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CCNA Cheat Sheet

Table of Contents

Configure basic Networking

Command Description
(config)# interface g1/0 Enter their interface config mode
(config-if)# description Link to Somehost Human readable link description
(config-if)# ip address 10.23.42.5 255.255.0.0 Add IPv4 address to interface.
(config-if)# mac address 1234.5678.90AB Overwrite MAC address.
(config-if)# no mac address Remove MAC overwrite.
(config-if)# ipv6 address 2001:41d0:8:e115::ccc/64 Add IPv6 address to interface.
(config-if)# ipv6 address 2001:41d0:8:e115::/64 eui-64 Add IPv6 address based on MAC to interface.
(config-if)# ip address dhcp Get IPv4 address via dhcp.
(config-if)# ipv6 address autoconfig [default] Get IPv6 address [and default route] via autoconfig
(config-if)# ip dhcp client client-id asccii SW2 Set hostname transmitted as dhcp client to SW2
(config)# interface g1/0 - 2 Configure both interfaces at once.
(config-if)# [no] shutdown En- or Disable interface. Often shutdown is the default.
(config)# ip default-gateway 10.23.42.1 Set 10.23.42.1 as the default gateway
(config)# ip route 10.20.30.0 255.255.255.0 {1.2.3.4,e0/0} [ad] Add static route via next hop or interface
(config)# ipv6 route 2001:41d0:8:e115::/64 [g1/1] [next hop] Next hop is required for Ethernet interface in IPv6
(config)# ip host the-space.agency 178.32.222.21 Create a static host entry on this device.
(config)# ipv6 unicast-routing Globally enable ipv6 routing.

Troubleshoot basic Networking

Command Description
# show interfaces [if-name] Show interfaces mac, bandwidth, mtu, packet stats...
# show ip[v6] route [static] Show routes and how they were learned.
# show ip[v6] interface [if-name] Show interfaces ip/arp/icmp/nd... configuration
# show ip[v6] interface brief [if-name] Only show ip, status and operational status
# show protocols [if-name] Much like show ip int brief, w/ cidr, w/o ok/method
# show mac address-table Show the mac address table of a switch.
# clear mac address-table [dynamic] Clear the dynamically learned mac address table entries.
# show arp Show {ip,ipx,appletalk}-mac bindings
# show ip arp [{ip, mac, if-name}] Show ip-mac bindings
# clear [ip] arp 192.168.1.1 Remove arp entry for ip
# debug arp Show debug messages when receiving/sending arp packets
# undebug all Disable all previously enabled debugs
# show ipv6 neighbors Show neighbor discovery table cache
# ping 1.2.3.4 [source g1/1]
# traceroute 1.2.3.4 [source g1/1]
# show control-plane host open-ports netstat -tulpn on this cisco device, basically

Troubleshoot networks with SPAN

Command Description
(config)# monitor session 23 source interface g1/1 {rx,tx,both} Define SPAN #23 input as g1/1
(config)# monitor session 23 destination interface g1/2 Define SPAN #23 output as g1/2
# show monitor Show all configured SPANs

Port Security

Command Description
(config-if)# switchport mode {access, trunk}
(config-if)# [no] switchport port-security En/Disable port-security
(config-if)# switchport port-security maximum 1 Number of allowed MACs.
(config-if)# switchport port-security mac-address 1234.5678.9abc Manually allow a MAC on this port.
(config-if)# switchport port-security mac-address sticky Allow learning of connected macs until mac reached.
(config-if)# switchport port-security violation shutdown Shutdown port when other device gets connected.
(config-if)# shutdown (config-if)# no shutdown Reenable if after port-security violation.
(config)# errdisable recovery cause psecure-violation Reenable if automatically after problem is fixed.
(config)# errdisable recovery interval 42 Recheck every 42 seconds. (min 30, default 300)

Port-security violation terms

Term Definition
protect Drops packets, no alert
restrict Drops packets, increments security-violation count
shutdown Shuts down the port (default)

Troubleshooting Port Security

Command Description
# show port-security [interface g1/1] port status, violation mode, max/total MACs,...
# show port-security address Secure MACs on ports.
# show errdisable recovery Check if autorecovery is enabled. Disabled by default.

