Training and Certification

Hi All,

What is the different between modified preference and metric? And which one more preferable? Please have a look at the attached picture, Why is traffic forwarded to 192.168.200.1 to access the destionation network -10.10.10.0/24?

Thanks

Hi,

Qualified next-hops are used, when you want to configure mulitple possible next-hop addresses for a particular static route and have them treated differently. Qualified next hops allow you to associate one or more properties with a particular next-hop address. You can set an overall preference for a particular static route and then specify a different preference for the qualified next hop.

In your example, for the nexthop 192.168.100.1, it is using default preference and metric of 10. However for the qualified-next-hop 192.168.200.1, the preference is set to 6. So when it comes to next-hop selection for this static route 10.10.10.0/24, it will check first the preference value and if it is same, metric will decide which nexthop will be selected.

For more details, you can refer to following links:

https://www.juniper.net/documentation/en_US/junos15.1/topics/concept/routing-protocol-static-security-route-preference-and-qualified-next-hop-understanding.html

https://www.juniper.net/documentation/en_US/junos15.1/topics/example/routing-protocol-static-security-route-selection-controlling-cli.html

Hope this helps,

Thanks

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sarathiraos' explanation is correct. Just to add, preference is related to the protocol itself while metric is a cost associated with the interface speed and you have the flexibility to modify both, So you have to remember the Junos route selection algorithm. So Juniper does not use the concept of weight, as long as the next hop is reachable then it checks the protocol preference first , lowest is better

http://www.juniper.net/documentation/en_US/junos15.1/topics/reference/general/routing-protocols-default-route-preference-values.html

Then as sarathiraos indicate, if preference is same/tied, it needs a tiebreaker, next it checks the metric, lower is better and if that is a tie, in the case of static routes it is random selection. So in you example the static route with the lowest preference is selected and made active. If it fails then your second static route will become active. Hopefully both our responses should clear up your question and understanding.