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{loadposition adposition5}Dynamic Trunking Protocol (DTP) is a Cisco proprietary protocol, that when enabled and configured correctly, uses advertisements to contact the switch on the other end of the link, and auto-negotiate a switchport to either an access or trunk link. When a switchport on either end of the link is misconfigured you will end up with a broken link (see chart below). DTP is enabled by default on the Cisco switches that are commonly used in the CCNA curriculum. There are four switchport modes that DTP will negotiate with in order to determine whether the link will be a trunk or an access link, the four modes are: Access, Trunk, Dynamic Auto, and Dynamic Desirable. The default switchport mode when DTP is enabled is Dynamic Auto. If both switches on either end of a link have DTP enabled, and both switchports are by default in Dynamic Auto mode then the resulting link modes will be Access on both ends of the link. By contrast, if one switchport, on one end of the link, is in Dynamic Auto mode, and the other switchport on the other end of the link, is configured for Trunk mode, then the DTP negotiation will result in the Dynamic Auto switchport changing its mode to Trunk mode and the link will become a trunk. See the chart below for the result when two DTP enabled Cisco switches negotiate switchport modes. Since only Cisco switches support DTP, when connecting to a non-Cisco switch DTP should be disabled.



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The following chart shows how the link will auto negotiate when DTP is enabled on both switches and different DTP modes are configured on either end of the link. When DTP is enabled by default on a switch, the default switchport mode is Dynamic Auto.

DTP auto-negotiation resulting link states
Port Mode Access Trunk Dynamic Auto Dynamic Desirable
Access access not recommended access access
Trunk not recommended trunk trunk trunk
Dynamic Auto access trunk access trunk
Dynamic Desirable access trunk trunk trunk


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Video Tutorial – Packet Tracer

In the following video tutorial, I demonstrate in Packet Tracer, how two Cisco switches running DTP, are able to auto-negotiate the link to either a trunking or access state. If you have Packet Tracer and would like to follow along with the demonstration you can download my PT files here: DTP-1-begin.zipDTP-1-Finished.zip 

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Welcome to Cisco CCNA 3

CCNA 3 Introduction

The Cisco CCNA certification is the most well known computer networking certification in the industry. I recommend a Cisco course of study and the Cisco Academy Curriculum in particular to anyone who wants to learn about computer networking. It is the best foundation to teach how networks communicate, the protocols that are involved, network addressing, subnetting, routing, switching, VLANs, trunking and more!

As a Cisco Networking Academy instructor I have taught the Cisco CCNA curriculum for over 14 years. The Cisco Academy offers 4 classes that together map to the Cisco CCNA certification exam. The current CCNA exam is the 200-125 CCNA, which has a stronger emphasis on IPv6. Another option is to break the CCNA exam into two separate tests the 100-105 ICND1 and the 200-105 ICND2.

All students that are enrolled through the college will qualify to be enrolled in the Cisco Academy, and all Cisco Academy students will have access to online curriculum materials as well as the latest version of Packet Tracer (6.1), a great tool for creating simulated networked environments, complete with functioning routers, switches, and hosts.

The Cisco Networking Academy regularly releases updates to their curriculum to reflect the current CCNA exam objectives. The currently updated curriculum coincides with the new 200-125 CCNA exam and includes many new area of study including IPv6, Virtualization and Cloud Computing, VPN technologies, Software Defined Networking, QoS, and more.

Course Materials

All of the course materials are available through the Cisco Academy website through their learning management system. This includes the complete text, the Packet Tracer software program, interactive activities, multiple choice exams, and plenty of labs with complete instructions. If you prefer a paper copy of the text you can purchase one online from Cisco Press or Amazon. Make sure you order a current version of the text. I have provided a link to the text at Cisco Press and the ISBN number:
Scaling Networks Companion Guide – Print: 9781587133282 / eBook: 9780133476408 

Class Availability

  • How can I enroll in a class?
    I teach the Cisco CCNA through Central Oregon Community College. To sign up for the class and attend remotely online, look for new student registration at http://www.cocc.edu
  • Where can I do my labs?.
    Some labs will be done in class, some labs will be done at home using Packet Tracer, and some labs can be done by remotely by connecting to the CIS Department Netlab+ server.
  • What if I am an online student, and I can’t come to the lab?
    If you are an online student, I recommend that you login to Blackboard and attend the class online through the Blackboard video conferencing tool. The is always available through video conference and it will also be recorded and available for watching later.
  • How will I turn in assignments?
    Exams will be taken online through the Cisco Academy website and learning management system. Labs will be turned into me directly.
  • What are the assignments and how will I be graded?
    I grade on a point system. Every week you will have the opportunity to
    earn points from chapter exams and chapter labs. At the end of the class there is a cumulative multiple choice final exam and a cumulative lab final.