Configure vlans

Note: Even when a switch port is changed from access to trunk, its access vlan is maintained in the config. When automatic trunk negotiation fails (e.g. because I unplug a link between to switches and put it into my laptop) the configured access vlan becomes active once again and I might be able to reach network parts I'm not supposed to. Always disable DTP / trunk auto negotiation.

Layer2 Switch Vlan Config

Command Description
(config)# [no] vlan 23 [delete vlan or] create vlan and enter config-vlan mode
(config-vlan)# name TelephoneSanitizer Name this vlan TelephoneSanitizer
(config)# int g1/1
(config-if)# switchport mode access Make frames out this port untagged
(config-if)# switchport access vlan 23
(config)# int g1/2
(config-if)# switchport mode trunk Make frames out this port tagged by default
(config-if)# switchport trunk encapsulation dot1q Sometimes the default is ciscos old isl.
(config-if)# switchport trunk native vlan 256 Except for vlan 256, which is still untagged.
(config-if)# switchport nonegotiate Disable DTP

Layer3 Switch Vlan Config

Command Description
(config)# interface vlan 23 enter interface config mode
(config-if)# ip address 1.2.3.4 255.255.255.0 set device ip in vlan 23
(config-if)# no shutdown virtual interfaces are disabled by default
(config-if)# int g
(config)# no vlan 23 delete vlan 23

Router (on a Stick) Vlan Config

Command Description
(config)# interface g1/1.10 Create subinterface g1/1.10 on g1/1
(config-subif)# encapsulation dot1q 10 enable ieee 802.1Q vlan tagging with vlan 10 on the subinterface
(config-subif)# ip address 10.0.10.1 255.255.255.0
# show vlans Show vlans and their trunk interfaces

Troubleshoot Vlans on a switch

Command Description
# show vlan [{id 23, name TelephoneSanitizer}] [brief] Show vlan settings for all switch ports
# show interfaces g1/1 switchport Verify mode and vlan of g1/1
# show interfaces g1/1 trunk Show trunk settings and state
# show run interface vlan 1 Quick way to search the running config.
# show interface status Show trunk mode / access vlan
# show dtp interface g1/1 Show current DTP mode for g1/1

VTP

Command Description
(config)# vtp mode [server, client, transparent]
(config)# vtp domain
(config)# vtp password
(config)# vtp pruning

Troubleshoot VTP

Command Description
show vtp status show vtp domain, pruning, mode and more
show vtp password

STP

Spaning Tree Protocol (802.1D) blocks ports with redundant links to prevent layer 2 loops and broadcast storms.

Command Description
(config)# spanning-tree vlan 1 root {primary, secondary} Make this device the primary/secondary root bridge.
(config)# spanning-tree portfast bpduguard default Enable bpdu guard for all portfast enable interfaces
(config)# spanning-tree portfast default Enable portfast for all non-trunk interfaces
(config-if)# spanning-tree bpduguard enable Enable gpduguard on this interface
(config-if)# spanning-tree portfast Enable portfast on this interface
(config-if)# spanning-tree guard root Enable root guard on this interface

Troubleshoot STP

Command Description
# show spanning-tree [vlan 1] Who's the root and how do I get there?
# show spanning-tree summary Is global portfast/bpduguard configured?
# show running-config interface g1/1 Is portfast/bpduguard configured on this interface?
# show spanning-tree interface g1/1 portfast Is portfast active on this interface?

RSTP

Rapid Spanning Tree Protocol (802.1w) reduces convergence time after a topology change compares to STP.

Command Description
(config)# spanning-tree mode rapid-pvst Change spanning-tree mode to RSTP

Etherchannel (Link Aggregation)

How to set LACP? TODO: Look at modes again

Command Description
(config)# interface range g1/1 - 2 configure g1/1 and g1/2 at the same time
(config-if-range)# channel-group 1 mode {auto, desirable} Add both interfaces to etherchannel 1 (PAgP)
(config-if-range)# channel-group 1 mode {active, passive} Add both interfaces to etherchannel 1 (LACP)
(config-if-range)# channel-group 1 mode on Add both interfaces to etherchannel 1 (Static)
(config)# interface port-channel 1 Configure virtual interface for etherchannel 1
(config-if)# switchport mode trunk Put etherchannel 1 in trunk mode
(config-if)# switchport trunk allowed vlan 10,20,30 Add tagged vlans 10,20,30 on ethercahnnel 1

Troubleshoot Etherchannel (Link Aggregation)

Command Description
# show interface port-channel 1 Has the combined bandwidth and members as extra info.
# show etherchannel summary Show etherchannel protocols and members as a list
# show etherchannel port-channel 1 Show per member state and stats

Configure a Serial

Layer 1 link speed is dictated by a CSU/DSU, in a lab without an external CSU/DSU a DTE (Data Terminal Equipment) cable and DCE (Data Communications Equipment) cable are used.

