project or work with a specialized application easier, such as a development platform for e-learning for teachers.

The benefits of VLANs are great but they must have extensive knowledge on the subject when you enter the communication between VLANs. This section will explore further in subsequent articles.

The performance of the network can be an important factor in the productivity of an organization and its reputation for broadcasting as planned. One of the technologies that contribute to the excellent performance of the network is the division of large broadcast domains into smaller domains, which is done with VLANs.

The smaller broadcast domains limit the number of devices involved in the broadcasts and allow devices to be separated into functional groups, such as database services for an accounting department and transfer data at high speed for an engineering department.

Introducing VLANs

A VLAN allows a network administrator to create groups of devices connected to the network logically acting as if they were its own separate network, even if they share a common infrastructure with other VLANs. When you configure a VLAN, you can put a name to describe the main function of the users of that VLAN. As another example, all student computers are configured in the VLAN "student." Using VLANs can logically segment switched networks based on project teams, functions or departments. You can also use a VLAN to structure your network geographically to support the growing confidence of business on domestic workers. In the figure, creating a VLAN for students and another for faculty. These VLANs allow network administrators implement access policies and security for particular groups of users. For example, you can allow the faculty, but students get access to server management e-learning to develop online course materials.

• A VLAN is an independent LAN Network.
• A VLAN allows student and faculty PCs  to be separated although they share  the same infrastructure.
• A VLAN can be named for easier identification.

A VLAN is a separate IP subnet in a logical manner. VLANs allow multiple IP networks and subnets exist in the same switched network. For computers to communicate on the same VLAN, each must have an IP address and subnet mask consistent with that VLAN. The switch must register VLANs, and each port must be assigned to the appropriate VLAN.

A switch port with a single VLAN configured on it, is called the access port. Remember that if two computers are physically connected on the same switch does not mean that they can communicate. The devices in two separate networks and subnets must communicate via a router (Layer 3) is used or not VLANs.

Later, will be the subject of VLAN configurations

Ethernet Communications

Communications in a switched LAN network occur in three ways: unicast, broadcast and multicast:

Unicast: Communication in which a host sends a frame to a specific destination. In the unicast transmission, there are only a transmitter and a receiver. Unicast transmission is the predominant mode of transmission in the LAN and the Internet. Some examples of protocols that use unicast transmissions are: HTTP, SMTP, FTP and Telnet.

Broadcast: Communication in which a frame is sent from an address to all other directions. In this case, there is only one source but the information is sent to all connected receivers. The broadcast transmission is essential when sending the same message to all devices on the LAN. An example of broadcast transmission is the address resolution query sent by the Address Resolution Protocol (ARP) to all computers on a LAN.

Multicast: Communication in which it sends a frame to a specific group of devices or clients. The multicast transmission customers must be members of a logical multicast group to receive the information. An example of multicast transmission is voice and video related to business meetings in web-based conference.

Duplex Settings

There are two types of parameters for duplex communications in an Ethernet network: half duplex and full duplex.

Half Duplex: half-duplex communication is based on a unidirectional data flow in the sending and receiving data do not occur simultaneously. This is similar to the role of two-way radios or two walkie-talkies where only one person can talk at once. Half-duplex communication implements the CSMA / CD in order to reduce the chances of collisions and detect them if they arise.

Half-duplex communications malfunctions occur due to the constant expected, since the flow of data occurs only in one direction at a time. Half-duplex connections are often the oldest hardware devices such as hubs.

Full duplex: In full-duplex communications, data flow is bidirectional, so the information can be sent and received simultaneously. The bidirectional capacity improves performance, because it reduces the wait time between transmissions. In full-duplex mode, the collision detection circuit is disabled. The frames sent by the two end nodes connected can not collide because they use two separate circuits on the cable network.

Switch port configuration

The switch port must be configured with a duplex settings match the type of medium.

The auto option sets the duplex mode autonegotiation. When this mode is enabled, both ports are communicated to decide the best mode of operation.

The full option sets the full-duplex mode.

The half option sets the half-duplex mode.

For Fast Ethernet and 10/100/1000 ports, the default is auto. For 100BASE-FX ports, the default is full. The 10/100/1000 ports operate in both half-duplex mode and in full-duplex when set to 10 or 100 Mb / s, but they work in full-duplex mode when set to 1000 Mb / s.

MAC address and MAC address tables of the switches

The switches use MAC addresses for routing the communications network through the port structure corresponding to the destination node. The structure of the switch are integrated circuits and attached machine programming that lets you control the data paths through the switch. The switch must first know which nodes exist in each of its ports to define what the port used to transmit a unicast frame.

The switch determines how to handle incoming data frames by a MAC address table. The switch generates its MAC address table recording MAC addresses of the nodes that are connected in each of its ports. Once the MAC address of a specific node in a given port is registered in the address table, the switch knows to send traffic destined to that specific node from the port assigned to that node for subsequent transmissions.

Methods of the switch packet forwarding

This topic will describe how switches forward Ethernet frames across a network. The switches can work in different modes and they can have both positive or negative.