OSPF Overview

Open Shortest Path First (OSPF) is a link-state routing protocol that is designed to work with large, more complex networks. OSPF is a classless routing protocol that supports VLSM and CIDR, and uses the Shortest Path First (SPF) algorithm to calculate the best path to a network. OSPF uses a routing metric of “cost” that in Cisco’s implementation is based mainly on the bandwidth of a link. OSPF is able to support hierarchical and scalable network designs through its ability to handle multiple OSPF routing areas.

The Cisco CCNA curriculum requires students to know how to implement and configure only a single-area OSPF network.

OSPF has some similarities to EIGRP, especially in regards to configuration, like requiring a process-id number, using wildcard bits for the subnet mask, hello packets, neighbor relationships or adjacencies, triggered updates, and the use of multiple tables like the neighbor and topology tables.

OSPF Characteristics

  • Algorithm – Dijsktra’s SPF algorithm
  • Metric – Cost, which is based on the bandwidth of a link
  • Administrative Distance – 110
  • Process-ID number – the process-id number is declared when OSPF is started/configured and is a number from 1 to 65535. The process id number does NOT need to match other OSPF routers in the area in order to create adjacencies (see commands below).
  • Wildcard bits/mask – The wildcard mask is the inverse of a network subnet mask (e.g. is It is declared after the network number in the network command (see commands below)
  • Area number – The area number is a number from 0-255, declared at the end of the network command after the wildcard bits. Routers in the same area will exchange routing information or Link State Updates or LSUs (see commands below)
  • Hello Interval – Hello packets are sent every 10 seconds by default. In order for OSPF routers to establish neighbor adjacencies and exchange routing information successfully, the hello interval needs to match all OSPF routers in the OSPF area.
  • Dead Interval – The dead interval is 40 seconds by default. The dead interval should be 4 times the hello interval, and needs to match all OSPF routers in the area
  • Multiple Tables – Routing Table, Topology Table, and Neighbor Adjacency Table
  • DR and BDR Elections – In broadcast multi-access networks (Ethernet), routers in the OSPF area will elect a Designated Router (DR) and a Backup Designated Router (BDR). The DR will be the receiver and distributor of Link-State Packets to other routers in the OSPF area. The BDR will wait, and be ready to take over the duties of the DR in case it fails.

IOS CLI Commands

The router ospf command starts the OSPF routing process. The process ID number can be a number between 1 and 65535:

 router(config)# router ospf <process-id>

  EXAMPLE: router(config)# router ospf 1

The network command will add a connected network to the routing process. In addition to the network IP address you need to provide the wildcard mask, which is the inverse of the subnet mask and the area parameter and number. Typically in single area OSPF the area is often set to area 0.

 router(config-router)# network <network-number> <wildcard-mask> area <area-number>

  EXAMPLE: router(config-router)# network area 0
EXAMPLE: router(config-router)# network area 0
  EXAMPLE: router(config-router)# network area 0 //for a /30 subnet mask

In OSPF, the router-id command will manually set the router’s router-id. In broadcast multi-access networks the router with the highest router-ID will become the designated router (DR) and the router with the second highest router-ID will become the backup designated router (BDR).

 router(config-router)# router-id <ip-address>

  EXAMPLE: router(config-router)# router-id

The passive-interface command can be used to stop OSPF packets from being sent out of a network interface where there are no other OSPF routers present.

 router(config-router)# passive-interface <interface-number>

  EXAMPLE: router(config-router)# passive-interface fastEthernet 0/0

Cisco’s OSPF cost metrics do not account for links faster than 100 Mbps. For example, a 100 Mbps Ethernet interface will calculate to an OSPF cost of 1, but what if you have a 1000 or 10000 Mbps Ethernet interface? The auto-cost reference-bandwidth can adjust the cost metrics to account for links fast than 100 Mbps.

 router(config-router)# auto-cost reference-bandwidth <megabits-per-second>

  EXAMPLE: router(config-router)# auto-cost reference-bandwidth 10000


The default-information originate command will distribute a default route to other OSPF area routers.

 router(config-router)# default-information originate 

You can use either of the following commands to exit out of router configuration mode.