Command Description
(config)# interface serial 1/0 Configure interface serial 1/0
(config-if)# clock rate 128000 Set clock rate on DCE router side to 128 kbps
(config)# show controllers serial 1/0 Verify clock rate for serial interface 1/0

ACLs

#1-#99, #1300-#1999: Standard IPv4 ACL

#100-#199, #2000-#2699: Extended IPv4 ACL

Default mask for standard ACLs: 0.0.0.0

Command Description
(config)# access-list 23 permit 1.2.3.4 [0.0.255.255] Create ACL #23 or append a rule to ACL #23, allow 1.2.x.x
(config)# no access-list 23 Delete entire ACL #23
(config)# ip[v6] access-list resequence local_only 5 10 Renumber ACL Rules, put first on #5, increment by 10.
(config)# ip access-list {standard, extended} 23 Create ACL and/or enter config mode for ACL #23
(config)# ip access-list {standard, extended} local_only Create ACL and/or enter config mode for ACL 'local_only'
(config-std-nac1)# permit 10.20.30.0 0.0.0.255 Append rule to standard ACL 'local_only'
(config-std-nac1)# 5 permit 10.20.30.0 0.0.0.255 Append rule to ACL at sequence number 5.
(config-std-nac1)# no <sequence#> Remove rule with sequence# from ACL
(config-ext-nac1)# deny tcp any any
(config-ext-nac1)# permit udp host 10.20.30.40 any lt 1024
(config-ext-nac1)# permit udp host 10.20.30.40 any eq dns
(config-ext-nac1)# deny udp host 10.20.30.40 any
(config-ext-nac1)# permit ip any any

Interface ACLs

Command Description
(config)# inter g1/1 Enter if-config mode for g1/1
(config-if)# ip access-group 23 out Apply ACL #23 to outgoing packets, not send by the router
(config-if)# ip access-group 42 in Apply ACL #42 to incoming packets
(config-if)# ip access-group local_only in Overwrite the used ACL, only one ACL per if + proto + direction!
(config-if)# ipv6 traffic-filter 23 out The v6 syntax of course differs...
# show ip interface g1/1 | incl access list Show ACLs on g1/1 (When none set shows not set for v4 and nothing for v6)

Troubleshooting ACLs

Command Description
# show [ip[v6]] access-lists Show all configured ACLs
# show access-list 10 Display all rules in ACL #10 and how often they matched.

NAT

Local addresses are any address as it appears inside the network. Global addresses are any address as it appears outside the network.

Term Definition
inside local IP address assigned to a host inside the network, non-routable
inside global IP address assigned by Network Information Center or ISP, routable
outside local IP address of a remote host as it appears inside the network, non-routable
outside global IP address of a remote host assigned by the host owner, routable
Command Description
(config)# int g1/1 Enter if-config mode for g1/1
(config-if)# ip address 1.2.3.4 255.255.255.240 configure 1.2.3.4/28 on g1/1
(config-if)# ip nat outside Packets going out, need to change their src, incoming their dest ip.
(config)# int g1/2 Enter if-config mode for g1/2
(config-if)# ip address 10.10.23.1 255.255.255.0 configure 10.10.23.1/24 on g1/2
(config-if)# ip nat inside Packets going out, need to change their dest, incoming their src ip.

SNAT

Command Description
(config)# ip nat inside source static 10.10.23.2 1.2.3.5 SNAT - statically map an internal ip 1:1 to an external ip.

DNAT

Command Description
(config)# access-list 42 permit 10.10.23.0 0.0.0.255 Create an ACL identifying 10.10.23/24
(config)# ip nat pool POOL 1.2.3.5 1.2.3.10 netmask 255.255.255.240 Create an IP Address Pool for NATing
(config)# ip nat inside source list 42 pool POOL DNAT IPs matching ACL #42 1:1 with IPs from nat pool 'POOL'.

Note the missing overload.

