Router interfaces

The term interface on Cisco routers refers to a physical connector on a router whose primary purpose is to receive and transmit packets. Routers have multiple interfaces that are used to connect to multiple networks. Typically, interfaces connect to different types of networks, which means that different types of media and connectors are needed. Often a router will have different types of interfaces. For example, a router usually has FastEthernet interfaces to connect to different LANs, and various types of WAN interfaces to connect multiple serial links, including T1, DSL, and ISDN. The figure shows the FastEthernet and serial interfaces on the router.

Like interfaces on a PC, ports and interfaces on a router are located on the outside of the router. Their external arrangement allows convenient connection to appropriate network cables and connectors.

14. How is dynamic routing formed?

15. What allows you to collect individual fragments into a single package?

16. What information does an OSPF packet contain in updates?

The OSPF packet is placed inside the IP packet immediately following the header. The basic information of an OSPF packet is:

– package type,

– router ID (Router ID),

– area number (area 0),

– network or subnet mask,

– time intervals (Hello Interval, Dead Interval),

– identifiers of the main designated router (Designated Router – DR ) and backup (Backup Designated Router – BDR ) of the defining router of this area,

– list of neighboring devices.

17. What devices divide the network into broadcast domains?

Since each virtual network represents a broadcast domain, the routers in the VLAN topology provide broadcast filtering, security, traffic control, and communication between VLANs. Switches do not provide inter-VLAN traffic because this violates the integrity of the VLAN broadcast domain. Traffic between VLANs is provided by routing, i.e. communication between nodes of different virtual networks occurs only through a router .

18. What parameters does the routing table and label forwarding table contain?

Routing table:

· Network;

· The address;

· Gateway;

The MPLS technology uses frames of different link layer technologies: PPP, Ethernet, Frame Relay, ATM. These frames contain an IP packet with an MPLS header. The MPLS header contains 32 bits, of which 20 bits are occupied by the label number field, 8 bits are the TTL time to live field, duplicating the corresponding field of the IP packet header, 3 bits are the CoS class of service field for the transmitted traffic type, 1 bit is the sign S of the bottom label stack (Fig.16.15). The MPLS header is placed between the PPP, Ethernet, Frame Relay frame header and the IP packet header.

19. How many binary digits contain the logical addresses of nodes in IP networks of the IPv4 version? What determines the high and low bits of a network address?

20. What information do topology tables contain?

21. How is communication between nodes of different virtual networks ensured?

22. What is the CSU/DSU used for?

The router is usually the user equipment and the DCE is provided by the ISP. Services provided by the provider for DTE terminal devices are available through a modem or channel-forming equipment, matching the channel device (Channel Service Unit / Data Service Unit – CSU / DSU ), which are DCE equipment

The DCE is the master of the DCE-DTE pair, providing synchronization and setting the data rate.

23. How are routing tables formed?

24. How are messages addressed when using the HDLC protocol?

25. What do the terms DTE, DCE mean?

In most cases, the connection of the LAN router to the Internet is made through the provider’s network. Terminal (terminal) equipment ( Data Terminal Equipment – DTE ), which includes the router, is connected to the global network (or to the provider’s network) through channel telecommunications equipment ( Data Communications Equipment or Data Circuit-terminating Equipment – DCE ). The router is usually the user equipment and the DCE is provided by the ISP. Services provided by the provider for DTE terminal devices are available through a modem or channel-forming equipment, matching the channel device (Channel Service Unit / Data Service Unit – CSU / DSU ), which are DCE equipment

26. What are network filters or access lists used for?

27. What are private addresses in local networks used for? What are the private address ranges?

The addresses of all Internet users must be unique. Initially, address uniqueness was provided by the Internet Network Information Center (InterNIC), which was replaced by the Internet Assigned Numbers Authority (IANA). IANA manages IP addresses to avoid duplication of public addresses by allocating them to five Regional Address Registrars: ARIN (North America), RIPE (Russia and Europe), APNIC (Asia and Australia), LACNIC (Latin and South America), AfriNIC (Africa). Thus, all public (public) addresses must be registered with the Regional Internet Registry (RIR), which allocates addresses to network operators and providers, who, in turn, allocate addresses to network administrators and individual users.

Due to the rapid growth of the Internet, there is a shortage of public addresses. The created new sixth version ( IPv6 ) of addressing in IP networks can radically solve the problem of the shortage of IP addresses. Prior to its widespread adoption, new addressing schemes such as Variable Length Mask Addressing (VLSM) and Classless Inter-Domain Routing (CIDR) were developed to alleviate the shortage of public addresses.

