Problem in EIGRP!!!!!

[Anand Solgama]

Hi,

I read sometime.. most of topics I know but sometime forget basics. May be good way yo use book again.

I left some topics because they were too tuff and tired with them multicast etc ...but now ...want to do one good point ...anyone can help

1- EIGRP variance and

2-load balance in good way:)

[JohnTylerPearce]

Arand.

By default, EIGRP supports equal cost load balancing. So if two routes, in the EIGRP Topology Table, have an equal composite metric.

For example:

Route_A: 192.168.1.0/24 (512000)

Route_B: 192.168.1.0/24 (512000)

In this situation, the router will install two routes into the routing table, and with load balance between these two.

There are several parameters you need to understand.

Feasible Distance: The best composite metric to a destination prefix

Successor: The best path for a specific destination prefix

Feasible Successor: A route to a destination prefix, that has passed the Feasibility Condition, and can be marked as a possible backup route

Feasibility Condition: EIGRP provides a loop-free path to a particular destination, by ensuring that loops cannot form within its own AS. If a route that has been learned from a neighbor has a Reported Distance that is less than the Sucessor, than this will pass the Feasibility Condition.

So let's say you have two paths to reach a particular network, but one route from a neighbor does not pass the Feasibility Condition and is therefore not installed in the EIGRP Topology Table. You will obviously not be able to use this as a Feasible Successor.

So with EIGRP, you can include the 'variance ' command where x is a multiple. If you run the following command 'variance 2' it will take the Feasible Distance and multiply it by two.

You will need the "multiple" to be enough, to include all the routes you want included in unequal cost load-balancing.

So if you have the following

(192.168.1.0/24) - 512000

(192.168.1.0/24) - 600000

(192.168.1.0/24) - 650000

A variance of two will be 512000 * 2 = 1024000. This will include all of those routes.

[paul driver]

Hello

Just like to add to johns comments-

EIGRP variance is used in unequal load sharing - when you have multiple routes to the same destination prefix witch different route metrics.

Lowest compositc metric * by variance ^

eg = lowest composite metric = 30 * 3 (variance) = 90

Any next hop router closer to the destination prefix whcih meets the FC and has a lower compostitic metric than 90 will be added to the RIB.

Compositic metric =BW+DLY*256

BW = 10^7/(lowest BW to destination)

Delay = total sum of delay to destination /10

BW = 10^7/1544 = 6476

Delay - 120000/10 - 12000

6476 + 12000 = 18476

18476*256 = 4729856

Feasible Distance:

The lowest composite metric to a destination prefix from the source router

Feasible condition = FC is meet when a next hop routers advertised distance is less then the FD of source router

However if the FC isnt met, the route will not be selected as a Feasible successor.

Successor= a next hop router advertising a route prefix which has the lowest reported distance path for a specific destination prefix

Feasible Successorr =a next hop router to the destination prefix which meets the FC but has a less attractive reported distance than the successor.

If for some reason the FS cannot be found in the topology table then eigrp will try and find another FS and if successful will use that route, however if not sucessfull eigrp will change the route to an active state (SIA) whilst trying to find a FS

res

Paul

Please don't forget to rate any posts that have been helpful.

Thanks.

[Anand Solgama]

I want to understand this example :::

I understood that variance 2 ...20*2=40 so

E-C-A and E-B-A path come under 40 so they both succe. and feas. succe. !!!!

But why in variance 3 ...20*3=60 E-D-A can not come in use which is just 45 that is lower than variance 60 ???

And if u can pls explain me little abt that load balancing 1:2 type example of this page ...

check I am doing all study from this site/page!!!

http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/13677-19.html

[JohnTylerPearce]

Anand,

The link you provided, is taken from the view point of RouterE, and network X on Router A.

1. If you look at all the composite metrics costs, you get this to start to reach Network X on Router A.

E-B-A: 20+10 = 30

E-C-A: 10+10 = 20

E-D-A: 20+25 = 45

So by default, on all Cisco routers, you have a default variance of 1 which provides equal cost load balancing.

So if you look at the numbers above, you will see the best composite metric to reach Network X on Router A is through path E-C-A with a Composite Metric cost of 20. This (20) becomes the Feasible Distance and Successor for this route to network X on Router E.

The others two costs are (30) through E-B-A and (45) through E-D-A.

Remember to become a Feasible Successor you must pass the Feasibility Condition. Which states the Report Distance of a route must be lowest then the current Successor. So as you can see the Reported Distance that Router E receives from Router B is (10). The Reported Distance that Router E receives from Router D is (25). So the path through Router B will be installed as a Feasible Successor. The path through Router D will not.

So as of right now, you have one path installed in the RIB(Main Routing Table), so to get both of these other routes installed to do unequal cost load balancing, you will need to change the default variance multiple from 1 to another number.

So if you give a "Variance of 2" on Router E, You will multiply the FD which is 20 to 20(2) which = 40.  This will install the path through E-B-A, so you now have the following paths installed in the RUB(Main Routing Table)

E-C-A and E-B-A.

You will notice that the other route has not been installed, this is beacuse it did not pass the Feasibility Conditon, and will therefore not be included.

Also, by default, when you do unequal cost load balancing with EIGRP, it will use the 'traffic-share' value, which by default, will didvide the "load balancing" proportionally based upon bandwidth I believe. You can also change this option as well. Use "traffic-share ?" to see additional options.