Cisco IOS command set quick reference

When I was studying for my CCNA afew years back, I needed some reference. There are plenty of books to read, and I decide to make some notes myself, and store them online where I could easily find them.

Below, are a list of the more common things you need to remember. If you want actual full blown explananations, then it isn’t for you. They are more meant for revision – the common buzz word is ‘cram’ 😉

Cisco IOS commands

IOS Commands:
Privileged Mode
Setting Passwords
the Router

Routing Protocols:
Access Lists
WAN Configurations

Static and Dynamic

and Link-State Routing

Interior and
Exterior Routing Protocols

with Routing Protocols

Encapsulation Types
WAN Service

WAN Devices
Classful and
Classless Protocols

Distances for IP Routes


Access Lists

Accessing Router
with Terminal Emulation
Router Startup

Miscellaneous Notes



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 - s
et encrypted password for privileged
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:
aux 0

Configuring the Router

sh running-config -
details the running configuration file (RAM)
sh startup-config -

displays the configuration stored in NVRAM

- 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 NVRAM to running config (RAM) or "write memory" for IOS under ver.11

copy startup-config running-config
- copies current running config (RAM) to non-volatile (NVRAM)

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)

- 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

& 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

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
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
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


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
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
- view arp cache; shows MAC address of connected routers
ip address secondary
configure a 2nd ip address on an interface
sh ip protocol

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


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


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

- show previous command

show next command

- Break




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


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.


Vector and Link-State Routing


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

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
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
4) IS-IS

II) Exterior
1) EGP (Exterior Gateway Protocol)
2) BGP (Border Gateway Protocol)

Routing Protocols used for each Routed Protocol
IP -




AppleTalk -


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 network

Solutions to Problems with Routing
1) 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
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 up


802.2 sap
802.3 novell-ether
Ethernet II arpa (Internet Standard)
Snap snap

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."

- the router side and receive clocking
DCE - the CSU/DSU side and provide clocking

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
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


(Basic Rate Interface) - 2 64K B channels, plus 1 16K D channel

(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)

and Classless Protocols

Classful - summarizes routing info by major
network numbers; ex. RIP,
Classless - BGP, OSPF


Administrative Distances for IP Routes
Administrative Distances are configured using ip route command:
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

? copies entire frame into buffer, checks for CRC errors
before forwarding. Higher latency.

? 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

1-99 IP Standard Access List
100-199 IP Extended Access List
200-299 Protocol Type-code Access List
300-399 DECnet Access List
600-699 Appletalk Access List
700-799 48-bit MAC Address Access List
800-899 IPX Standard Access List
900-999 IPX Extended Access List
1000-1099 IPX SAP Access List
1100-1199 Extended 48-bit MAC Address Access List
1200-1299 IPX Summary Address Access List


Filters Wildcard
Standard IP Source IP address field in the packet's IP header To 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. everything
Extended IP Source IP or Destination IP, or TCP or UDP Source or
Destination Ports, or Protocol
Same as standard The key word ANY implies any
IP value is allowed, the
keyword HOST implies the IP exactly has to match
Standard IPX Packets sent by clients and servers, and SAP updates sent
by servers and routers
Configured as a hexadecimal number instead of binary -1 means any and all network
numbers ( works like ANY)
Extended IPX Source Network or Node, or Destination Network or Node, or
IPX Protocol, or IPX Socket, or SAP
Match multiple networks with one statement, again in
The most practical use of the protocol type is for
SAP Sent and received SAP traffic N/A Updates its own SAP tables. Again uses -1 to mean



Ping Results

! success
, timeout
U destination unreachable
? unknown packet type
& TTL exceeded

Traceroute Results

!H router rec'd, but didn't forward
because of access-list
P protocol unreachable
N network unreachable
U port unreachable
, timeout


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



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

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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