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1608 Views 2 Replies Latest reply: Sep 14, 2012 4:57 AM by BIGEVIL RSS

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Track - metric threshold

Sep 11, 2012 3:18 AM

BIGEVIL 347 posts since
Jun 28, 2008

Hi all -

 

What is the rule of thumb for BGP tracking and metric threshold?

I have set up a multihoned layout to resprents two links to an ISP (primary/backup), with the ISP sending me only a deafult route.

 

I currently amd trying this -

 

track 1 ip route 0.0.0.0 0.0.0.0 metric threshold

threshold metric up 199 down 200

 

The way i see this is - my current metric is 20 over the primary link and 199 is below this, if the link to the primary drops the metric would be 200 and so the track would go to down correct?

 

My real point of confustion is with the metic and this output - which makes me think the metric i am using above are wrong?

 

Router#sh track resolution
Route type     Metric Resolution
static         10
EIGRP       2560

OSPF       1
ISIS           10
BGP          2560

 

I read the last post here - https://supportforums.cisco.com/thread/2119326 which gave me some idea.

 

Thanks.

 

BE.

  • borzol CCNP (CCIE R&S candidate) 276 posts since
    Apr 6, 2012
    Currently Being Moderated
    1. Sep 12, 2012 4:32 AM (in response to BIGEVIL)
    Re: Track - metric threshold

    hi,

     

    I don't think you use wrong metric, because as I know metric resolution is important only for scaled metrics. Because bgp has a standard metric for ebgp and ibgp connections you can use the value of 200 and 199.

     

    In my router ( 3700 Software (C3725-ADVIPSERVICESK9-M), Version 12.4(15)T6) I can't see bgp in sh track resol

     

    R3#sh track resolution

    Route type     Metric Resolution

    static         10

    EIGRP          2560

    OSPF           1

    ISIS           10

    R3#

     

    Did you read this article?

     

    A part from it:


    Tracking the Threshold of IP-Route Metrics

    Perform this task to track the threshold of IP route metrics.

    Scaled Route Metrics

    The track ip route command enables tracking of a route in the routing table. If a route exists in the table, the metric value is converted into a number. To provide a common interface to tracking clients, route metric values are normalized to the range from 0 to 255, where 0 is connected and 255 is inaccessible. Scaled metrics can be tracked by setting thresholds. Up and down state notification occurs when the thresholds are crossed. The resulting value is compared against threshold values to determine the tracking state as follows:

    State is up if the scaled metric for that route is less than or equal to the up threshold.

    State is down if the scaled metric for that route is greater than or equal to the down threshold.

    Tracking uses a per-protocol configurable resolution value to convert the real metric to the scaled metric. Table 1 shows the default values used for the conversion. You can use the track resolution command to change the metric resolution default values.

    Table 1     Metric Conversion

     

     


    Route Type1

     

    Metric Resolution

    Static

    10

    Enhanced Interior Gateway Routing Protocol (EIGRP)

    2560

    Open Shortest Path First (OSPF)

    1

    Intermediate System-to-Intermediate System (IS-IS)

    10

     

    1 RIP is scaled directly to the range from 0 to 255 because its
    maximum metric is less than 255.

     

     

    For example, a change in 10 in an IS-IS metric results in a change of 1 in the scaled metric. The default resolutions are designed so that approximately one 2-Mbps link in the path will give a scaled metric of 255.

    Scaling the very large metric ranges of EIGRP and IS-IS to a 0 to 255 range is a compromise. The default resolutions will cause the scaled metric to go above the maximum limit with a 2-Mbps link. However, this scaling allows a distinction between a route consisting of three Fast-Ethernet links and a route consisting of four Fast-Ethernet links.

     

     

    Regards,

     

    borzol

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