In addition, the problem of lack of public addresses can be somewhat weakened by the use of private addresses (Private IP addresses). Networks with private addresses that are not connected to the Internet can have any address, as long as they are unique within the private network. Access to the Internet for packets with private addresses is blocked by the router. Thus, these addresses cannot be used directly on the Internet, because routers drop packets with private addresses. In order for nodes with private addresses to be able to connect to the Internet if necessary, special private-to-public address translators are used, for example, Network Address Translation ( NAT ). This translator translates one private address into one public address. Therefore, IP address savings can only be achieved by not allowing all hosts on a private network to access the Internet.

28. On the basis of what is the prohibition or permission of network traffic through the router interface formed?

The network administrator must be able to manage traffic to ensure that authorized users have access to required resources and to prevent unauthorized access to the network. Access control lists ( ACLs ) are an effective means of filtering traffic. They are also called network filters or simply access lists . Access lists are used to allow ( permit ) or deny ( deny ) the forwarding of packets through a router, i.e. allow or deny access to information from other local networks or from the Internet to the protected network, as well as remote access using Telnet commands.

29. What functions do global networks perform?

Leased line networks are economically expensive because they are not always fully loaded. A shared common line in circuit-switched and packet-switched networks reduces economic costs.

Circuit-switched networks were created for public telephone networks. To improve the performance of their highways, PDH, SDH technologies were developed. The networks were designed for uniform streaming traffic. Therefore, with the advent of computer networks, new network technologies were required.

Packet-switched networks designed for elastic (bursting) traffic have been widely developed recently because they provide a more cost-effective wide area network technology compared to circuit-switched networks designed for uniform (streaming) traffic.

30. Can I use private addresses on the Internet? What translates private addresses into public ones?

31. What is the “last mile” or local (local) loop?

32. What are variable length masks for?

variable length masks (Variable-length subnet mask – VLSM ) allow you to create subnets of different sizes, flexibly setting the boundaries between the network address field and the host address field. VLSMs allow more than one subnet mask to be used within an allocated network address space.

33. Which table contains complete information about the network topology?

For efficient operation, in addition to the Neighbor Table, the EIGRP protocol builds and maintains a Topology Table and a Routing Table. For any topology changes that are recorded in the neighbor and network topology tables, the DUAL algorithm either includes spare routes from the topology table in the routing table, or calculates new routes and then includes them in the routing table . The DUAL algorithm provides calculation of routes free from route loops (loop-free).

34. How do routers exchange routing tables with neighbors when using the distance vector algorithm?

Routing protocols that operate within autonomous systems are in turn subdivided into distance-vector protocols ( distance-vector ) and link-state protocols ( link-state ). Distance vector protocols define distance and direction, i.e. the connection vector in the composite network on the way to the destination. Distance can be expressed in terms of the number of hops ( hop count ) or routers in the connection on the path from the source node to the destination, as well as other metric values.

When using the distance vector protocol, routers send all or part of the routing table to neighboring (adjacent) routers at regular intervals. In protocols such as RIP , the exchange of updates (update) or modifications occurs periodically, even if there are no changes in the network , which consumes a fairly large part of the bandwidth. Once the routing information has been updated, the router can recalculate all known paths and make changes to the routing table.

35. In what case does the router in response to a request send an ARP response with the MAC address of its input interface on which the request was received?

36. What is a routing control protocol?

37. How to manage VLANs?

38. Which protocols automatically assign device IP addresses? What IP parameters does the administrator assign manually?

The assignment of an IP address can be done manually by the administrator or automatically by a DHCP server. To assign an address manually, usually in the main menu of the computer, you must select the following options in sequence: “ Start ”, “ Settings ”, “ Control Panel ”, “ Network Connections ”, “ Local Area Connection

Of the protocols for automatically assigning an IP address to devices (hosts – host) , the Dynamic Host Configuration Protocol ( DHCP ) is currently used, which allows a host to dynamically obtain an IP address without administrator intervention. You only need to define the range of IP addresses on the DHCP server.

39. What is the function of Time to Live (TTL)?

There is a TTL time-to-live field in the IP network protocol header , which is decremented as it passes through each router. Thus, the number of devices a packet can pass through is limited. When the TTL value is set to zero, the router discards the packet, and a network unreachable message is sent to the sender using the ICMP protocol.

40. What packet parameters can be parsed in the access list? Where are access lists set?

The network administrator must be able to manage traffic to ensure that authorized users have access to required resources and to prevent unauthorized access to the network. Access control lists ( ACLs ) are an effective means of filtering traffic. They are also called network filters or simply access lists . Access lists are used to allow ( permit ) or deny ( deny ) the forwarding of packets through a router, i.e. allow or deny access to information from other local networks or from the Internet to the protected network, as well as remote access using Telnet commands.

