CCNA Notes by shuifanglj


									James’ Rough CCNA Crib Notes

Here is a jumble of stuff I crammed for the exam. You may find it useful. You
may not.

Show Commands:
Configuration             sh run                     The Running Config
IOS & Register            sh ver                     IOS version, IOS file
                                                     used, Config Register
Layer-1 (Physical)        sh controller s 0          DTE cable or DCE cable
Layer-2 (Data-Link)       sh int                     MAC address, first IP
                                                     address for an interface,
                                                     encapsulation. Is it up?
                          sh cdp n                   Show directly connected
Layer-3 (Network)         sh ip int                  All IP details. Access-list
                                                     on or off ?
                          sh access-list             Detail on access lists
                          sh ip access-list          IP access lists
                          sh ip route                Networks with C, I, S, R
                                                     to show where learnt
                          sh ip prot                 Which routing protocols
                                                     are active and which
                                                     RIP version etc.
                          sh cdp n d                 Show IOS and IP
                                                     address of directly
                                                     connected neighbours
Other:                    sh line                    Who is logged on here ?
                          sh session                 Where am I logged on ?

Things to remember before you go into the exam:

   1. Always type copy run start when you change a router config.

   2. Access-lists have an implied “deny all” at the end, so always add:
         access-list 1 permit any               to standard lists
         access-list 101 permit any any         to extended lists

   3. Access-groups are set outgoing by default, but it is more likely you will
      want them to be active incoming. Note that to remove an access-group,
      you have to include in or out as appropriate too:
         access-group 100 in to add it or
         no access-group 100 in to remove it

   4. Always remember to type no shut on an interface you have just
      configured in order for it to come up.
5. The copy command only ever has two parameters. It will prompt if it
   wants more. ie. copy tftp flash

6. The boot system tftp command has filename before ip address:
      boot system tftp blahblahblah.bin

7. The configuration register is usually 0x2102. To reset the password,
   change it to 0x2142. Third digit: 0=Load the startup config, 4=Don’t
   load the startup config.

8. The last digit in the configuration register tells the router where to boot
   from: 0=POST, 1=ROM, 2=First IOS in flash, 3=Second IOS in flash
   etc. NOTE: settings 0 and 1 will aways take effect. Settings of 2 or
   above will be ignored if a boot system command exists in the startup
   config and it is being loaded (ie. third digit is 0).

9. In any IP subnet, consider the last octet :
       Subnet number is EVEN
       First IP address is ODD
       Last IP address is EVEN
       Broadcast number is ODD

10. Class A addresses 1-126 (/8), Class B 128-191(/16), Class C 192-

11. ISDN:
       TE2 (RS232 into a TA)
       TA (ISDN talking device)
       S (TE1 hangs off here – UK ISDN interface for routers)
       NT2 (PBX – only for Primary Rate)
       NT1 (BT wallbox)
       U (Wires – US ISDN interface for routers)

12. Use frame-relay map ip 100 when inverse-arp inactive for
    multipoint PVCs

13. Use frame-relay inverse-arp to disable inverse-arp

14. Use frame-relay interface-dlci 100 on a sub-interface when using
    point-to-point PVCs and LMI is playing up. For safety, always use it.

15. encapsulation frame-relay is only set on the interface, not the sub-

16. LMI types: cisco, ansi, q933a

17. Example Frame config:
      conf t
      interface serial 0/0
       encapsulation frame-relay
       no frame-relay inverse-arp
       no shut
      interface serial 0/0.103 point-to-point
      descr blah blah blah
      ip address
      frame-relay interface-dlci 103
      bandwidth 768
      no shut

   18. Add a static route with ip route
      where is the next hop router

   19. CHAP – encrypted. PAP – plain text. In both cases, there is a global
       username <target router hostname> password <password> and
       add ppp authentication chap to the relevant interface.

   20. no cdp run turns off cdp on the router, no cdp enable turns off cdp for
       an interface.

   21. Spanning tree valid modes: blocking, forwarding, listening, learning

   22. Switching methods: store&forward (get whole frame, check fcs), cut-
       through (get destination and start forwarding), fragment-free (get first
       64bytes and start forwarding)

Useful Lists:

   Packet switched                   FR, X.25, ATM
   Circuit switched                  ISDN, Analogue dial-up

   Connection                        TCP, FR, SPX, PPP, X.25
   Connection-less                   UDP, IP, IPX, TFTP

   Classful                          RIPv1, IGRP
   Class-less                        RIPv2, EIGRP, OSPF

   Distance Vector                   RIPv1, IGRP
   Link State                        OSPF
   Balanced Hybrid                   EIGRP

