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Matt's Cisco Page                                                         

This page is very young but you have got to start somewhere!  I created this to help me study for my CCNA exam.


I hope you find all of this useful.  Cheers  Matt.

 

  Index

 

 

    Web Links

 

These are some more Cisco web links.

 

 

    IP Conversion Tables

 

Below is the subnet table I would memorise before going in to do an exam with TCP/IP topics in it.

 

Mask Number
of
Subnets 		Interval Number of Hosts
      Class A Class B Class C
192 2 64 4M 16K 62
224 6 32 2M 8K 30
240 14 16 1M 4K 14
248 30 8 512K 2K 6
252 62 4 256K 1K 2
254 126 2 128K 510  
255 254 1 64K 254  

 

Click Here to download IPCalc207.exe   This is an excellent Binary Subnetting Tool.

 

Decimal-Binary-Hexadecimal

Decimal Binary Hexadecimal
0 0000 0
1 0001 1
2 0010 2
3 0011 3
4 0100 4
5 0101 5
6 0110 6
7 0111 7
8 1000 8
9 1001 9
10 1010 A
11 1011 B
12 1100 C
13 1101 D
14 1110 E
15 1111 F

 

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   STP - Spanning Tree Protocol 802.1d
 

STP stops frames looping endlessly in networks that have redundant paths (links).  STP blocks chosen ports and stop them forwarding frames making sure that only one active path exists between any LAN segments.

How does STP choose what state a port is in?

  • STP Elects a ROOT BRIDGE.  All interfaces are set to forward on this ROOT BRIDGE..
  • All other Bridges place their "least cost to root" interface called the ROOT PORT into a forwarding state.
  • If there are multiple Bridges on the same network segment.  The Bridge with the lowest cost to the root bridge becomes the DESIGNATED BRIDGE.
  • The port on the DESIGNATED BRIDGE attached to this segment is called the DESIGNATED PORT
  • All other interfaces are set to Blocking.

 

Catalyst 2950 - Use the spanning-tree vlan global configuration command to configure Spanning Tree Protocol (STP) on a per-VLAN basis.  

 

Catalyst 2950 - Use the show spanning-tree privileged EXEC command to display spanning-tree information for the specified spanning-tree instances.

 

 
Port STP State Set to Description
Any Root Bridge Ports Forwarding The root bridge is the designated bridge on all the networks it's connected to
Any Non-Root Bridge's Root Port Forwarding The root port is the port receiving the lowest cost BPDU from the root bridge
Each LAN's designated port Forwarding The bridge with the lowest cost BPDU is the designated bridge for that segment
All other Ports Blocking These ports cannot send or receive frames

 

Transition-Intermediate States

STP State Port State Learns
MAC addresses		 Description
Blocking Blocking No  
Listening Blocking No Wait to make sure there are no better Hellos from a better Root
Learning Blocking Yes Learn the MAC's without forwarding
Forwarding Forwarding Yes  

STP has a 50 second default convergence time.

  • 20 sec - No BPDU's for MaxAge before transitioning from Blocking to Forwarding
  • 15 sec - Forward Delay - Listening
  • 15 sec - Forward Delay - Learning

 

 

How does STP elect the ROOT?

  • Each Bridge sends STP messages (BPDU's) stating "I am the bridge".
  • The Bridge with the lowest Bridge ID becomes the ROOT.
  • Bridge Protocol Data Units (BPDU), they contain:
    • Root Bridge's ID - Priority Number + MAC address
    • Cost to reach the Root from this Bridge
    • Bridge ID of the sender of this BPDU
  • hello BPDU every 2 seconds by default to check topology is connected

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    RSTP - Rapid Spanning Tree 802.1w
 

RSTP shares many features of STP and works well along side it on a topology. 

RSTP has a much shorter convergence below 10 seconds but as low as 1 or 2 seconds.

  • 6 sec - Loss of path to a Root Bridge, in 3 times the hello timer (2 secs default).
 
Port STP State Set to Description
All the the Root Bridge's Ports Forwarding The root bridge is always the designated bridge on all connected segments
Non-Root Bridge's Root Port Forwarding The root port is the port receiving the lowest cost BPDU from the root bridge
Each LAN's designated port Forwarding The bridge forwarding the lowest cost BPDU onto the segment is the designated bridge for that segment
All other Ports Blocking These ports cannot send or receive frames
Alternate Port Discarding Lower root BPDU. Used if the designated port fails.
Backup Port Discarding Used if the forwarding port fails.

 

Transition-Intermediate States

STP State RSTP State
Blocking Discarding
Listening Discarding
Learning Learning
Forwarding Forwarding
Disabled Discarding

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    VLAN Trunking - ISL and 802.1Q
Multiple switches can host the same VLAN.  So how does VLAN specific traffic flow between multiple switches? Trunking. Trunking is the term used to describe passage of VLAN tagged frames between multiple switches.

