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Use Of Access Control List In Networking

An Access Control List in networking is a series of commands that control whether a device forwards or drops packets based on information found in the packet header. When configured, ACLs perform the following tasks:

  • They limit network traffic to increase network performance. For example, if a corporate policy does not allow video traffic on the network, ACLs that block video traffic could be configured and applied. This would greatly reduce the network load and increase network performance.
  • They provide traffic flow control. ACLs can restrict the delivery of routing updates to ensure that the updates are from a known source.
  • They provide a basic level of security for network access. ACLs can allow one host to access a part of the network and prevent another host from accessing the same area. For example, access to the Human Resources network can be restricted to authorized users.
  • They filter traffic based on traffic type. For example, an ACL can permit email traffic but block all Telnet traffic.
  • The screen hosts permit or deny access to network services. ACLs can permit or deny a user to access file types, such as FTP or HTTP.

In addition to either permitting or denying traffic, ACLs can be used for selecting types of traffic to be analyzed, forwarded, or processed in other ways. For example, ACLs can be used to classify traffic to enable priority processing. This capability is similar to having a VIP pass at a concert or sporting event.


The VIP pass gives selected guests privileges not offered to general admission ticket holders, such as priority entry or being able to enter a restricted area.

What Is an ACL?

ACLs: Important Features

Two types of Cisco IPv4 ACLs are standard and extended. Standard ACLs can be used to permit or deny traffic only from source IPv4 addresses. The destination of the packet and the ports involved are not evaluated.

Extended ACLs filter IPv4 packets based on several attributes that include:

  • Protocol type
  • Source IPv4 address
  • Destination IPv4 address
  • Source TCP or UDP ports
  • Destination TCP or UDP ports
  • Optional protocol type information for finer control


Standard and extended ACLs can be created using either a number or a name to identify the ACL and its list of statements.


Using numbered ACLs is an effective method for determining the ACL type on smaller networks with more homogeneously defined traffic. However, a number does not provide information about the purpose of the ACL. For this reason, a name can be used to identify a Cisco ACL.


By configuring ACL logging, an ACL message can be generated and logged when traffic meets the permit or deny criteria defined in the ACL.


Cisco ACLs can also be configured to only allow TCP traffic that has an ACK or RST bit set, so that only traffic from an established TCP session is permitted. This can be used to deny any TCP traffic from outside the network that is trying to establish a new TCP session.


Simple Network Management Protocol (SNMP) allows administrators to manage end devices such as servers, workstations, routers, switches, and security appliances, on an IP network.
It enables network administrators to monitor and manage network performance, find and solve network problems, and plan for network growth.
SNMP is an application layer protocol that provides a message format for communication between managers and agents.
As shown in the figure, the SNMP system consists of two elements.
  • SNMP manager that runs SNMP management software.
  • SNMP agents are the nodes being monitored and managed.


The Management Information Base (MIB) is a database on the agents that stores data and operational statistics about the device.


To configure SNMP on a networking device, it is first necessary to define the relationship between the manager and the agent.


The SNMP manager is part of a network management system (NMS). The SNMP manager runs SNMP management software.


As shown in the figure, the SNMP manager can collect information from an SNMP agent by using the “get” action and can change configurations on an agent by using the “set” action. In addition, SNMP agents can forward the information directly to a network manager by using “traps”.


NetFlow is a Cisco IOS technology that provides statistics on packets flowing through a Cisco router or multilayer switch.
While SNMP attempts to provide a very wide range of network management features and options, NetFlow is focused on providing statistics on IP packets flowing through network devices.
NetFlow provides data to enable network and security monitoring, network planning, and traffic analysis to include the identification of network bottlenecks, and IP accounting for billing purposes. For example, in the figure, PC 1 connects to PC 2 using an application such as HTTPS.
The figure shows 3 devices connected to a switch, p c 1, a pc labelled net flow collector and analyzer software and router R1 labelled net flow enabled router. R1 also connects to a switch that also has p c 2 attached. Across the top of the diagram is a line with arrows at both ends and the words net flow analyzed traffic flow.

NetFlow in the Network

NetFlow can monitor that application connection, tracking byte and packet counts for that individual application flow. It then pushes the statistics over to an external server called a NetFlow collector.