PAT

The overload keyword means, that one or a couple of external IPs are to be used for multiple internal IPs. Higher level information like connection port numbers are used to identify the correct internal destination for incoming packets. Cisco calls this PAT, while this is what your average joes home router would call NAT.

Command Description
(config)# access-list 10 permit 10.10.0.0 0.0.255.255 Create an ACL identifying 10.10/16
(config)# ip nat inside source list 10 interface g1/1 overload PAT IPs matching ACL #10 many:1 with g1/1s public IP

Troubleshooting NAT

Command Description
# show ip nat translations Show nat table entries if any
# show ip nat statistics Show translations are actually used and interfaces are marked in/out correctly.
# clear ip nat translation {ip, *} Clear dynamic translations. Doesn't mess with SNAT!
# debug ip nat [detailed]

Is the ACL correct? Is there a route to the address? Note: NAT Table entries are kept for 24h after the last use by default.

DHCP Server

Command Description
(config)# ip dhcp excluded-address 10.30.4.1 10.30.4.100 Don't distribute these IPs in leases
(config)# ip dhcp pool PCs Creat and/or enter dhcp config for pool 'PCs'
(dhcp-config)# network 10.30.4.0 /24 define pool addresses
(dhcp-config)# default-router 10.2.1.1 define default-gateway to be distributed in the leases
(dhcp-config)# dns-server 10.30.4.1
(dhcp-config)# domain-name acme.com
(dhcp-config)# lease lease validity time
(config)# int g1/1 Enter interface config mode on client facing interface
(config-if)# ip helper-address 192.168.1.1 Relay DHCP Requests to this host

Troubleshooting DHCP

Command Description
# debug ip dhcp server packet
# show dhcp lease Show dhcp lease information
# show ip dhcp pool Show pool size and addresses in use
# show ip dhcp binding Show which mac got which ip
# sh run | section dhcp See if ip dhcp exclude-address / pool stuff is wrong.
# sh run int g1/1 See if ip helper-address is wrong.

HSRP

Command Description
(config-if)# standby [group-number] ip Join HSRP Group
(config-if)# standby [group-number] priority (optional) Set prio of this router.
(config-if)# standby [group-number] preempt (optional) Preempt other routers when this router becomes active
(config-if)# standby {1,2} (optional) Set HSRP Version

Troubleshooting HSRP

Command Description
# show standby HSRP Groups, their VIPs, state, active router, standby router, preemption.

SLAs

Command Description
(config)# ip sla 23 Create ip sla test #23 and enter its config mode.
(config-ip-sla)# icmp-echo 1.2.3.4 Define icmp-echo test.
(config-ip-sla)# frequency 42 frequency in seconds.
(config)# ip sla schedule 23 life {forever, seconds} start-time now Start test #23 now and until manually stopped.

Troubleshooting SLAs

Command Description
# show ip sla configuration Show all configured ip sla configs
# show ip sla statistics Show sla results

Device Management

Command Description
(config)# hostname R1 Set hostname to R1
(config)# enable password Set enable passwort.
(config)# enable secret Same, but with hashing.
(config)# service password-encryption Very weak encryption of passwords passwords.
# copy flash0: tftp: Copy something from flash to tftp. Wizard asks for details. Works both ways.
# write # copy running-config startup-config
# write erase # erase startup-config
# reload restart the device and load the startup-config
# copy running-config tftp: copy running-config to an tftp server. (interactive)
# copy running-config Merge source config into the running config.
# setup initial configuration dialog
# show version ios, bootloader and hardware infos, uptime, configuration register
# show {running,startup}-config

Firmware Management

Note: flash: is the main flash memory on all iOS devices

Command Description
(config)# boot system flash:filename.bin Boot filename.bin from flash memory.
(config)# boot system tftp:https://10.20.30.40/filename.bin Boot filename.bin from tftp.
(config)# boot system rom Boot ROM monitor as a backup.
(config)# config-register 0x2342 Set the 16bit Configuration Register value used after reboot.
# show file systems Lists available file systems
# show flash0: List fs content and free space.

