Administrative Distance

Administrative Distance (AD) is a value that routers use in order to select the best path when there are two or more different routes to the same destination from two different routing protocols. Administrative Distance counts the reliability of a routing protocol. Administrative Distance (AD) is a numeric value which can range from 0 to 255. A smaller Administrative Distance (AD) is more trusted by a router, therefore the best Administrative Distance (AD) being 0 and the worst, 255.

Administrative Distance (AD) Route Type
0 Connected interface
0 or 1 Static Route
90 Internal EIGRP Route (within the same Autonomous System (AS))
100 IGRP Route
110 OSPF Route
115 IS-IS
120 RIP Route
255 Unknown Route

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Autonomous System | Autonomous System Number

                        An Autonomous System (AS) is a group of networks under a single administrative control which could be an Internet Service Provider (ISP) or a large Enterprise Organization. An Interior Gateway Protocol (IGP) refers to a routing protocol that handles routing within a single autonomous system. IGPs include RIP, IGRP, EIGRP, and OSPF. An Exterior Gateway Protocol (EGP) handles routing between different Autonomous Systems (AS). Border Gateway Protocol (BGP) is an EGP. BGP is used to route traffic across the Internet backbone between different Autonomous Systems.

When BGP (Border Gateway Protocol) was at development and standardization stage, a 16-bit binary number was used as the Autonomous System Number (ASN) to identify the Autonomous Systems. 16-bit Autonomous System Number (ASN) is also known as 2-Octet Autonomous System Number (ASN). By using a 16 bit binary number, we can represent (2 16) numbers, which is equal to 65536 in decimals.

The Autonomous System Number (ASN) value 0 is reserved, and the largest ASN value 65,535, is also reserved. The values, from 1 to 64,511, are available for use in Internet routing, and the values 64,512 to 65,534 is designated for private use.

Available 16-bit (2-Octet) Autonomous System Numbers (ASN) were in verge of depletion by middle of 2011. To provide more Autonomous System Numbers (ASN), IETF published RFC 4893 in May 2007, which introduced 32-bit AS numbers. 32-bit Autonomous System Number (ASN) is also known as 4-Octet Autonomous System Number (ASN). 32-bit (4-Octet) AS numbers are represented as either as simple integers, or in the form x.y, where x and y are 16-bit numbers. Numbers of the form 0.y are exactly the previous 16-bit AS numbers.

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Difference Between Routable Protocol and Non-Routable Protocol

Routable protocol

A Routable protocol is a network protocol which can carry data from one network and can pass through the router to reach another network and be delivered to a computer in that remote network.

Examples of routable protocols: Internet Protocol (IP -IPv4 and IPv6), IPX, AppleTalk, VINES Internetwork Protocol (VIP), DECnet

Routable Protocol

Non-routable protocols

A non-routable protocol’s data cannot be passed through a router to reach a remote network. This is mainly because of the lack of capability of protocol (almost all non-routable protocols are designed long back which will not fit well in current networks) and the addressing scheme the non-routable protocol is using.

Non-routing protocols reachability limit is its own network and they are designed in such a way to think that all computers they communicate are on the same network as the source computer.

Non Routable Protocol

Examples of non-routable protocols: Local Area Transport Protocol (LAT), NetBios Extended User Interface (NetBEUI).

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Router interface naming convention

Cisco Routers different types of interfaces like Serial, Ethernet, Fast Ethernet, Gigabit Ethernet, Tokenring,  FDDI are some of them (A single router may not have all these). The speed of Ethernet, Fast Ethernet and Gigabit Ethernet are different. The speed of Ethernet is 10 Mbps (Megabits per second), Fast Ethernet is 100 Mbps (Megabits per second) and Gigabit Ethernet is 1 Gbps (Gigabits per second)

Most of the latest routers are modular routers. Modular routers are expandable routers by using plug-in components.

The following naming convention is followed for a Cisco Router.


Slot numbers begin with 0 and port numbers begin with 0. Hence the name of the first interface of a WIC2T (modular card with two smart serial interfaces) is serial0/0 and the name of the second port is serial0/1. The short form of the two interface is s0/0 and s0/1.

Old 2500 series routers are not modular, and they had fixed ports. The interface naming convention of these routers is

<Interface_Type>< Port_Number>

Hence the name of the first Ethernet interface for non-modular router is ethernet0 or e0, and first Serial interface is serial0 or s0.


Difference Between Routing Protocols and Routed Protocols

Routed Protocols

A Routed Protocol is a network protocol which can be used to send the user data from one network to another network. Routed Protocol carries user traffic such as e-mails, file transfers, web traffic etc.

