Cisco IOS Commands

Cisco IOS Commands

Posted on 02-22-2003

This is a list of Cisco IOS commands and information. IOS is the software used on the vast majority of Cisco Systems routers and most Cisco network switches.

Quick Links

IOS Commands:
    Privileged Mode
    Setting Passwords
    Configuring the Router
    Routing Protocols:
    Access Lists
    WAN Configurations
    Keyboard Shortcuts

    Static and Dynamic Routing
    Distance-Vector and Link-State Routing
    Interior and Exterior Routing Protocols
    Problems with Routing Protocols
    Encapsulation Types
    WAN Service Providers
    WAN Devices
    Classful and Classless Protocols
    Administrative Distances for IP Routes
    Switching Terminology
    Access Lists
    Troubleshooting Tools
    Accessing Router with Terminal Emulation
    Router Startup Sequence
    Miscellaneous Notes


IOS Commands

Privileged Mode
   enable – get to privileged mode
   disable – get to user mode
   enable password  – sets privileged mode password
   enable secret  – sets encrypted privileged mode password

Setting Passwords
   enable secret – set encrypted password for privilegedaccess
   enable password  – set password for privileged access (used when there is no enable secret and when using older software)
Set password for console access:
   (config)#line console 0
Set password for virtual terminal (telnet) access (password must be set to access router through telnet):
    (config)#line vty 0 4 
Set password for auxiliary (modem) access:
    (config)#line aux 0

Configuring the Router 
   sh running-config – details the running configuration file (RAM)
   sh startup-config – displays the configuration stored in NVRAM
   setup – Will start the the automatic setup; the same as when you first boot the router
   config t – use to execute configuration commands from the terminal
   config mem – executes configuration commands stored in NVRAM; copies startup-config to running-config
   config net – used to retrieve configuration info from a TFTP server
   copy running-config startup-config – copies saved config in running config (RAM) to NVRAM or “write memory” for IOS under ver.11
   copy startup-config running-config – copies from non-volatile (NVRAM) to current running config (RAM)
   boot system flash  – tells router which IOS file in flash to boot from
   boot system tftp – tells router which IOS file on the tftp server to boot from
   boot system rom – tell router to boot from ROM at next boot
   copy flash tftp – Copies flash to tftp server
   copy tftp flash – Restores flash from tftp server
   copy run tftp – Copies the current running-config to tftp server
   copy tftp run – Restores the running-config from tftp server 

General Commands
   no shutdown – (enables the interface)
   reload – restarts the router
   sh ver – Cisco IOS version, uptime of router, how the router started, where system was loaded from, the interfaces the POST found, and the configuration register
   sh clock – shows date and time on router
   sh history – shows the history of your commands
   sh debug – shows all debugging that is currently enabled
   no debug all – turns off all debugging
   sh users – shows users connected to router
   sh protocols – shows which protocols are configured
   banner motd # Your_message # – Set/change banner
   hostname  – use to configure the hostname of the router
   clear counters  – clear interface counters

Processes & Statistics
   sh processes – shows active processes running on router
   sh process cpu – shows cpu statistics
   sh mem – shows memory statistics
   sh flash – describes the flash memory and displays the size of files and the amount of free flash memory
   sh buffers – displays statistics for router buffer pools; shows the size of the Small, Middle, Big, Very Big, Large and Huge Buffers
   sh stacks – shows reason for last reboot, monitors the stack use of processes and interrupts routines

CDP Commands (Cisco Discovery Protocol uses layer 2 multicast over a SNAP-capable link to send data):
   sh cdp neighbor – shows directly connected neighbors
   sh cdp int – shows which interfaces are running CDP
   sh cdp int eth 0/0 – show CDP info for specific interface
   sh cdp entry  – shows CDP neighbor detail
   cdp timer 120 – change how often CDP info is sent (default cdp timer is 60)
   cp holdtime 240 – how long to wait before removing a CDP neighbor (default CDP holdtime is 180)
   sh cdp run – shows if CDP turned on
   no cdp run – turns off CDP for entire router (global config)
   no cdp enable – turns off CDP on specific interface

Miscellaneous Commands
   sh controller t1  – shows status of T1 lines
   sh controller serial 1 – use to determine if DCE or DTE device
   (config-if)#clock rate 6400 – set clock on DCE (bits per second)
   (config-if)#bandwidth 64 – set bandwidth (kilobits)