 router(config-router)# end
router(config-router)# exit

Since OSPF relies on bandwidth for the metric, it is a good idea to set the specific bandwidth of the serial interface, otherwise the Cisco serial interfaces will default to a speed of 1544 Kbps, which may lead to an inaccurate measurement of the cost of the link. It is important to remember that this command is applied to the network interface in interface configuration mode.

 router(config)# interface serial <interface-number>
router(config-if)# bandwidth <speed-in-kbps>

  EXAMPLE: router(config)# interface serial 0/0/0
router(config-if)# bandwidth 384

Another command that is applied to a network interface is the ip ospf priority command. This command can be used to manipulate the DR/BDR election process. By default, the Cisco router’s interfaces are all given an OSPF priority of 1, by changing this value to a higher number you can effect the DR/BDR elections. An OSPF priority of 0 will insure the router is never the DR, but an OSPF priority number of 255 will insure that the router will be elected as the designated router or DR.

 router(config-if)# ip ospf priority <0-255>

  EXAMPLE: router(config-if)# ip ospf priority 255

The following commands are all applied to a network interface, but they all effect the OSPF routing protocol operation. Instead of configuring the bandwidth of the link, which will subsequently effect the calculation of the cost metric, you can configure the cost value directly. To do this you need to know how to manually calculate the cost metric. The cost metric of a network link is calculated by the following method: cost equals 10^8 power divided by the network interface speed in bits per second, e.g. the cost for Fast Ethernet is 10^8/100,000,000 = 1..

 router(config-if)# ip ospf cost <cost-value>

  EXAMPLE: router(config-if)# ip ospf cost 781 //for a 128kbps link

For neighboring OSPF routers to achieve adjacencies the OSPF hello interval and dead interval, on each OSPF router needs to match. In a multi-access, broadcast network the default hello interval is 10 seconds, and the dead interval is set to four times the hello interval, or 40 seconds. You can manipulate these times to, for example: have less hello packets on the network, but if you adjust the hello interval, you also need to adjust the dead interval, and you need to do so, for all OSPF routers in the OSPF area.

 router(config-if)# ip ospf hello-interval <seconds>
router(config-if)# ip ospf dead-interval <seconds>

  EXAMPLE: router(config-if)# ip ospf hello-interval 10
router(config-if)# ip ospf dead-interval 40


The following show commands are useful in verifying and troubleshooting OSPF operation and configuration, as well as identifying the router-ids and the identities of the DR and BDR. 

 router# show ip ospf neighbor
router# show ip ospf interface
router# clear ip ospf process
router# show running-config
router# show ip protocols
router# show ip route

Sample Command Usage

router(config)#router ospf 1
router(config-router)#network area 0
router(config-router)#network area 0
router(config-router)#passive-interface fa0/1
router(config-router)#default-information originate
router#show ip ospf neighbor
router# show ip ospf interface
router# clear ip ospf process

OSPF Show Commands

Example OSPF Network

     router#show ip ospf neighbor

In the “show ip ospf neighbor” command above you can see that the router R0 has established three neighbor relationships or adjacencies with the other routers. The “Neighbor ID” above is the neighbor router’s Router ID#. The Router ID# can be different than the neighbor router’s IP address on the network. In the example above the first router listed has a Router or Neighbor ID of but its IP address on the network is You can also see that router at (R3) is the current BDR or Backup Designated router and that the “Pri” or Router Priority Number has been changed from the default number of 1 to 50. The “State” shows that all three routers have current “FULL” adjacency or neighbor relationships. DROTHER routers will only form FULL adjacencies with DR and BDR routers and 2WAY adjacencies with each other. You can see this in the image of R1’s “show ip ospf neighbor” output above. Even if we only look at the output of R0’s show command above (top router output image) we can infer that the Designated Router or DR must be the router that issued the command (R0), because of the fact that there is no neighbor listed as a DR, only a BDR and two DROTHERs.

     router#show ip route

In the image above, the “show ip route” command has been issued, displaying router R0’s routing table. From the routing table we can tell that the R0 router has two connected networks “c on FastEthernet0/1” and “c on FastEthernet0/0” and that it has learned from OSPF about routes to three additional “o” networks:,, and a “o*E2” candidate default route/gateway of last resort.


OSPF Lab Overview

In this lab, the goal is to set up multiple routers in a multi-access network to observe and control the OSPF DR/BDR election process. The video tutorials below illustrate the process using Packet Tracer to simulate the network environment.