NetFlow technology has seen several generations that provide more sophistication in defining traffic flows, but “original NetFlow” distinguished flows using a combination of seven fields. Should one of these fields vary in value from another packet, the packets could be safely determined to be from different flows:

  • Source IP address
  • Destination IP address
  • Source port number
  • Destination port number
  • Layer 3 protocol type
  • Type of Service (ToS) marking
  • Input logical interface


The first four of the fields NetFlow uses to identify a flow should be familiar. The source and destination IP addresses, plus the source and destination ports, identify the connection between the source and destination application.



The Layer 3 protocol type identifies the type of header that follows the IP header (usually TCP or UDP, but other options include ICMP). The ToS byte in the IPv4 header holds information about how devices should apply quality of service (QoS) rules to the packets in that flow.


Port Mirroring

A packet analyzer (also known as a packet sniffer or traffic sniffer) is typically software that captures packets entering and exiting the network interface card (NIC). It is not always possible or desirable to have the packet analyzer on the device that is being monitored. Sometimes it is better on a separate station designated to capture the packets.
Because network switches can isolate traffic, traffic sniffers or other network monitors, such as IDS, cannot access all the traffic on a network segment. Port mirroring is a feature that allows a switch to make duplicate copies of traffic passing through a switch, and then send it out to a port with a network monitor attached.
The original traffic is forwarded in the usual manner. An example of port mirroring is illustrated in the figure.

Traffic Sniffing Using a Switch

Syslog Servers

When certain events occur on a network, networking devices have trusted mechanisms to notify the administrator with detailed system messages.



These messages can be either non-critical or significant. Network administrators have a variety of options for storing, interpreting, and displaying these messages, and for being alerted to those messages that could have the greatest impact on the network infrastructure.


The most common method of accessing system messages is to use a protocol called Syslog.


Many networking devices support Syslog, including routers, switches, application servers, firewalls, and other network appliances. The Syslog protocol allows networking devices to send their system messages across the network to Syslog servers.



The Syslog logging service provides three primary functions:

  • The ability to gather logging information for monitoring and troubleshooting
  • The ability to select the type of logging information that is captured
  • The ability to specify the destination of captured Syslog messages


It is important to synchronize the time across all devices on the network because all aspects of managing, securing, troubleshooting, and planning networks require accurate and consistent timestamping.
When the time is not synchronized between devices, it will be impossible to determine the order of the events that have occurred in different parts of the network.

Typically, the date and time settings on a network device can be set using one of two methods:

  • Manual configuration of the date and time
  • Configuring the Network Time Protocol (NTP)


As a network grows, it becomes difficult to ensure that all infrastructure devices are operating with synchronized time. Even in a smaller network environment, the manual method is not ideal. If a device reboots, how will it get an accurate date and timestamp?


A better solution is to configure the NTP on the network. This protocol allows routers on the network to synchronize their time settings with an NTP server. A group of NTP clients that obtain time and date information from a single source have more consistent time settings.




When NTP is implemented in the network, it can be set up to synchronize to a private master clock or it can synchronize to a publicly available NTP server on the Internet.
NTP networks use a hierarchical system of time sources.



Each level in this hierarchical system is called a stratum. The stratum level is defined as the number of hop counts from the authoritative source. The synchronized time is distributed across the network using NTP. The figure displays a sample NTP network.


The figure shows the words stratum 0 to the left and two alarm clocks. Each alarm clock has an arrow that points down to a server. To the left of these servers is the words stratum 1. Below the server to the left on stratum 1 are two more servers and arrows point from the server on stratum 1 to each of the two servers on stratum 2. The stratum 1 server on the right has a stratum 2 server below it and an arrow pointing to it.
This server has a line with arrows at each end pointing toward the adjacent server to the left. There is also an arrow pointing to a stratum 3 server. The leftmost stratum 2 server has an arrow pointing to a stratum 3 server. The middle stratum 2 server has two strata 3 servers below it and an arrow going to each of them. There is also a line with an arrow on both ends between these two strata 3 servers.

NTP Stratum Levels

NTP servers are arranged in three levels known as strata:

  • Stratum 0 – An NTP network gets the time from authoritative time sources. These authoritative time sources, also referred to as stratum 0 devices, are high-precision timekeeping devices assumed to be accurate and with little or no delay associated with them.
  • Stratum 1 – The stratum 1 devices are directly connected to the authoritative time sources. They act as the primary network time standard.
  • Stratum 2 and lower strata – The stratum 2 servers are connected to stratum 1 devices through network connections. Stratum 2 devices, such as NTP clients, synchronize their time using the NTP packets from stratum 1 servers. They could also act as servers for stratum 3 devices.