License Management

Command Description
# license save flash:licenses.lic Save a copy of all licenses.
# license install flash0:license.xml Install a license.
(config)# license boot module technology-package active a evaluation right-to-use license.
# reload Reboot to activate the package and right to use license.
(config)# license boot module technology-package disable deactive a technology-package.
# reload Reboot without that technology-package.
# license clear Remove license from the license storage.
(config)# no license boot module technology-package disable Remove the no longer needed line from the config.
# reload I don't even know why this is needed. Fu cisco.
# show license active licenses
# show license feature technology packe and feature licenses supported.
# show license udi product id and serial number needed to order licenses

Reset Password

Command Description
> confreq Show the configuration register in rom monitor
> confreq 0x2142 Set the configuration register in rom monitor to not load startup-conf
> reset Reboot in rom monitor
# copy startup running
(config)# enable secret foobar Overwrite forgotten password
(config)# config-register 0x2102 Do load startup-config after boot again.
# save

Telnet / Console

Command Description
(config)# banner login "Insert snarky banner." Make sure to include legal terms to sound smart.
(config)# banner motd "Insert snarky banner." Set Login Banner.
(config)# line vty 0 4 Enter config mode for vty 0 to 4 (up to 15 allowed).
(config)# line console 0 Enter config mode for the console port
(config-line)# login Require login on telnet/console connection.
(config-line)# password Enable Telnet and set vty login password.
(config-line)# access-class 10 in Set ACL to limit inbound IPs allowed to access vty
(config-line)# access-class 42 in Overwrite the used ACL, only one ACL per vty + direction!
(config-line)# exec-timeout 10 Autologout after 10 Minutes
(config-line)# login local Require login on telnet/console connection via local users.
(config)# username h.acker secret C1sco123 Create local user with encrypted password.

SSH

Command Description
(config)# hostname Foobar Required to generate SSH keys.
(config)# ip domain-name example.com Required to generate SSH keys.
(config)# crypto key generate rsa modulus 2048 Generate keys like it's 1995! Potentially takes forever.
(config)# ip ssh version 2 Force SSHv2
(config-line)# transport input ssh Force ssh, disable telnet.
# show ip ssh SSH version, timeout time, auth retries..
# show ssh List of active connections

Clock

Command Description
# show clock Show time and date
(config)# clock set 23:50:42 10 Jan 2017 Update clock
(config)# clock timezone EST 0 Update timezone to EST
(config)# ntp server 10.20.30.40 Configure upstream ntp server.
(config)# ntp master [stratum] Enable ntp server.
# show ntp associations ntp connections.
# show ntp status synchronized?, statum, ...

Disable unused services

Command Description
# show control-plane host open-ports Show open ports
(config)# no ip http server Stop the http server (but not https).
(config)# no cdp enable Stop CDP
# auto secure

Radius

Command Description
(config)# username password Local backup user.
(config)# aaa new-model Enable aaa services.
(config)# radius server Add and define Radius conf.
(config-radius-server)# address ipv4 [auth-port ] Use this hostname/ip of server.
(config-radius-server)# key Radius PSK
(config)# aaa group server radius Create authentication group.
(config-sg-radius)# server name Using the radius config.
(config)# aaa authentication login group local Allow that group and local users in.

TACACS+

Command Description
(config)# username password Local backup user.
(config)# aaa new-model Enable aaa services.
(config)# tacacs server Add and define TACACS conf.
(config-server-tacacs)# address ipv4
(config-server-tacacs)# [port ]
(config-server-tacacs)# key
(config)# aaa group server tacacs+ Multiple possible.
(config-sg-tacacs+)# server name
(config)# aaa authentication login group local Allow that group and local users in.

Syslog

Command Description
# logging 10.20.30.40 Log to this syslog server (name or ip)
# logging trap informational Only log messages with min. informational sev.

service sequence-number | Needed for seqence number in syslog messages service time stamps log [datetime, log] | Needed for date and time in syslog messages

Command Description
# show logging syslog status, local logging buffer

SNMP

Command Description
(config)# snmp-server contact [email protected] Contact email
(config)# snmp-server location RZ-Hamburg Where is the device
(config)# snmp-server community [ro, rw] Add community
(config)# snmp-server host 10.20.30.40 SNMP notifications recipient
Command Description
# show snmp community
# show snmp location
# show snmp contact
# show snmp host

CDP - Cisco Discovery Protocol

Command Description
# [no] cdp run Enables cdp globaly and on all interfaces (default)
# (config-if)# [no] cdp enable Enable cdp on an interface
# show cdp neighbors [detail] List connected cisco devices (name, local/remote port, [ip] ..)
# show cdp entry *