If you have Packet Tracer, and you would like to follow along with the videos, download my Packet Tracer starter file: click here

Video Tutorials

Link-State Routing Protocols

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Link-State Routing Protocols Overview

Link-State Routing protocols are routing protocols whose algorithms calculate the best paths to networks differently than Distance Vector routing protocols. Whereas Distance Vector protocols know routes by measures of distance and vector(direction) as reported by neighboring routers, Link-State routing protocols calculate their network routes by building a complete topology of the entire network area and then calculating the best path from this topology or map of all the interconnected networks.

{loadposition adposition5}There are two link-state routing protocols, OSPF and IS-IS. The Cisco CCNA curriculum covers the Open Shortest Path First or OSPF link-state routing protocol, and the IS-IS routing protocol is part of the CCNP curriculum.

Link-State Characteristics

  • SPF algorithm – Link-State routing protocols are designed around Dijkstra’s Shortest Path First Algorithm (SPF) in which the shortest path from point A to point B is build around a metric of cost.
  • Cost metric – SPF algorithm finds the shortest path based on a metric network link costs. Each router measures the cost of its own directly connected networks or "links." Cost is a measure of the quality of a link based mostly on bandwidth.
  • Hello packets – Link-State routing protocols establish adjacencies with neighboring routers using hello packets.
  • Link State Packets (LSP) – Initial flooding of link-states to all routers in the network.
  • Topology or SPF Tree – Link-State routing protocols build and maintain a complete map or topology of the network area.   {loadposition adposition6}

Link-State Advantages

  • Faster Convergence – Unlike Distance Vector routing protocols which run algorithm calculations before sending updates, Link-State routing protocols send link-state updates to all routers in the network before running route calculations
  • Triggered Updates – Unlike Distance Vector routing protocols (except EIGRP) which send periodic updates at regular intervals, Link-State routing protocols send LSPs during router startup (flooding) and when a link changes states like going up or down. If their are no changes in the network the protocol only sends hello packets to maintain adjacencies.
  • Scalability – Link-State routing protocols support the ability to configure multiple routing "areas" which allows an administrator to segment a routing protocol processes to defined areas which supports the expansion and troubleshooting of much larger networks.

Link-State Disadvantages

  • Greater Processing Requirements – Link-State routing protocols typically demand greater processing power and memory resources from the router.
  • Greater Administrator Knowledge – Link-State routing protocols can demand advanced administrator knowledge to configure and troubleshoot the network area


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EIGRP Packet Tracer Lab Part 2

EIGRP LAB Part 2 – Overview

In the second part of this Packet Tracer EIGRP lab, I build off of the network created in the first 3 video tutorials. In video tutorial part 4: I expand the EIGRP network by adding an ISP router with a default route out of the network. I distribute that route with the “redistribute static” command and observe the learned EIGRP external route in the routing table.

In video tutorial part 5: I add another router to the EIGRP network to create a scenario to show EIGRP’s default summarizing behavior and the problems it can cause by creating summary routes to null0 interfaces bypassing the router’s IP classless behavior.

In video tutorial part 6: Using Packet Tracer’s simulation mode I test the network using PING to demonstrate EIGRP’s auto-summarizing behavior and the problems it causes by dropping packets to a summary route null0 interface instead of continuing to search the routing table for a default route. The problem is fixed with the no auto-summary command.

In video tutorial part 7: I change the bandwidth on two serial interfaces to demonstrate EIGRP’s ability to prioritize routes based on a route’s bandwidth. This allows me to demonstrate how EIGRP and DUAL can calculate successor and feasible successor routes when multiple routes are available. This time the “no auto-summary” command is used on all routers in the network in order to fix entries in the topology table.


You can download the Packet Tracer file to follow along with this second series of EIGRP video tutorials: basicEIGRPstep2.zip Note: You will need to have the Packet Tracer program installed on your computer for the downloadable file to work. Packet Tracer is free to all students enrolled in a Cisco Academy course. Feel free to contact me if you would like to enroll in a Cisco Academy CCNA course.

Video Tutorials

EIGRP Packet Tracer Lab

EIGRP Lab – Overview

In the series of video tutorials below, I walk through the process of configuring a network to work with the EIGRP routing protocol. In the first video, I subnet the network into six subnets of various sizes; in the second video, I wire the network and configure the router’s network interfaces with IP addresses; in the third video, EIGRP is configured on all of the routers and the learned EIGRP routes are verified in the routing tables.


Download the Packet Tracer file to follow along with the EIGRP video tutorials below: basic-EIGRP-beginning.zip Note: You will need to have the Packet Tracer program installed on your computer for the downloadable file to work. Packet Tracer is free to all students enrolled in a Cisco Academy course. Feel free to contact me if you would like to enroll in a Cisco Academy CCNA course.