Previously, the switches used to use one of the following methods to switch data forwarding between the ports of the network: switching method of cutting or storage and shipping. Button switch forwarding method shows these two methods. However, store and forward is the only forwarding method used in current models of Cisco Catalyst switches.

Store and forward switching

In this type of switching, when the switch receives the frame, this is stored in data buffers to receive the frame in its entirety. During storage, the switch analyzes the frame to find information about your destination. In this process, the switch also performs error checking using the trailer portion of the Cyclic Redundancy Check (CRC) of the Ethernet frame.

When you start working in the area of ​​networks is important to know some basics about the operation of Ethernet networks. Important aspects in the design and implementation of switching networks.

CSMA / CD

Ethernet signals are transmitted to all hosts that are connected to the LAN through a set of special rules to determine which station can access the network. The set of rules that uses Ethernet technology is based on carrier sense multiple access and collision detection (CSMA/CD) IEEE. You may remember CCNA Exploration: Network Fundamentals, CSMA/CD is used only half-duplex communication is usually found in the hubs. The switches do not use full-duplex CSMA/CD.

Carrier Detect

In the access method CSMA/CD, all network devices that have a message to send should listen before transmitting.

If a device detects a signal from another device, wait a certain period before attempting to transmit.

When not detected any traffic, the device transmits its message. While such a transfer occurs, the device continues monitoring traffic or collisions on the LAN. After sending the message, the device returns to the default listener.

Multiple Access

If the distance between the devices is such that the latency of the signals of a device is no detection of these by a second device, it could also begin to transmit. Thus, the medium would have two devices transmitting signals at the same time. The messages are spread across the medium until they meet. At that time, the signals are mixed and the messages are destroyed: a collision occurs. Although messages are damaged, the mixed signals continues to spread throughout the medium.

Collision Detection

When a device is in listening mode, can detect when a collision on the shared medium. This is because all devices can detect an increase in the amplitude of the signal is above the normal level.

When a collision occurs, other devices are in listening mode, as well as all transmitting devices, detect the increase in amplitude of the signal. All devices are transmitting at that time, they will do to ensure that all devices on the network can detect the collision.


Congestion signal and delay random

When a collision is detected, the transmitting devices send a signal of congestion. Congestion signal, notifies the other devices on the collision so that they invoke a Backoff Algorithm. The function of this is to make all devices stop transmission during a random period, thereby reducing the signs of a collision.

After ending the delay assigned to a device, the device returns to "listen before transmit." A random delay period ensures that the devices involved in the collision does not attempt to send traffic back at the same time, leading to repeat the whole process. However, during the period of postponement is possible that a third device to transmit before either involved in the collision have a chance to re-transmit.

VTP Concepts

The challenge of managing the VLAN

As the number of switches in a network of small and medium enterprises, general administration required to manage VLANs and trunking on a network becomes a challenge.

What is VTP?

VTP allows a network administrator to set up a switch so that propagate the VLAN configurations to other switches in the network. The switch can be configured in the function of VTP server or client.

VTP Benefits

VTP maintains consistency in VLAN configuration by managing the addition, removal and renaming of VLANs across multiple Cisco switches in a network. The VTP provides many benefits to network administrators.

Components of VTP

There are a number of key components that need to be familiar with learning about the VTP. Here is a brief description of the components will be explained later as progress is made in the chapter.

VTP domain: consists of one or more interconnected switches. All switches in a domain share the details of VLAN configuration with VTP publications. A router or Layer 3 switch defines the boundary of each domain.

Publications VTP: VTP uses a hierarchy of publications to distribute and synchronize VLAN configurations across the network.

VTP Modes: A switch can be configured in one of three modes: server, client, or transparent.

VTP server: VTP servers publish the VLAN information from the VTP domain to other VTP-enabled switches in the same VTP domain. VTP servers save VLAN information for the entire domain in NVRAM.

Client VTP: VTP clients function the same way as VTP servers, but can not create, change, or delete VLANs on a VTP client. A VTP client only stores the VLAN information for the entire domain while the switch is activated. A reset switch erases the information of the VLAN.

VTP transparent: transparent switches send VTP advertisements to VTP clients and servers. Transparent switches do not participate in VTP. VLANs are created, renamed or deleted on the local switches are transparent to the switch only.

Use or creation of aliases in Cisco equipment

Sometimes I found some commands that use a lot but are still long to write using the tab in these cases you can set an alias to reduce the command. However, it is necessary to know that this must be configured on each computer on the network. Here are the commands needed to complete the setup.


Make use of aliases is very common in Unix users, to reduce the time to write long or difficult commands to remember. You can create aliases for commands that contain lots of information as widely used for commands.


The key to making a good alias is to select a password that is short and easy to remember, of course it is important to select an alias that do not conflict with any command that already exists. Aliases can be used as part of another command. For example the following command is used as shrinking the show vlan summary command.


You can find lots of uses for the alias, for example when working with scripts you can use alias to standardize some computers.