IP Commands
Configure IP on an interface:
    int serial 0
    ip address
    int eth 0 
    ip address 2008.1.1.4 
Other IP Commands:
    sh ip route – view ip routing table
    ip route [administrative_distance] – configure a static IP route
    ip route  – sets default gateway
    ip classless – use with static routing to allow packets destined for unrecognized subnets to use the best possible route
    sh arp – view arp cache; shows MAC address of connected routers
    ip address secondary – configure a 2nd ip address on an interface
    sh ip protocol     sh ip interface – Show all IP interfaces
    sh ip interface brief – Show brief overiew of IP interfaces
    sh ip nat translations – Show current IP NAT translations

IPX Commands
Enable IPX on router:
    ipx routing
Configure IPX + IPX-RIP on an int: 
    int ser 0
    ipx network 4A
Other Commands:
    sh ipx route – shows IPX routing table
    sh ipx int e0 – shows ipx address on int
    sh ipx servers – shows SAP table
    sh ipx traffic – view traffic statistics
    debug ipx routing activity – debugs IPS RIP packets
    debug ipx sap – debugs SAP packets

Routing Protocols
Configure RIP:
    router rip
Other RIP Commands: 
    debug ip rip – view RIP debugging info
Configure IGRP:
    router IGRP 200
Other IGRP Commands:
    debug ip igrp events – view IGRP debugging info
    debug ip igrp transactions – view IGRP debugging info

Access Lists (see notes below for details)
sh ip int ser 0 – use to view which IP access lists are applies to which int
sh ipx int ser 0 – use to view which IPX access lists are applies to which int
sh appletalk int ser 0 – use to view which AppleTalk access lists are applies to which int
View access lists:
    sh access-lists
    sh ip access-lists
    sh ipx access-lists
    sh appletalk access-lists
Apply standard IP access list to int eth 0:
    access-list 1 deny
    access-list 1 permit any
    int eth 0
    ip access-group 1 in
Apply Extended IP access list to int eth 0:
    access-list 100 deny tcp host host eq 23
    access-list 100 deny tcp any eq 80
    int eth 0
    ip access-group 100 out
Apply Standard IPX access list to int eth 0:
    access-list 800 deny 7a 8000
    access-list 800 permit -1
    int eth 0
    ipx access-group 800 out
Apply Standard IPX access list to int eth 0:
    access-list 900 deny sap any 3378 -1
    access-list 900 permit sap any all -1
    int eth 0
    ipx access-group 900 out


Wan Configurations (see notes below for more details)

PPP Configuration
   encapsulation ppp
   ppp authentication
   ppp chap hostname
   ppp pap sent-username
   sh int ser 0 – use to view encapsulation on the interface

Frame-Relay Configuration
   encapsulation frame-relay ietf – use IETF when setting up a frame-relay network between a Cisco router and a non-Cisco router
   frame-relay lmi-type ansi – LMI types are Cisco, ANSI, Q933A; Cisco is the default; LMI type is auto-sensed in IOS v11.2 and up
   frame-relay map ip 100 broadcast – if inverse ARP won’t work, map Other IP to Your DLCI # (local)
   keepalive 10 – use to set keepalive
   sh int ser 0 – use to show DLCI, LMI, and encapsulation info
   sh frame-relay pvc – shows the configured DLCI’s; shows PVC traffic stats
   sh frame-relay map – shows route maps
   sh frame-relay lmi – shows LMI info

Keyboard Shortcuts
   CTRL-P – show previous command
   CTRL-N – show next command
   SHIFT-CTRL-6 – Break



Static and Dynamic Routing 

Static Routing – manually assigned by the Admin user entering the routes (Routed Protocols – IP, IPX and AppleTalk)
Dynamic Routing – generated/determined by a Routing Protocol (Routing Protocols – RIP I, RIP II, IGRP, EIGRP, OSPF, NLSP, RTMP)

1) With Dynamic Routing, routers pass information between each other so that routing tables are regularly maintained. 
2) The routers then determine the correct paths packets should take to reach their destinations. 
3) Information is passed only between routers. 
4) A routing domain is called an Autonomous System, as it is a portion of the Internetwork under common admin authority. 
5) Consists of routers that share information over the same protocol. Can be split into routing areas. 