LLDP - Link Layer Discovery Protocol

Command Description
# [no] lldp run Enables lldp globaly and on all interfaces
(config-if)# [no] lldp transmit Enable lldp packet transmission on interface
(config-if)# [no] lddp receive Enable lldp packet reception on interace

PPP

Command Description
(config)# username fnord password pass Create users for pap auth.
(config)# inteface S0/0/0
(config-if)# clock rate 125000 Baud rate. Only on DCE cable!
(config-if)# bandwidth 125 Logical speed used for routing cost calc, RSVP...
(config-if)# encapsulation ppp Default is HDLC
(config-if)# ppp authentication pap Require remote to authenticate via pap
(config-if)# ppp pap sent-username fnord password pass Authenticate to remote pap
(config)# hostname routy1 Required for CHAP, used as chap client username
(config)# username routy2 password foobar Create users for chap auth for routy2
(config)# inteface S0/0/0
(config-if)# no ppp authentication pap Remove in favor of chap
(config-if)# no ppp pap sent-username fnord password pass Remove in favor of chap
(config-if)# ppp authentication chap Require remote to authenticate via chap

Note: When routy1 connects to routy2 it looks in it's local user database for a user named routy2 and uses that users password. This means the passwords have to be the same on both sides and the usernames must be the other sides hostname.

Troubleshooting PPP

Command Description
# show controllers S0/0/0 interface, connected type of cable, clock rate
# show interfaces encapsulation, logical bandwidth
# show ppp all session state, auth type, peer ip and name
# debug ppp authentication

MLP

Command Description
(config)# interface Multilink23 Create and configure virtual if
(config-if)# ip address 10.20.30.40 255.255.255.0
(config-if)# ppp multilink Enable mlp
(conifg-if)# ppp multilink group 23 Make phys ifs with mlp #23 join.
(config)# interface s0/0/0 Configure phys ifs
(config-if)# no ip address Remove ip addrs.
(config-if)# encapsulation ppp
(config-if)# ppp multilink
(config-if)# ppp multilink group 23 Join mlp group #23.

Troubleshooting MLP

Command Description
show ppp multilink Physical IFs,

PPPoE

Command Description
(config)# interface Dialer23 Create and configure virtual dialer interface.
(config-if)# ip address negotiated Get IP via PPP/IPCP
(config-if)# encapsulation ppp
(config-if)# dialer pool 23 The dialer interface is a member of one dialer pool...
(config)# interface s0/0/0
(config-if)# no ip address
(config-if)# pppoe-client dial-pool-number 23 ... the pool is a group of one or more physical interfaces.

Troubleshooting PPPoE

Command Description
# show ip interface brief is the dialer if up? Does the dialer have an IP via IPCP?
# show pppoe session Are PPPoE sessions established? Which ports.

GRE

Note: We can run OSPF and other routing protocols through this gre tunnel, as gre supports multicast.

Command Description
(config)# interface tunnel23
(config-if)# ip address 192.168.1.1 255.255.255.0 transit net
(config-if)# tunnel source 10.20.30.40 local, can be linklocal
(config-if)# tunnel destination 6.5.4.3 remote, can be linklocal

tunnel mode gre ip ip mtu

Troubleshooting GRE

Command Description
# show ip interface brief tunnel23 Line hould be up, given a route to the destination.
# show inteface tunnel23 Tunnel source, dest, protocol
# show ip route Should include the transit net as directly connected.

RIPv2

Command Description
(config)# router rip Enable RIP and enter it's config mode
(config-router)# version 2 Set RIPv2, which is Classless
(config-router)# network 192.168.0.0 Advertise connected networks which are within .
(config-router)# network 0.0.0.0 Advertise all connected networks.
(config-router)# timers basic
(config-router)# no auto-summary Don't summarize a smaller subnet route in a bigger one.
(config-router)# passive-interface g1/1 Don't send RIP updates out this interface
(config-router)# passive-interface default Don't send RIP updates on any if by default
(config-router)# no passive-interface g1/2 Overwrite passive-interface default
(config-router)# default information originate Advertise the default route.
(config-if)# no ip rip advertise 123

Troubleshooting RIPv2

Command Description
# show ip[v6] protocols Show rip timers, interfaces, networks,
# show ip rip database Routes learned by rip, used to combile the routing table
# show ip route Show learned routes
# clear ip route * Get rid of all routes

EIGRP

Note: The network command enables any interface with an ip in that net to send and receive EIGRP updates. Also it enables routes to this nets to start beeing advertised.

