Tag Archives: 3.5c

3.5.c Implement and troubleshoot loop free path selection

3.5.c [i] RD, FD, FC, successor, feasible successor

Feasible distance is the best metric along a path to a destination network, including the metric to the neighbor advertising that path.

Reported distance is the total metric along a path to a destination network as advertised by an upstream neighbor. A feasible successor is a path whose reported distance is less than the feasible distance (current best path).

A feasible successor for a particular destination is a next hop router that is guaranteed not to be a part of a routing loop. This condition is verified by testing the feasibility condition.

Thus, every successor is also a feasible successor. However, in most references about EIGRP the term feasible successor is used to denote only those routes which provide a loop-free path but which are not successors (i.e. they do not provide the least distance). From this point of view, for a reachable destination there is always at least one successor, however, there might not be any feasible successors.

The feasibility condition (FC) is a sufficient condition for routing loop prevention in EIGRP-routed network. It is used to select the successors and feasible successors that are guaranteed to be on a loop-free route to a destination. Its formulation is strikingly simple:

● If, for a destination, a neighbor router advertises a distance that is strictly lower than our feasible distance, then this neighbor lies on a loop-free route to this destination. or in other words,

● If, for a destination , a neighbor router tells us that it is closer to the destination than we have ever been, then this neighbor lies on a loop-free route to this destination.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3095-3098).  . Kindle Edition.

http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/16406-eigrp-toc.html#basictheory

 

3.5.c Implement and troubleshoot loop free path selection

3.5.c [ii] Classic metric

EIGRP historically has used five vector metrics: minimum throughput, latency, load, reliability, and Maximum Transmission Unit (MTU).

These values are accumulated from destination to source as follows:

● Throughput-minimum value

● Latency-accumulative

● Load-maximum

● Reliability-minimum

● MTU-minimum

● Hop count-accumulative

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3116-3125).  . Kindle Edition.

http://books.google.com/books?id=NoY7BAAAQBAJ&pg=PA360&lpg=PA360&dq=eigrp+classic+metric&source=bl&ots=3c0vcVdCdy&sig=SP_xKJKk4lNu7RcSfWXJZEGElGY&hl=en&sa=X&ei=cqA9VNWgKMfJggTYwoG4BA&ved=0CFgQ6AEwCQ#v=onepage&q=eigrp%20classic%20metric&f=false

3.5.c Implement and troubleshoot loop free path selection

3.5.c [iii] Wide metric

To this, there are two additional values being added: jitter and energy. These two new values are accumulated from destination to source:

Jitter-accumulative

Energy-accumulative

These extended attributes, as well as any future ones , will be controlled through K6. If K6 is non-zero, these will be an additive to the path’s composite metric. Higher jitter or higher energy usage will result in paths which are worse than those paths that either do not monitor these attributes, or which have lower values. EIGRP will not send these attributes if the router does not provide them. If the attributes are received, then EIGRP will use them in the metric calculation (based on K6) and will forward them with that router’s values, assumed to be “zero”, and the accumulative values will be forwarded unchanged. Of these vector metric components, by default, only minimum throughput and latency are traditionally used to compute the best path. Unlike most metrics, minimum throughput is set to the minimum value of the entire path, and it does not reflect how many hops or low throughput links are in the path, nor does it reflect the availability of parallel links. Latency is calculated based on one-way delays, and is a cumulative value, which increases with each segment in the path.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3127-3138).  . Kindle Edition.

http://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/enhanced-interior-gateway-routing-protocol-eigrp/whitepaper_C11-720525.html

 

3.5.c Classic metric

Composite Metric

 

EIGRP uses a composite metric of bandwidth and delay by default similar to IGRP but modifed by a factor of 256. This combination is known as the feasible distance. http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#eigrpmetrics

 

Although bandwidth and delay are the recommended values commonly used for manipulation, there are a total of 5:

 

Bandwidth: in kilobytes, a value that can be adjusted per interface using the bandwidth command. Use with caution as this can affect other protocols as well.

 

Delay: in microseconds, adjusted using the delay command. Delay is the preferred value adjustment.

 

Reliability: as an integer value between 1 and 255. 1 is unreliable; 255 most reliable.

 

Load: as an integer value between 1 and 255. 1 is least loaded.

 

MTU: as a byte value; the least value recorded in the path. MTU is maximum transmission unit and is not used in the calculation.

 

Default integer values:

 

K1 = 1 K2 = 0 K3 = 1 K4 = 0 K5 = 0

 

Metric calculation (actual):

 

metric = [K1 * bandwidth + (K2 * bandwidth) / (256 – load) + K3 * delay] * [K5 / (reliability + K4)]

 

Equation after default K5 (0) value:

 

[K1 * bandwidth + (K2 * bandwidth)/(256 – load) + K3 * delay]

 

And further given K2 = 0 (per the default):

 

metric = bandwidth + delay

 

Important; the reasons an EIGRP adjacency will not form other than a physical link problem:

 

Mismatched AS, not in the same subnet and mismatched K values.