   MAC Address                       6 Bytes (3 OUI, 3 Card)
   IP Address                        4 Bytes (32 bits) Net and Host
   IPX Address                       10 Bytes (4 Host, 6MAC) Net and Node
    Device               Collision Domains        Broadcast Domains
    Hub                  One                      One
    Bridge               One per port             One
    Switch               One per port             One (unless VLANned)
    Router               One per port             One per port

7   Application    Telnet, HTTP, SMTP, FTP       Data
6   Presentation   JPEG, ASCII, Encryption       Data
5   Session        RPC, NFS                      Data
4   Transport      TCP, SPX                      Segments
3   Network        IP, IPX                       Packets     Routers
2   Data-Link      HDLC, FR, PPP, 802.2,         Frames      Switches &
                   802.3                                     Bridges
1   Physical       Ethernet, 802.2, FDDI, V.35   Bits        Hubs &
OSI 7 Layer Model

    Electrical
    Plugs and pins
    Voltage levels

    Transmit data across a single piece of the network (ie. segment or
      cable, rather than the whole network.)
    Error detection, but usually no error recovery.
    Split into LLC and MAC
    MAC
          o Hardware specific
          o Arbitration – ie. CSMA/CD
          o Addressing – ie. MAC, DLCI
    LLC
          o Error Detection – FCS in trailer
          o Fragmentation and flow control – FRF.12, FECN & BECN,
          o Identifying the L3 data – Protocol field in header (inc. SNAP)

    Routing & Addressing
        o Network-wide, not just the segment
        o Ability to learn routes
        o ARP needed to get L2 address from L3 address in order to
           actually send a frame on the network.
        o Routing vs Routed protocols

    Connection-orientated or Connection-less
    Error recovery (TCP uses “send and wait”)
    Re-ordering of data for out-of-order segments
    Flow control:
          o Buffering – store bursts. No attempts to slow incoming
          o Windowing - how much data to send before ACK required
          o congestion avoidance – source quench, stop/start
    Multiplexing (IP port numbers)

  Given an IP address and asked to generate subnet numbers, broadcast
  numbers etc for x hosts per subnet or x subnets.

                               128    192        224   240     248       252        254   255
                               128    64         32    16      8         4          2     1
  n      2n        2n -2
  1      2         0
  2      4         2
  3      8         6
  4      16        14
  5      32        30
  6      64        62
  7      128       126
  8      256       254
  9      512       510
  10     1024      1022

Number of ticks if x subnets    Count the ticks from    Values above the last tick is
required                        the left and crosses    the subnet number and
Number of crosses if x hosts    from the right          magic number
per subnet required

       1. Decide what class the address is and thus which octet you want to play
          with (ie. Class A has default mask, so we need to play with
          the second octet to get 255.x.0.0)
       2. Put in the ticks or crosses as required
       3. Read the subnet octet and magic number.
       4. The octet for the first subnet is the same as the magic number
       5. The octet for the second subnet is the first subnet octet plus the magic
       6. The first valid host is first subnet plus 1
       7. The broadcast address is the next subnet number minus 1
       8. The last valid host is the broadcast address minus 1

  Given an IP address and subnet mask and asked to find the subnet number
  and all valid hosts:

       1. Decide what class the address is and thus which octet you want to play
          with (ie. Class A has default mask, so we need to play with
          the second octet to get 255.x.0.0)
       2. Look-up the octet on the top row to get the magic number.
       3. The multiple of this number which is closest to the same octet in the IP
          address without going higher is the subnet number of the block.
       4. Carry on as before
Password Recovery:

To recover the password, you simply need to convince the router to not load
the startup-config, but still give you a prompt you can work from. On routers,
you have to set the startup register to a value that tells the router to do just
that. You can then set the passwords, reset the register and boot normally.
The Arrowpoints are a bit easier.

On Arrowpoint:
Restart the unit and just before it finishes the boot it says “Press Ctrl-C to skip
loading Startup-Config”. Do that and goto the “After the reboot” section.

On Older routers:
Press BREAK key (or Shift-f5) during first 60 seconds of power-on to get the
rommon prompt and type the following commands:
o/r 0x2142

On Newer routers:
Press BREAK key during first 60 seconds of power-on to get the rommon
prompt and type the following commands:
(say y to change the config and to ignore system config info. Say n to
everything else. In other words: ynnnnnynn)

After the reboot:
copy startup-config running-config
show startup-config (in order to see an unencrypted password)
conf t (goes into config mode)

Change the secret:
enable secret squirrel (sets the enable secret to the word squirrel)

Change the enable password (the secret always supercedes this):
enable password plop (sets the enable password to the word plop)

Change the console password (if required):
Line con 0
password plop (sets the console password to the word plop)

Change the telnet password (if required):
Line vty 0 4
password plop (sets the telnet password to the word plop)

config-reg 0x2102 (set the startup register back to normal)
<ctrl-z> to leave config mode
copy run start (save the new password, if you changed it)

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