         

ISL - Inter-Switch Link

 

  • Cisco-proprietary protocol, thus only between Cisco switches.
  • Fully encapsulates the original frame.

 

802.1Q

 

  • IEEE standard, thus multi-vendor support.
  • Doesn't encapsulate the original frame.
  • Adds a 4 byte header to the original header.
  • Uses a Native VLAN

 

VTP - VLAN Trunking Protocol is used by Cisco switches to swap VLAN configuration data between Cisco switches.  This allows the configuration of VLANs to be done once on one switch (VTP Server) instead of multiple times on multiple switches.

  • VTP sends advertisements throughout the VTP domain every 5 minutes.
  • VTP advertisements contain a revision number that specifies the version of the information.
  • VTP has three modes:
    • Server -  create, modify and delete. Stored in NVRAM.
    • Client - cannot create, modify and delete. Not stored in NVRAM.
    • Transport - this mode of operation disables VTP and switches ignore VTP advertisements.  Local settings are configured only. Stored in NVRAM.
  • VTP Pruning is used to make sure that only switches who are participating in the VLANs will receive advertisements about the VLAN.

 

VLAN Command Summary example from - http://www.ciscopress.com/articles/article.asp?p=102157&seqNum=7

Command Description
vtp mode [server | client | transparent] In global configuration mode, this command sets the operational VTP mode for the switch. The default is server.
vtp domain name In global configuration mode, this command assigns a VTP domain name, which allows the switch to send VTP advertisements out trunk links. The default is NULL, which would allow a switch to join the first domain it received an update from.
show vtp status Displays VTP status information including configuration revision number, domain name, and switch mode.
switchport mode [trunk | access | dynamic [desirable | auto | nonegotiate]] In interface configuration mode, this configures the behaviour of the interface. Trunk mode will force frame tagging. Dynamic mode can become a trunk if it negotiates with the other side of the link. Access mode is a nontrunk port.
switchport trunk encapsulation [isl | dot1q] Used in interface configuration mode to specify a trunking protocol. For some switches, before you can set an interface to trunk mode, you must first specify the encapsulation.
show interface [type module/port] trunk Displays trunking information about the active or specified trunk links on the switch.
show interface [type module/port] switchport Displays Layer 2 configuration and operational parameters of the switch. This includes VLAN membership and trunking status.
vlan number In global configuration mode, this command defines a VLAN and puts the switch into VLAN configuration mode. In VLAN configuration mode, commands such as name can be used to further define the VLAN.
show vlan [id vlan#] Displays VLAN information. The id option allows you to specify a particular VLAN.
switchport access vlan [1-4096 | dynamic] In interface configuration mode, this command assigns an access port to a VLAN or makes it a dynamic port.
show vlan brief Displays a brief table of the VLANs, including the port membership for each VLAN.
show spanning-tree [vlan number] Displays Spanning Tree information for the switch or a VLAN if the vlan option is used.

 

 

Links?

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    Distance Vector Routing Protocols

 

Class of routing algorithms that iterate on the number of hops in a route to find a shortest-path spanning tree. Distance vector routing algorithms call for each router to send its entire routing table in each update, but only to its neighbours.

Distance Vector Routing protocols advertise spare information about routes.  When a distance vector router receives an update, the update says nothing about the routers beyond the neighbouring router that sent the update.

 

 

RIP- Routing Information Protocol

IGRP- Interior Gateway Routing Protocol

 

Distance Vector Routing Protocols prevent problems by doing the following:

Feature RIP
(Default Setting)		 IGRP
(Default Setting)
Update Timer 30 Seconds 90 Seconds
Metric Hop Count Bandwidth and Delay.
May also be reliability, load and MTU.
Hold-Down Timer 180 280
Infinite-Metric Value 16 4,294,967,295


 

Route Problem Problem Defined Solution Description
Routing loops  Updates pass each other over the same link Split Horizon Only advertise a route out an interface if it wasn't learned on that interface
Split Horizon
with Poison Reverse		 If a route fails then we can advertise the route on all interfaces with an infinite distance metric.
Routing loops

Routing information loops through alternative paths

 Route Poisoning When a route to a subnet fails, the subnet is advertised with an infinite-distance metric, in RIP this would be a metric of 16.  This means the route is still known but is represented with such a large metric it is considered invalid.
Counting to infinity

The problem of counting to infinity is normally caused by gossip and wrong information being communicated between the routers. The hop count keeps on increasing each time the packet goes through a router.

Therefore to solve this problem there is need to define a maximum hop count. Distance Vector (RIP) permits a hop count of up to 15, so anything that needs 16 hops is unreachable.

 Hold-down timer After finding out that a route has failed, a router waits a certain period of time before believing any other routing information about that subnet.
Triggered updates When a route fails, an update is sent immediately rather than waiting on the update timer to expire.  This ensures that all the routers know of failed routes before any hold-down timers can expire.