Routed protocols use an addressing system (example IP Address) which can address a particular network and a host (a computer, server, network printer etc) inside that network. In other words, the address which is used by a Routed Protocol (Example IP (Internet Protocol)) has a network address part and a host (a computer inside a network) part.

IP (Internet Protocol) is the most widely used Routed Protocol. Internet is using IP (IPv4 or IPv6) as its Routed Protocol. Other Routed protocols are vanishing from network industry.

A Routed Protocol is an integral part of network protocol suit and it is available in every device which is participating in network communication (Example, Routers, Switches, Computers etc).

Routing Protocol

A Routing Protocol learns routes (path) for a Routed Protocol and IP (Internet Protocol), IPX (Internetwork Packet Exchange) and Appletalk are the examples of Routed Protocols.

Routing Protocols are network protocols used to dynamically advertise and learn the networks connected, and to learn the routes (network paths) which are available. Routing protocols running in different routers exchange updates between each other and most efficient routes to a destination. Routing Protocols have capacity to learn about a network when a new network is added and detect when a network is unavailable.

Routing Protocols normally run only in Routers, Layer 3 Swithes, End devices (firewalls) or Network Servers withNetwork Operating Systems. Routing Protocols are not available in a normal computer or a printer.

Examples of Routing Protocols are RIP (Routing Information Protocol) , EIGRP (Enhanced Interior Gateway Routing Protocol) and OSPF (Open Shortest Path First).

Following table lists important Routing Protocols related Routed Protocols.

Routed Protocol Routing Protocols

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Difference Between Static Route and Dynamic Route

Routing is the process of selecting paths in a network along which to send network traffic and route is the path to send the network traffic.

There are two ways a router learn a route: static and dynamic. The difference between static route and dynamic route is as below. A static route is a route that is manually configured on the router. Simply we can say a static route is a route that is created manually by a network administrator. The information about the networks that are directly connected to the active router interfaces are added to the routing table initially and they are known as connected routes. The second way that the router can learn static routes are by configuring the routes manually.

Dynamic routes are routes that a router learns by using a routing protocol. Routing protocols will learn about routes from other neighbouring routers running the same routing protocol. Dynamic routing protocols share network numbers a router knows about and how to reach these networks. Through this sharing process, a router can learn about all of the reachable network numbers in the network.

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Difference between Routing and Switching

Routing and switching are the basic functions of network communication. Routing and Switching are different functions of network communications. The main differences between Routing and Switching are as below.

The function of Switching is to switch data packets between devices on the same network (or same LAN – Local Area Network). The function of Routing is to Route packets between different networks (between different LANs – Local Area Networks).

Switches operate at Layer 2 of the OSI Model (Datalink Layer). A switch knows where to send a data packet by using Layer 2 addresses (MAC address – hardware address of a network adapter). A switch maintains a table of MAC addresses (MAC Address table or CAM Table) and what physical switch port they are connected to. The Switching function can be explained more simply that, Switching is the function of moving data packets (Ethernet Frames) within the same LAN (Local Area Network).

Routers operate at Layer 3 of the OSI Model (Network layer). A Router knows where to send a packet by using Network part of the destination IP address. A Router maintains a table called Routing Table and uses the routing table to determine the route to the destination network.

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Important Cisco Discovery Protocol (CDP) IOS commands

Important Cisco Discovery Protocol (CDP) IOS commands are listed below.

To see the summary of Cisco device neighbors, run the “show cdp neighbors” IOS command from privilege mode.

router01#show cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone
Device ID    Local Intrfce   Holdtme    Capability   Platform    Port ID
Ser 0/0          156            R       C2600       Ser 0/0
Fas 0/0          149            S       2950        Fas 0/1

To see the detailed information of Cisco device neighbors, run the “show cdp neighbor detail” IOS command fromprivilege mode.

router01#show cdp neighbor detail
Device ID:router02
Entry address(es): 
IP address :
Platform: cisco C2600, Capabilities: Router
Interface: Serial0/0, Port ID (outgoing port): Serial0/0
Holdtime: 143
Version :
Cisco Internetwork Operating System Software
IOS (tm) C2600 Software (C2600-I-M), Version 12.2(28), RELEASE SOFTWARE (fc5)
echnical Support:
Copyright (c) 1986-2005 by cisco Systems, Inc.
Compiled Wed 27-Apr-04 19:01 by miwang
advertisement version: 2
Duplex: full
Device ID: switch01
Entry address(es): 
Platform: cisco 2950, Capabilities: Switch
Interface: FastEthernet0/0, Port ID (outgoing port): FastEthernet0/1
Holdtime: 136
Version :
Cisco Internetwork Operating System Software
IOS (tm) C2950 Software (C2950-I6Q4L2-M), Version 12.1(22)EA4, RELEASE SOFTWARE(fc1)
Copyright (c) 1986-2005 by cisco Systems, Inc.
Compiled Wed 18-May-05 22:31 by jharirba
advertisement version: 2
Duplex: full