Distance Vector and Link-State Routing

Routing Protocols
I) Interior (within an autonomous system – AS – group of routers under the same administrative authority) 
    a) Distance Vector – understands the direction and distance to any network connection on the internetwork. Knows how 
    many hops (the metric) to get there. All routers w/in the internetwork listen for messages from other routers, which are sent 
    every 30 to 90 seconds. They pass their entire routing tables. Uses hop count for measurement. 1) Used in smaller networks 
    that are have fewer than 100 routers.  2) Easy to configure and use.  3) As routers increase in number, you need to consider 
    CPU utilization, convergence time, and bandwidth utilization.  4) Convergence is due to routing updates at set intervals.  5) When
    a router recognizes a change it updates the routing table and sends the whole table to all of its neighbors. 
            1) RIP – 15 hop count max 
            2) IGRP – 255 hop count max, uses reliability factor (255 optimal), and bandwidth
            3) RTMP
    b) Link State – understands the entire network, and does not use secondhand information. Routers exchange LSP?s (hello 
    packets). Each router builds a topographical view of the network, then uses SPF (shortest path first) algorithm to determine the 
    best route. Changes in topology can be sent out immediately, so convergence can be quicker. Uses Bandwidth, congestion for measurement; Dijkstra’s algorithm;
    1) Maintains Topology Database.  2) Routers have formal neighbor relationship.  3) Exchanges LSA (Link State Advertisement) or 
    hello packets with directly connected interfaces.  4) These are exchanged at short intervals (typically 10 sec).  5) Only new info is 
    exchanged.  6) Scales well, however link?state protocols are more complex. 7) Requires more processing power, memory, and bandwidth.
            1) OSPF – decisions based on cost of route (metric limit of 65,535)
            2) EIGRP – hybrid protocol (both Distance-Vector and Link State), Cisco proprietary
            3) NLSP
            4) IS-IS
II) Exterior 
            1) EGP (Exterior Gateway Protocol) 
            2) BGP (Border Gateway Protocol) 

Routing Protocols used for each Routed Protocol

Problems with Routing Protocols
1) Routing Loops – occur when routing tables are not updated fast enough when one of the networks becomes unreachable. Due to the slow convergence (updates of routing table between all routers), some routers will end up with incorrect routing table and will broadcast that routing table to other routers. This incorrect routing tables will cause packets to travel repeatedly in circles.
2) Counting to infinity – occurs when packets end up in a routing loop; hop count increases with every pass through a router on the networkSolutions to Problems with Routing Protocols1) Define the maximum number of hops – When the number of hops reaches this predefined value, the distance is considered infinite, thus the network is considered unreachable. This does stop routing loops, but only limit the time that packet can travel inside the loop.
2) Split horizon – The packets can not be sent back to the same interface that they originally came from. During the updates, one router does not send updates to the router that it received the information from.
3) Route poisoning – The router sets the cost/distance of routes that are unreachable to infinity.  Used with hold-down timers
4) Triggered updates – The router sends updates of the routing table as soon as it detects changes in the network.  Does not wait for the prescribed time to expire.5) Hold-Downs – After the router detects  unreachable network, the routers waits for a specified time before announcing that a network is unreachable. The router will also wait for a period of time before it updates its routing table after it detects that another router came online (Router keeps an entry for the network possibly down state, allowing time for other routers to re-compute for this topology change).  Hold-downs can only partially prevent counting to infinity problem. Prevents routes from changing too rapidly in order to determine if a link has really failed, or is back upnovell-etherEthernet IIarpa (Internet Standard)Snapsnap

Wan Service Providers
1) Customer premises equipment (CPE) – Devices physically located at subscriber?s location; examples: CSU/DSU, modem, wiring on the customer’s location
2) Demarcation (or demarc) – The place where the CPE ends and the local loop portion of the service begins. (Usually in the “phone closet”). 
3) Local loop – Cabling from the demarc into the WAN service provider?s central office; wiring from customer’s location to the nearest CO
4) Central Office switch (CO) – Switching facility that provides the nearest point of presence for the provider?s WAN service; location of telephone company’s equipment where the phone line connects to the high speed line (trunk); Regional Telco Office where the local loop terminates (the Telco location nearest you)
5) Toll network – The switches and facilities, (trunks), inside the WAN provider?s “cloud.” 

DTE – the router side and receive clocking
DCE – the CSU/DSU side and provide clocking

WAN Devices
Routers – Offer both internetwork and WAN interface controls
ATM Switches – High-speed cell switching between both LANs and WANs
X.25 and Frame-Relay Switches – Connect private data over public circuits using digital signals
Modems – Connect private data over public telephone circuits using analog signals
CSU/DSU (Channel Service Units/Data Service Units) – Customer Premises Equipment (CPE) which is used to terminate a digital circuit at the customer site
Communication Servers – Dial in/out servers that allow dialing in from remote locations and attach to the LAN
Multiplexors – Device that allows more than one signal to be sent out simultaneously over one physical circuit