Command Description
# show run &#124 section eigrp Show EIGRP settings.
# show interfaces g1/1 Show configured/default bandwith and delay.
(config-if)# bandwidth Overwrite bandwidth used for eigrp metric.
(config-if)# delay Overwrite deplay used for eigrp metric.
(config)# router eigrp 23 Add and conf EIGRP AS#23
(config-router)# network 10.20.30.0 0.0.0.255 Announce routes to 10.20.30.0/24
(config-router)# no shutdown On some iOS versions it's off by default.
(config-router)# [no] eigrp router-id Defaults to highest loopback ip
(config-router)# [no] passive-interface g1/2 Disable EIGRP here. Ignore incoming pkgs.
(config-router)# [no] passive-interface default Disable EIGRP on all ifs by default.
(config-router)# maximum-paths Default 4, must match, number of loadbalanced paths.
(config-router)# variance 4 Default 1, Max 4:1 variance for unequal lb.
(config-router)# no auto-summary Don't summarize a smaller subnet route in a big one.
# show ip[v6] eigrp neighbors Neighbor addr, if, hold time, uptime, queued pkgs
# show ip[v6] eigrp interfaces [if-name] If, Number of peers, pending routes, queued pkgs
# show ip[v6] route [eigrp] Routes starting with D were learned via EIGRP
# show ip[v6] eigrp topology [all-links] Topology table, as#, router-id

EIGRP with ipv6

Command Description
(config)# ipv6 unicast-routing Enable v6 routing on the router
(config)# ipv6 router eigrp 23 Configure eigrp as #23
(config-rtr)# no shutdown Enable this eigrp routing process.
(config-if)# [no] ipv6 eigrp 23 Enable eigrp with ipv6 for as #23 on this if.

OSPF

cost = reference bandwidth / interface bandwidth

The default reference bandwith is 100Mbps. Everything faster has a cost of 1.

Command Description
(config)# router ospf 1 1 is the pid, not the area.
(config-router)# router-id 1.2.3.4 Defaults to highest IPv4 on lo, then other ifs.
(config-router)# network 10.20.30.0 0.0.0.255 area 0 enable interfaces for ospf with matching IPs
(config-router)# (no) passive-interface g1/1 Stop in- and egress ospf hello packets.
(config-router)# passive-interface default Mark all ifs passive by default.
(config-router)# default-information originate (always) Advertise default routes into a normal area
(config-router)# auto-cost reference-bandwidth <refbw in Mb/s> Change reference bandwidth speed
(config-if)# ip ospf cost 23 Overwrite interface cost to 23
(config-if)# bandwidth <bw in kb/s> Change interface bandwidth

Router Types

Term Definition
Internal Router All OSPF interfaces in one area
Backbone Router Has one or more OSPF interfaces in the backbone
Area Boundary Router (ABR) Has at least one interface in the backbone area and at least one in another area
Autonomous System Boundary Router (ASBR) Injects routes into OSPF via redistribution from other routing protocols

OSPF with ipv6 (OSPFv3)

Command Description
(config)# ipv6 unicast-routing
(config)# ipv6 router ospf
(config-router)# router-id Required if we don't have any v4 addrs configured.
(config-if)# ipv6 ospf area Required for OSPFv3.

The networks command does not exist, non mentioned commands are the same.

Troubleshooting OSPF

Command Description
# show run | sect ospf
# show ip(v6) protocols Other protocols with lower AD?
# show ipv6 ospf reference bandwidth, router id, networks, interface per area
# show ip(v6) ospf neighbor neighbor IDs, IPs and via interface.
# show ip(v6) ospf neighbor detail dr, bdr, timers, ...
# show interface brief admin down? link?
# show ip(v6) ospf interface brief ospf enabled interfaces
# show ip(v6) ospf interface g1/1 ospf related infos for g1/1, passive?
# show ip(v6) route (ospf) ospf routes are marked O, show route ad and cost

BGP

Note: In other routing protocols the network statement is used to determin the interfaces over which the protocol should talk to its neighbors. In BGP it indicates only which routes should be advertised to the BGP neighbors. The network needs to match an exact route in the routing table or it will still not be announced.