41. Can routers connect local area networks of different technologies?

42. Description and principles of operation of the RIP routing protocol?

43. What is static and dynamic routing?

44. Why are masks of variable length necessary? What will radically solve the problem of shortage of IP addresses?

variable length masks (Variable-length subnet mask – VLSM ) allow you to create subnets of different sizes, flexibly setting the boundaries between the network address field and the host address field. VLSMs allow more than one subnet mask to be used within an allocated network address space.

The use of IPv6 class addresses will help solve the shortage of IP addresses.

45. Description and principles of operation of the OSPF routing protocol?

Open Shortest Path First ( OSPF ) is a link-state link-state protocol that responds quickly to changes in the network by broadcasting changes in the network topology to all routers within a certain area of the network. OSPF is designed to work in large flexible composite networks and can work with equipment from different manufacturers, so it has become widespread.

The OSPF protocol uses five types of packets to exchange routing information between devices:

1. Hello package

2. Database description package DataBase Description – DBD

3. Link-State Request Packet – LSR

4. Link-State Update – LSU

5. Link-State Acknowledgment Packet – LSAck.

46. What does the term convergence mean?

Convergence is the process of negotiating between all routers in a network information about available routes. When the network state changes, it is necessary for the modification exchange to restore consistent network information.

47. What is the maximum number of nodes that class C addresses can specify? What address is used for self-test?

The most significant bit of the class A address is always 0, so network addresses can range from 1 to 127. However, the address 127.0.0.1 is for self-testing , at this address the host refers to itself, checking if the TCP / IP protocol is installed on this host .

48. Why are virtual local networks created? Their dignity?

Virtual networks are created to implement network segmentation on switches, i.e. at the second layer of the OSI model. The creation of virtual local area networks (Virtual local area networks – VLAN ), which are a logical association of groups of network stations (Fig.), is one of the main methods of protecting information in networks on switches.

Typically, VLANs are grouped by functional features of operation, regardless of the physical location of users. Data exchange occurs only between devices that are in the same VLAN. Data exchange between different VLANs is performed only through routers.

49. What are the additional features of the PPP protocol compared to the HDLC protocol??

When equipment from different manufacturers operates in a network based on leased channels, messages are transmitted between routers over leased lines of global networks using the Point -to-Point Protocol ( PPP ). Unlike HDLC, the PPP protocol supports authentication when establishing a connection.

The functions of the PPP protocol cover the physical and link layers, and also allow you to establish relationships with the network layer. At the physical layer, synchronous and asynchronous connections can be used through RS-232-C, V.35, or other DTE/DCE interfaces that determine the data rate.

The PPP protocol retains the HDLC frame format, but additional header fields are placed in the data field. Unlike the HDLC protocol, the PPP protocol does not provide reliable data transfer and flow control procedures. However, the PPP protocol is supplemented with a procedure for accepting connection parameters (line quality, frame size, authentication type, network layer protocol).

50. Description and principles of the EIGRP routing protocol?

Currently, the Distance Vector Routing Protocol Interior Gateway Routing Protocol (IGRP) has been replaced by an improved (extended) version of Enhanced IGRP . Both protocols are developed by Cisco and are designed to work with Cisco equipment. Routing protocols use the metric to determine the shortest route to a destination device. The value of the metric determines the desirability of the route. The EIGRP protocol metric takes into account a number of parameters. The protocol’s DUAL algorithm calculates a metric value for each path through the network. A lower number indicates a better route. Bandwidth and delay are static parameters, they remain the same for each interface until the network is rebuilt or the router is reconfigured. The load and reliability parameters are dynamic and can be calculated by the router for each interface in real time.

The more factors that make up the metric, the more flexibility to take into account the characteristics of the network. By default, the EIGRP protocol uses the static bandwidth and delay parameters to calculate the metric value. But the metric calculation can also use dynamic load and reliability factors, i.e. the router can make a decision based on the current state of the network. If the connection becomes heavily loaded or unreliable, the metric will increase. In this case, an alternative route can be used.

51. How are messages addressed when using the PPP protocol?

The PPP protocol retains the HDLC frame format, but additional header fields are placed in the data field. Unlike the HDLC protocol, the PPP protocol does not provide reliable data transfer and flow control procedures. However, the PPP protocol is supplemented with a procedure for accepting connection parameters (line quality, frame size, authentication type, network layer protocol).

The data field of the PPP protocol frame contains:

– a code field (Code) with a length of one byte defines the type of LCP packet, for example, configuration request, configuration confirmation or rejection.

– the identifier field (Identifier) with a length of one byte determines the match of the request and response packets;

– length field (Length) occupies 2 bytes and sets the total size of the LCP packet;

– data field (Data) of variable length is determined by the code.

