Free Cisco GNS3 Lab EIGRP Challenge

Posted by: admin  :  Category: CCNP, EIGRP, GNS3 Dynamips, Other

EIGRP-FINALThis EIGRP challenge has been created to test your knowledge of EIGRP and include such topics as EIGRP Stub network, EIGRP timers, Metric calculations, Authentication, offset-list, Access-list, Prefix-list, and route maps. The final goal of this lab is full connectivity. The topology of the lab has been created using GNS3 and consist of seven 3725 Cisco routers, five switched and a frame relay cloud.

Learning Objectives:

  • Configure basic EIGRP.
  • Configure EIGRP Timers.
  • Adjust Metric calculation.
  • Configure EIGRP Router-IDs
  • Understand load balancing.
  • Configure stub routing.
  • Configure offset-list.
  • Configure EIGRP authentication.
  • Configure access-list
  • Configure prefix-list
  • Configure route maps
  • Configure summary address
  • Configure default routing.

Hello Interval & Hold Time (dead interval):

Routing devices periodically send hello packets to each other to dynamically learn of other routers on their directly attached networks. This information is used to discover neighbors and to learn when neighbors become unreachable or inoperative. By default, hello packets are sent every 5 seconds. The exception is on low-speed, nonbroadcast multiaccess (NBMA) media, where the default hello interval is 60 seconds. Low speed is considered to be a rate of T1 or slower, as specified with the bandwidth interface configuration command.

Router(config-if)# ip hello-interval eigrp autonomous-system-number seconds

The hold time is advertised in hello packets and indicates to neighbors the length of time they should consider the sender valid. The default hold time is three times the hello interval, or 15 seconds. For slow-speed NBMA networks, the default hold time is 180 seconds.

Router(config-if)# ip hold-time eigrp autonomous-system-number seconds

EIGRP Metrics:

EIGRP uses the metric weights along with a set of vector metrics to compute the composite metric for local RIB installation and route selections. Default values are K1=1, K2=0, K3=1, K4=0 & K5=0. The EIGRP composite metric is calculated using the formula:

EIGRP Metric = 256*((K1*Bw) + (K2*Bw)/(256-Load) + (K3*Delay)*(K5/(Reliability + K4)))

  • Bandwidth:

Minimum bandwidth of the route in kilobits per second. It can be 0 or any positive integer. The bandwidth for the formula is scaled and inverted by the following formula:

(10^7/minimum Bw in kilobits per second)

  • Delay

Route delay in tens of microseconds.

  • Reliability

Likelihood of successful packet transmission expressed as a number between 0 and 255. The value 255 means 100 percent reliability; 0 means no reliability.

  • Load

Effective load of the route expressed as a number from 0 to 255 (255 is 100 percent loading).

  • MTU

Minimum maximum transmission unit (MTU) size of the route in bytes. It can be 0 or any positive integer.

Once the K value is changed on a router, the neighbor adjacency will go down because K values have to match between two routers for them to establish an adjacency. Following values must match between two routers for them to become adjacent:

  • K values
  • AS numbers
  • Must share same datalink and must belong to the same IP address space
  • If using authentication, it must be enabled on both routers and passwords must match


The ip summary-address eigrp command is used to configure interface-level address summarization. EIGRP summary routes are given an administrative distance value of 5. The administrative distance metric is used to advertise a summary without installing it in the routing table.

By default, EIGRP summarizes subnet routes to the network level. The no auto-summary command can be entered to configure subnet level summarization.

EIGRP Support for Leaking Routes:

Configuring the leak-map keyword allows to advertise a component route that would otherwise be suppressed by the manual summary. Any component subset of the summary can be leaked. A route map and access list must be defined to source the leaked route.  Leak-map option is only available under physical interface or a virtual-template interface. So if you don’t find this under the sub-interface then don’t freak out.

The following is default behavior if an incomplete configuration is entered:

  • If the leak-map keyword is configured to reference a nonexistent route map, the configuration of this keyword has no effect. The summary address is advertised but all component routes are suppressed.
  • If the leak-map keyword is configured but the access-list does not exist or the route map does not reference the access list, the summary address and all component routes are sent.

EIGRP Variance Command:

EIGRP is the only routing protocol that allows unequal cost load balancing with the help of variance command.  We have to check one condition before we can perform unequal cost load balancing. AD of the worst route should be lower than the FD, this is known as the feasibility condition. 

To get the value for variance, let’s divide the FD of the worst route by the best route. Round off this number and this is your variance value.

EIGRP Authentication:

EIGRP route authentication provides Message Digest 5 (MD5) authentication of routing updates from the EIGRP routing protocol. The MD5 keyed digest in each EIGRP packet prevents the introduction of unauthorized or false routing messages from unapproved sources.

Step 1: Enable MD5 authentication

Router(config-if)# ip authentication mode eigrp autonomous-system md5

Step 2: Enable authentication for EIGRP packets

Router(config-if)# ip authentication key-chain eigrp autonomous-system key-chain

Percentage of Link Bandwidth

 By default, EIGRP packets can consume upto 50% of the link bandwidth configured under the interface. We can change this by using the interface level command “ip bandwidth-percent”. Another way would be to change the bandwidth on the interface so that 50% of that bandwidth is the value that you want the routing protocol to use.

EIGRP Stub Routing:

EIGRP stub router is a feature used in a hub-spoke topology where there is only one way out of the spoke. Also this feature can help save precious cpu cycles since the router won’t have a large routing table. There are a few options available when configuring a router as a stub

  • receive-only
  • connected
  • static
  • summary

Offset List:

An offset list is the mechanism for increasing incoming and outgoing metrics to routes learned via EIGRP. An offset list provides a local mechanism for increasing the value of routing metrics. Optionally, you can limit the offset list with either an access list or an interface.

Router(config-router)# offset-list [access-list-number | access-list-name] {in | out} offset [interface-type interface-number]

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