Command Description
(config)# router bgp Create routing process.
(config)# neighbor remote-as BGP does not auto discover neighbors.
(config)# network [mask ] Advertise this network.
Command Description
# show run | sect bgp
# show ip bgp summary neighbors IPs, ASs and session states, bgp version
# show ip bgp neighbors [peer-ip] tcp sessions and timers, bgp parameters
# show ip bgp routing infos received from all peers

CLI

Default Behavior

Here I'll collect crazy default behaviors and how to fix them, I guess..

Command Description
(config)# no ip domain-lookup Don't try to telnet unknown single word commands

Modes

Mode Prompt enter
User > N/A
Exec # > enable
Config (config)# # configure terminal
Interface (config-if)# (config)# interface g1/0
Line (config-line)# (config)# line vty 0 4
DHCP (dhcp-config)# (config)# ip dhcp pool Foobar

Filters

Name Function
include hostname find a line including 'hostname'
section interface find a section including 'interface'
begin interface Show remaining config starting with the first line containing 'interface'
exclude ! exclude all line containing ! (comments)

Navigation

Sequence Function
Ctrl-Shfit-6 Kill many commands
Ctrl-Shift-6 x Move telnet session to background
Esc-B Ctrl-Left arrow
Esc-F Ctrl-Right arrow
Ctrl-R Redraw the current line
Ctrl-U Erase line
Ctrl-W Delete the word left of the cursor
Ctrl-C Drop back to Exec, does not kill processes..
Ctrl-A Move Cursor to the beginning of the line
Ctrl-E Move Cursor to the end of the line
Tab Autocompletion
? Help, can be entered mostly everywhere

Packet Types

Ethernet Frame

Field Field Length Description
Preamble 8 bytes Alternating 1s and 0s used to synchronize
Destination MAC (DA) 6 bytes MAC of recipient
Source MAC (SA) 6 bytes MAC of sender
802.1Q tag (optional) 4 bytes Optional vlan tag. Starts with 0x8100 to mark 802.1Q mode in type location.
Type or Length 2 bytes Layer three type OR length if smaler then 1536 bytes.
Data 46 - 1500 bytes Payload
Frame check sequence (FCS) 4 bytes 32 bit CRC Checksum

IPv4 Header

Field Field Length Description
Version 4 bits IP Version, always four
Internet Header Length (IHL) 4 bits Length of the header
Service Type 8 bits Desired QOS information (DSCP and ECN)
Total Length 2 bytes Packet length, including this header
Identification 2 bytes A unique ID
Flag 3 bits fragmentation behaviour
Fragment Offset 13 bits
TTL 1 byte TTL, decreased by every router by one.
Protocol 1 byte Layer four type
Header Checksum 2 bytes
Options (optional) 16 bytes
Padding max. 31 bits Pad to the nearest 32 bit boundary

TCP Segment

Field Field Length Description
Source Port 2 bytes
Destination Port 2 bytes
Squence Number 4 bytes Unique Number for this Segment
Acknowledgement Number 4 bytes Next expected sequence number, acknowledge all prior Segments.
Header Lenght 4 bits Header size in multiples of 4 bytes, sometimes also called Data Offset.
Reserved 3 bits N/A
Flags 9 bits Control Flags like SYN, ACK, FIN, RST and Flags for congestion control.
Window size 2 bytes bytes sender is currently willing to receive
Checksum 2 bytes Header Checksum
Urgent Pointer 2 bytes Points to the last 'urgent' byte in the Segment, used when URG flag is set.
Options 0 - 320 bits The Size is determined by Header length. TODO:
Data variable

UDP Segment

Field Field Length Description
Source Port 2 bytes
Destination Port 2 bytes
Length 2 bytes Length of the whole Segment
Checksum (optional) 2 bytes Checksum of the whole Segment
Data variable

To Sort and Misc

Command Description
# telnet 1.2.3.4 23 Telnet to 1.2.3.4 using port 23
# disconnect Disconnect background telnet session
# ssh -l h.acker 1.2.3.4 SSH to 1.2.3.4 using h.acker user
(config-if)# duplex {full, auto} Set duplex mode or set it to autonegotiation.
(config-if)# speed {100, auto} Set speed or set it to autonegotiation.

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A comprehensive CCNA CLI reference.

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