 

Some RIP and IGRP commands

 
Command Description
router rip Global Configuration Mode
router igrp as-number Enables IGRP.  An autonomous system number is needed (but does not need to be registered) so you can use any number as long as the number is used by all the routers.
network net-number Enables RIP or IGRP on a set of interfaces based on the network number specified
show ip route [ip-address [mask] [longer-prefixes]] | [protocol [process-id]] Shows entire routing table, or one entry if subnet is entered

 

Administrative Distances

Administrative Distance allows IGRP (100) to be more believable than RIP (120) by default.  In the case where a router learns the same route from two different protocols eg. RIP and IGRP by default it will believe IGRP first because it has the lower Administrative Distance.

10.10.40.0 [100/8890]

network [administrative distance/metric]

 
Route Type Default
Administrative
Distance
Connected 0
Static 1
IGRP 100
RIP 120

 

  

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    Link State Routing Protocols
 
Routing algorithm in which each router broadcasts or multicasts information regarding the cost of reaching each of its neighbours to all nodes in the inter-network. Link state algorithms create a consistent view of the network and therefore are not prone to routing loops; however, they achieve this at the cost of relatively greater computational difficulty and more widespread traffic (compared with distance vector routing algorithms).

Link State protocols advertise a large amount of topological information about the network.  They may even discover neighbours before they actually talk to them.  Unlike distance vector, link state protocols must calculate the metric instead of simply being told the metric received in the routing update.

 

OSPF- Open Shortest Path First

IS-IS- Intermediate System to Intermediate System

 

Terminology Description
LSU's - Link-State updates Routing updates sent between Link state routers
LSA's - Link-State advertisements Individual entries in a LSU which describe a sub-net number and mask,
the cost (metric) and other information about the subnet.
Dijkstra SPF The algorithm used to calculate routes. The inventor Dijkstra and Shortest Path First
Hello packets Used to keep in touch with their neighbouring routers.  Small packets which identify the subnet,
the router sending the update. Hello's tell the router that the link is still up and usage.
OSPF Areas OSPF areas break up the network so that the routers in one area only find out small pieces
of network information about other areas.  No information about the routers in the other
area is gained at all.  This allows the routers to use their CPU cycles to calculate their local routes only
, great for larger environments.
ABR - Area Border Router A router that is in multiple areas and thus does not gain the advantages from Areas.

 

Feature Link-State Distance Vector
Convergence Time Fast Slower, due to loop avoidance features
Loop Avoidance Built-in Extra Feature Set
Memory and CPU requirements Large but scalable Low
Standards OSFP-Public RIP-Public, IGRP-Cisco

 

Some OSPF commands
Command Description
router ospf process-id Global Configuration Mode
network ip-address wildcard-mask area area-id Router sub command
ip ospf cost interface-cost OSPF cost associated with the interface
bandwidth bandwidth Sets the interface bandwidth.  OSPF uses this to calculate the cost - 108 /bandwidth

 

Administrative Distances

Administrative Distance allows one route to be more believable than another.

 
Route Type Default
Administrative
Distance
Connected 0
Static 1
OSPF 110
IS-IS 115

 

  

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    Balanced Hybrid Routing Protocols
 
EIGRP is placed in this category all by itself because it shares some features that act like distance vector protocols and some that act like links state protocols.

 

EIGRP- Enhanced Interior Gateway Routing Protocol

EIGRP:

  • undertakes neighbour discovery and exchange routing information. 
  • sends and receives "EIGRP Hello" packets to ensure that the neighbour is still up and active.  When the link status changes or the topology is different routing updates are sent with the new information. 
  • uses a formula (the same as IGRPx256) based on bandwidth and delay to calculate the metrics. 
  • keeps loop free backup routes in its routing table and uses them if a route becomes unusable.
  • converges quickly.
  • proprietary only thus Cisco only routers.

 

Administrative Distances

Administrative Distance allows one route to be more believable than another.

 

Route Type Default
Administrative
Distance
Connected 0
Static 1
EIGRP summary route 5
EIGRP internal 90
EIGRP external 170

 

  

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    NAT

 

Most people have a little trouble with the NAT terminology used when they first see it.  I hope this diagram helps a little bit.

 

 

Command Description
ip nat {inside | outside} Interface Sub-Command
ip nat inside source {list {access-list-number | access-list-name} | route-map name} {interface type number | pool pool-name} | [overload] Global Command
ip nat inside destination list {access-list-number | name} pool name Global Command
ip nat outside source {list {access-list-number | access-list-name} | route-map name} pool pool-name} | [add-route] Global Command
show ip nat statistics Lists counters for packets and NAT table entries.
show nat translations [verbose] Displays the nat table

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    Point to Point Connections

 

 
     
     
     
     
     
     
     

 

 
 
   
   
   
   
   
   

 

  

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    Topic

 

 
     
     
     
     
     
     
     

 

 
 
   
   
   
   
   
   

 

  

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This page was last edited on Tuesday, 16 March 2010 08:04:19 PM