52. What does the term Default Gateway mean?

53. What devices divide the network into broadcast domains?

54. What parameters does the routing table and label forwarding table contain?

55. What are private addresses in local networks used for? What are the private address ranges?

56. What are the characteristics of pre-connected networks?

Virtual circuit technologies provide for the preliminary connection of end nodes (source and destination), while a route ( virtual circuit ) is laid, along which data is then transmitted. The receipt of data is acknowledged by the receiving party. X.25 technology is focused on unreliable analog communication lines , therefore it is characterized by a low data transfer rate (up to 48 Kbps). However, this technology has been used to date, for example, in ATM networks, due to its high reliability with unreliable lines. Frame Relay technology provides a higher data transfer rate compared to X.25 – up to 2-4 Mbps. But the links should be more reliable than X.25. The highest data transfer rates (155 Mbps, 620 Mbps, and 2.4 Gbps) are provided by ATM networks. However, the development of these networks is constrained by their high cost .

57. What can be done in privileged configuration mode?

58. What is an autonomous system?

The set of networks, represented by a set of routers under common administrative control, forms an autonomous system (Fig. 8.2). Provider networks are examples of autonomous systems. Autonomous systems are numbered and some protocols (IGRP, EIGRP) use these numbers. In this course of lectures, routing is considered only within an autonomous system. The BGP protocol, which provides routing between autonomous systems, is studied in the CCNP course of the Cisco International Networking Academy.

59. What can be done in the user configuration mode?

The user mode is used to check the status of the device, as well as to switch to privileged mode . No changes to the configuration file, including deleting and saving the current configuration, can be made in user mode. In this mode, only some show verification commands are available, i.e. commands to view device status.

60. What information do topology tables contain?

For efficient operation, in addition to the Neighbor Table, the EIGRP protocol builds and maintains a Topology Table and a Routing Table. For any topology changes that are recorded in the neighbor and network topology tables, the DUAL algorithm either includes spare routes from the topology table in the routing table, or calculates new routes and then includes them in the routing table . The DUAL algorithm provides calculation of routes free from route loops (loop-free).

61. What can be set in global configuration mode?

In global mode, changes are made that affect the router as a whole, which is why it is called global configuration mode . For example, in this mode, you can set the router name using the hostname command. The name of the router does not matter on the Internet and is significant only on the local one, it is convenient for configuration. In global configuration mode, passwords can be set on the router. There are several types of passwords for securing Cisco routers. The first two passwords, enable secret and enable password , are used to provide authorized access to privileged mode. One (or both) of these passwords is set on the router. Once a password is set, the system prompts the user for it when the enable command is issued.

62. Description and principles of operation of the RIP routing protocol?

63. What is the detailed configuration mode used for?

64. What is a routing control protocol?

The Routing Information Protocol (RIP) is widely used in small networks where the maximum number of hops between routers does not exceed 15 on the path from source to destination. However, in the case of a poorly designed network, the use of RIP can lead to routing problems.

The main disadvantage of the first version of RIPv1 is that no mask value is passed in updates, so RIPv1 does not support classless interdomain CIDR routing and VLSM variable length masks. The protocol of the second version of RIPv2 is free from this shortcoming, which in its update messages, in addition to the destination network address, transmits the mask value and the address of the next hop (next-hop). This uses the mask value of the interface to which the network is attached, so the mask is not specified during configuration. Routing information is exchanged using UDP segments (port address 250). A segment can contain up to 25 routes. The rest of the RIPv2 parameters are the same as for the RIPv1 protocol.

65. What parameters does the OSPF protocol metric take into account? What is the format of the OSPF protocol configuration commands?

The OSPF packet is placed inside the IP packet immediately following the header. The basic information of an OSPF packet is:

– package type,

– router ID (Router ID),

– area number (area 0),

– network or subnet mask,

– time intervals (Hello Interval, Dead Interval),

– identifiers of the main designated router (Designated Router – DR ) and backup (Backup Designated Router – BDR ) of the defining router of this area,

– list of neighboring devices.

The OSPF routing protocol uses the cost metric. OSPF protocol metrics are based on bandwidth . The protocol algorithm calculates the total value of the metric of all connections through the network. A lower number indicates a better route. The following formula is used to calculate the OSPF metric:

Metric (Cost) = 108 / Bandwidth.

66. What do standard access lists analyze? What do extended access lists parse?

Access control lists ( ACLs ) are an effective means of filtering traffic. They are also called network filters or simply access lists . Access lists are used to allow ( permit ) or deny ( deny ) the forwarding of packets through a router, i.e. allow or deny access to information from other local networks or from the Internet to the protected network, as well as remote access using Telnet commands.

Standard access lists – to make a decision in an IP packet, only the source address of the message is analyzed in order to filter the network.

Extended access lists check both the source and destination IP address, the protocol field in the Network Layer header of the packet, and the port number in the Transport Layer header.

Thus, for each protocol, for each direction of traffic and for each interface, a different access list can be created. Outbound filters do not affect traffic that originates from the local router.

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