Video Tutorials


EIGRP Overview

Enhanced Interior Gateway Routing Protocol or EIGRP is Cisco’s proprietary Distance Vector routing protocol that replaced the earlier IGRP routing protocol. EIGRP introduced significant improvements to the IGRP routing protocol including support for VLSM and CIDR, guaranteed “loop free” routes, and faster convergence times.

Protocol Administrative
connected route 0
static route 1
EIGRP summary 5
EIGRP internal 90
IGRP 100
OSPF 110
RIP 120
EIGRP external

Routing Enhancements

  • VLSM & CIDR – EIGRP has support for variable length subnet masks (VLSM) and classless inter domain routing (CIDR).
  • DUAL algorithm – The diffusing update algorithm or DUAL, provides guaranteed and optimized loop free routes.
  • Successor & Feasible Successor routes – The successor route is the best route to a destination network. If available, DUAL and the EIGRP topology database will also calculate a guaranteed loop free backup route called the Feasible Successor route.
  • Partial & Bounded Updates – for faster convergence times. No periodic updates like RIP. EIGRP only sends information when there is a change in the network, like a network link going down. EIGRP does not send the entire routing table, just the information that has changed and only to those routers that need the new information.
  • Routing Metrics – EIGRP uses a 32 bit routing metric that is backwards compatible with IGRP’s 24 bit metric. EIGRP’s routing metric is not based on hop count like RIP, it is based instead on: Bandwidth, Load, Delay and Reliability, with Bandwidth and then Delay being the most important factors. EIGRP also features MTU and Hop Count as metric vectors, though they are not used in route calculations.
  • RTP reliable transport protocol – EIGRP uses its own layer 3, layer 4 protocol to exchange routing updates, and information
  • PDMs protocol dependent modules – can be added to EIGRP so that it can route other routed protocols like Apple Talk and IPX/SPX
  • Unequal Cost Load Balancing – EIGRP is capable of being configured for unequal cost load balancing

    EIGRP Routing Tables

  • Routing Table – the best “loop free” network routes are placed in the routing table
  • Neighbor Table – neighbor adjacencies are maintained in this table
  • Topology Table – maintains “loop free” backup routes known as successor routes and feasible successor routes

IOS CLI Commands

The command to start the EIGRP routing process is router eigrp followed by the autonomous system number. The autonomous system number or (AS#) functions more as a process ID number. The AS number needs to be the same on all neighbor EIGRP routers.

router(config)# router eigrp <AS/ID-number>

  EXAMPLE: router(config)# router eigrp 1

The command to add a network and interface to the EIGRP routing process is: network <network number> <wildcard mask>. The network number is the network ip address and the wildcard bits is the inverse of the subnet mask in decimal, so a /24 subnet mask or in wildcard bits is and a /16 or would be

 router(config-router)# network <network-number> <wildcard-mask>

  EXAMPLE: router(config-router)# network
router(config-router)# network

If the EIGRP router is a boundary router it will auto-summarize routes by default. A boundary router is a router with multiple interfaces having different classful network ranges and/or different subnet mask lengths. This can cause problems by working against EIGRPs ability to handle VLSM, CIDR, and general routing to non-contiguous networks. The command to turn off auto summarization is no auto-summary.

 router(config-router)# no auto-summary

The redistribute static command will propagate all static routes including the default route to all other EIGRP routers in the network.

 router(config-router)# redistribute static

The passive-interface command can be used to stop EIGRP packets from being sent out of a network interface where there are no other EIGRP routers present.

 router(config-router)#passive-interface <interface>

 EXAMPLE: router(config-router)#passive-interface fastEthernet 0/0

The no auto-summary command is very useful to taking advantage of EIGRP’s ability to route to variable length and discontiguous subnets, however you may want to still use summary addresses in order to optimize your router’s routing tables. In this situation you can manually configure and advertise an EIGRP summary address with the ip summary-address command configured on a network interface.

 router(config)# interface <int-type> <int-num>
router(config-if)# ip summary-address eigrp <as-number> <ip-summary-address> <subnet-mask> <administrative-distance>

  EXAMPLE: router(config)# interface s0/0/0
router(config-if)# ip summary-address eigrp 1

The following commands will exit from router configuration mode

 router(config-router)# exit
 router(config-router)# end

The following show commands are useful in verifying and troubleshooting EIGRP operation and configuration, as well as identifying the successor and feasible successor routes

 router# show ip eigrp neighbor
router# show ip eigrp topology
router# show running-config
router# show ip protocols
router# show ip route

Video Tutorials