To view the Cisco Discovery Protocol (CDP) configuration on interface, run the “show cdp interface” IOS command from privilege mode.

router01#show cdp interface
FastEthernet0/0 is up, line protocol is up
Sending CDP packets every 60 seconds
Holdtime is 180 seconds
FastEthernet0/1 is administratively down, line protocol is down
Sending CDP packets every 60 seconds
Holdtime is 180 seconds
Serial0/0 is up, line protocol is up
Sending CDP packets every 60 seconds
Holdtime is 180 seconds
Serial0/1 is administratively down, line protocol is down
Sending CDP packets every 60 seconds
Holdtime is 180 seconds

To see the status of Cisco Discovery Protocol (CDP) on your Cisco device run the “show cdp” IOS command fromprivilege mode.

router01#show cdp
Global CDP information:
Sending CDP packets every 60 seconds
Sending a holdtime value of 180 seconds
Sending CDPv2 advertisements is enabled

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Cisco Discovery Protocol (CDP)

The Cisco Discovery Protocol (CDP) is a Cisco proprietary Layer 2 (Data Link Layer) network protocol developed by Cisco to share information about other directly connected Cisco devices, such as the operating system version and IP address.

Cisco Discovery Protocol (CDP) messages received from a neighbor Cisco device are not forwarded to any other devices by default. This means that Cisco Discovery Protocol (CDP) is passed only to directly connected Cisco devices. Each Cisco device (which supports Cisco Discovery Protocol (CDP)) stores the messages received from neighbor devices in a table that can be viewed using the show cdp neighbors command.

Cisco devices send Cisco Discovery Protocol (CDP) messages to the multicast destination address 01:00:0C:CC:CC:CC. CDP messages are sent every 60 seconds on interfaces that support Subnetwork Access Protocol (SNAP) headers. The support for Subnetwork Access Protocol (SNAP) is not available with every data link layer media type. The media types which are supported for Cisco Discovery Protocol (CDP) are Ethernet, Token Ring, FDDI, PPP, HDLC, ATM, and Frame Relay.

Cisco Discovery Protocol (CDP) message contain information about

IOS software version

Name of the device (configured with hostname command)

Hardware capabilities (routing/switching)

Hardware platform

The IP addresses of the device

The interface which generated the Cisco Discovery Protocol (CDP)  message

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Configure Router Serial Interfaces, DTE (Data Terminal Equipment) and DCE (Data Communications Equipment)

When connecting a serial cable to the serial interface of the router, clocking is provided by an external device, such as a CSU/DSU device. A CSU/DSU (Channel Service Unit/Data Service Unit) is a digital-interface device used to connect a router to a digital circuit. The router is the DTE (Data Terminal Equipment) and the external device is the DCE (Data Communications Equipment), where the DCE provides the clocking. However, in some cases we might connect two routers back-to-back using the routers’ serial interfaces (Example: Inside the router labs). Each router is a DTE by default. The cable decides which end to be DCE or DTE and it is usually marked on the cable. The picture below shows back to back cable.

Smart Serial to Smart Serial back to back cable

If is not marked, we can use the Cisco IOS show command “show controller” command to determine the interface is DTE or DCE. Since clocking is required to enable the interface, one of the two routers should function as DCE and should provide clocking. This can be done by using the “clock rate” command, from the interface configuration mode.

To find the possible clock rate values, get the command help by using a question mark after the “clock rate” from the serial interface configuration mode, as shown below.

Router(config-if)# clock rate ?

The output for above command is shown below.

R1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#interface serial 1/0
R1(config-if)#clock rate ?
With the exception of the following standard values not subject to rounding,

          1200 2400 4800 9600 14400 19200 28800 38400
          56000 64000 128000 2015232

  accepted clockrates will be bestfitted (rounded) to the nearest value
  supportable by the hardware.

  <246-8064000>    DCE clock rate (bits per second)

These values are in bites per second. You can find the possible values by using help.

Router(config)# interface s0/1
Router(config-if)# clock rate 64000

All interfaces have a bandwidth value assigned to them. This is used by certain routing protocols, such as IGRP,OSPF, and EIGRP, when making routing decisions.

For LAN-based interfaces, the speed of the interface becomes the bandwidth value (In kilobits per second (Kbps)). However, on synchronous serial interfaces, the bandwidth defaults to 1,554Kbps and this is not related with the clock rate set. To change the bandwidth value for an interface, use the bandwidth Interface Subconfiguration mode command:

Router(config)# interface  s0
Router(config-if)#bandwidth 64


Posted By – RamCruiseWalker