ISDN BRI (Basic Rate Interface) – 2 64K B channels, plus 1 16K D channel 
ISDN PRI (Primary Rate Interface) – 23 64K B channels, plus 1 64K D channel (North America & Japan), 30 64K B channels, plus 1 64K D channel (Europe & Australia) 

Classful and Classless Protocols
Classful – summarizes routing info by major network numbers; ex. RIP, IGRP
Classless – BGP, OSPF

Administrative Distances for IP Routes

Administrative Distances are configured using ip route command:

Example: ip route 85  (where 85 is the administrative distance)

IP Route

Administrative Distance

Directly connected interface


Static route using connected interface


Static route using IP address


EIGRP summary route


External BGP route


Internal EIGRP route


IGRP route


OSPF route


IS-IS route


RIP route


EGP route


External EIGRP route


Internal BGP route


Route of unknown origin


Switching Terminology 
Store-and-Forward ? copies entire frame into buffer, checks for CRC errors before forwarding. Higher latency. 
Cut-Through ? reads only the destination address into buffer, and forwards immediately; Low latency; “wire-speed”
Fragment free ? modified form of cut-through; switch will read into the first 64 bytes before forwarding the frame. Collisions will usually occur within the first 64 bytes. (default for 1900 series).

Access Lists

1-99IP Standard Access ListIP Extended Access ListProtocol Type-code Access ListDECnet Access ListAppletalk Access List48-bit MAC Address Access ListIPX Standard Access ListIPX Extended Access ListIPX SAP Access ListExtended 48-bit MAC Address Access ListIPX Summary Address Access List


Access ListWildcard MasksAdditional NotesStandard IPSource IP address field in the packet’s IP headerTo put simply, when the IP is broken down to binary, the 1’s allow everything and the 0’s must match exactly.Wildcard mask examples: address must match. the first octet must match, the rest will allow everything. everythingExtended IPSource IP or Destination IP, or TCP or UDP Source or Destination Ports, or ProtocolSame as standardThe key word ANY implies any IP value is allowed, the keyword HOST implies the IP exactly has to matchStandard IPXPackets sent by clients and servers, and SAP updates sent by servers and routersConfigured as a hexadecimal number instead of binary-1 means any and all network numbers ( works like ANY)Extended IPXSource Network or Node, or Destination Network or Node, or IPX Protocol, or IPX Socket, or SAPMatch multiple networks with one statement, again in hexadecimalThe most practical use of the protocol type is for NetBIOSSAPSent and received SAP trafficN/AUpdates its own SAP tables. Again uses -1 to mean “ANY”


Troubleshooting Tools:

Ping Results

!success,timeoutUdestination unreachable?unknown packet type&TTL exceeded

Traceroute Results

!Hrouter rec’d, but didn’t forward because of access-listPprotocol unreachableNnetwork unreachableUport unreachable,timeout


Accessing Router with Terminal Emulation
Using HyperTerminal on a Windows machine adjust the following settings:
    VT100 Emulation
    Connection Speed: 9600 Baud
    Data Bits: 8
    Parity: None
    Stop Bits: 1
    Flow Control: None
On a Linux machine you may use Seyon or Minicom (at least one should come with your distribution).


Router Startup Sequence
Bootstrap program loaded from ROM
IOS is loaded from either flash (default), TFTP, or ROM
IOS image loaded into low-addressed memory; hardware and software is determined
Config file is load from NVRAM; if no configuration exists in NVRAM, the initial configuration dialog will begin


Miscellaneous Notes
Multiple Loop Problems ? complex topology can cause multiple loops to occur. Layer 2 has no mechanism to stop the loop. This is the main reason for Spanning ? Tree Protocol. 

Spanning-Tree Protocol (STP) IEEE 802.1d. ? developed to prevent routing loops; uses STA (Spanning-Tree Algorithm) to calculate a loop-free network topology; allows redundant paths without suffering the effects of loops in the network

Virtual LAN?s (VLAN’s) ? sets different ports on a switch to be part of different sub-networks. Some benefits: simplify moves, adds, changes; reduce administrative costs; have better control of broadcasts; tighten security; and distribute load. Relocate the server into a secured location. 

HDLC (High-Level Data Link Control) – Link layer protocol for Serial links. Cisco Default. Supports the following modes: Normal Response Mode ? as per Secondary under SDLC; Asynchronous Response Mode allows secondary to communicate without permission; Asynchronous Balanced mode combines the two stations. Has lower overhead than LAPB but less error checking. 

Modular Switch/VIP Syntax
type slot/port   (example:  e 2/1)
type slot/port-adapter/port    (example:  e 2/0/1)


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