Smaller stratum numbers indicate that the server is closer to the authorized time source than larger stratum numbers. The larger the stratum number, the lower the stratum level.



The max hop count is 15. Stratum 16, the lowest stratum level, indicates that a device is unsynchronized. Time servers on the same stratum level can be configured to act as a peer with other time servers on the same stratum level for backup or verification of time.

AAA Servers

The table lists the three independent security functions provided by the AAA architectural framework.
AAA Provides Description
  • Users and administrators must prove that they are who they say they are.
  • Authentication can be established using a username and password combinations, challenge and response questions, token cards, and other methods.
  • AAA authentication provides a centralized way to control access to the network.
  • After the user is authenticated, authorization services determine which resources the user can access and which operations the user is allowed to perform.
  • An example is “User ‘student’ can access host server XYZ using SSH only.”
  • Accounting records what the user does, including what is accessed, the amount of time the resource is accessed, and any changes that were made.
  • Accounting keeps track of how network resources are used.
  • An example is “User ‘student’ accessed host serverXYZ using SSH for 15 minutes.”
Terminal Access Controller Access-Control System Plus (TACACS+) and Remote Authentication Dial-In User Service (RADIUS) are both authentication protocols that are used to communicate with AAA servers. Whether TACACS+ or RADIUS is selected depends on the needs of the organization.
While both protocols can be used to communicate between a router and AAA servers, TACACS+ is considered the more secure protocol. This is because all TACACS+ protocol exchanges are encrypted, while RADIUS only encrypts the user’s password. RADIUS does not encrypt usernames, accounting information, or any other information carried in the RADIUS message.
The table lists the differences between the two protocols.
Functionality Separates AAA according to the AAA architecture, allowing modularity of the security server implementation Combines authentication and authorization but separates accounting, allowing less flexibility in implementation than TACACS+
Standard Mostly Cisco supported Open/RFC standard
Transport TCP UDP
Protocol CHAP Bidirectional challenge and response as used in Challenge Handshake Authentication Protocol (CHAP) Unidirectional challenge and response from the RADIUS security server to the RADIUS client
Confidentiality Entire packet encrypted Password encrypted
Customization Provides authorization of router commands on a per-user or per-group basis Has no option to authorize router commands on a per-user or per-group basis
Accounting Limited Extensive

Virtual Private Network

Instead of using a dedicated physical connection, a VPN uses virtual connections that are routed through the internet from the organization to the remote site. The first VPNs were strictly IP tunnels that did not include authentication or encryption of the data. For example, Generic Routing Encapsulation (GRE) is a tunnelling protocol developed by Cisco that can encapsulate a wide variety of network layer protocol packet types inside IP tunnels. This creates a virtual point-to-point link to Cisco routers at remote points over an IP network.


A VPN is virtual in that it carries information within a private network, but that information is actually transported over a public network. A VPN is private in that the traffic is encrypted to keep the data confidential while it is transported across the public network.


A VPN is a communications environment in which access is strictly controlled to permit peer connections within a defined community of interest. Confidentiality is achieved by encrypting the traffic within the VPN.
Today, a secure implementation of VPN with encryption is what is generally equated with the concept of virtual private networking.
In the simplest sense, a VPN connects two endpoints, such as a remote office to a central office, over a public network, to form a logical connection.

The logical connections can be made at either Layer 2 or Layer 3. Common examples of Layer 3 VPNs are GRE, Multiprotocol Label Switching (MPLS), and IPsec. Layer 3 VPNs can be point-to-point site connections, such as GRE and IPsec, or they can establish any-to-any connectivity to many sites using MPLS.


IPsec is a suite of protocols developed with the backing of the IETF to achieve secure services over IP packet-switched networks.


IPsec services allow for authentication, integrity, access control, and confidentiality. With IPsec, the information exchanged between remote sites can be encrypted and verified. VPNs are commonly deployed in a site-to-site topology to securely connect central sites with remote locations.
They are also deployed in a remote-access topology to provide secure remote access to external users travelling or working from home. Both remote-access and site-to-site VPNs can be deployed using IPsec.

Action Point
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