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New Member

How does reliability get looked at when using it with EIGRP?

If I enable the use of reliability in the metric calculation for EIGRP, will DUAL kick off when the reliability of an interface changes?

I have two sites connected w/ two equal cost PVCs on seperate circuits. If one of these circuits begins to have problems, and thus decrease the reliability, I would like EIGRP to dynamically route around the problem circuit until the reliability returns to 100%.

My understanding is that EIGRP can do this by setting the K value for reliability to 1. However, reading the Cisco Press Book, "EIGRP Network Design Solutions" it indicates that since EIGRP only uses Event Triggered updates, reliability will only be looked at when an event occurs, and not dynamically.

Can someone confirm or deny this? I would appreciate links to any documentation that supports the answer.

Jason Coleman CCNP, CCDP

  • Other Network Infrastructure Subjects

Re: How does reliability get looked at when using it with EIGRP?

I guess, that, reliability is calculated, on the basis of keepalives exchanged between 2 routers.

Keepalives are exchanged every 10 seconds on the line.Keepalive Frames sent over a link will have CRC and if that CRC check fails, value of reliability is changed. This is again measured by taking samples calculated over a period of 5 minutes.

Reliability is calculated dynamically.

New Member

Re: How does reliability get looked at when using it with EIGRP?

I understand how reliability is calculated and that the calculation is dynamic.

The main focus of my question is will a dynamic change in the reliability of an interface cause DUAL to compute a new metric?

From what I have read it appears that from an EIGRP perspective reliability is only checked when an event occurs. This would mean that if an interface begins to decrease in reliability, but not completely go down that the routing table will not dynamically adjust.

Hope that makes more sense.

Re: How does reliability get looked at when using it with EIGRP?

No, by default a change in reliability won't cause a new metric to be formed. EIGRP updates contain five metrics: minimum bandwidth, delay, load, reliability, and maximum transmission unit (MTU). Of these five metrics, by default, only minimum bandwidth and delay are used to compute best path (default metric: {[100000000/min bandwidth]+sum of delays}*256). Unlike most metrics, minimum bandwidth is set to the minimum bandwidth of the entire path, and does not reflect how many hops or low bandwidth links are in the path. Delay is a cumulative value which increases by the delay value of each segment in the path.

To change this default use the command "metric weights tos k1 k2 k3 k4 k5". This will let you include reliability into the metric calculation. If you change the k# into a positive number (k2,k4,k5 are = 0 by default, others are 1) you will use the following metric:

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

See these links:



Hope it helps.