SRX

I'm reading about qualified next-hop and how this provides automated routing.

the examples i have seen use an ip next-hop, not an interface so how will this provide automated routing?, does the juniper 'probe' the next hop or something. I mean its not like when a static route points out of an interface, and the interface goes down therby removing the route from the table ...

I understand that ultimately the packet must leave an outgoing interface, and it does this by performing recursive lookup, so are we saying that a static route with next-hop ip will be removed if a recursive lookup fails i.e. the ultimate outgoing interface is in 'down' state? 

And how about the 'qualified next-hop', why is this necessary, can't the same result be achieved by using 'preference' only, where the lowest preference wins, or using metrics?. Whats the real magic behind 'qualified next-hop' as clearly i'm not getting it 

thanks

Hi Ajaz,

That's exactly what we're saying in Junos - when the outgoing interface is down, the next-hop will be marked as ineligible for route selection, and all routes that reference it will be marked as inactive.  Qualified next-hop allows you to provide alternatives for a particular route when this occurs.

Route preferencing can also be used, but there has to be something that causes the primary route to become inactive for this to be effective.  In the case of a static route, interface availabiity is one option.  Depending on the platform you are using, track-ip / ip-monitoring may also be available.

Ah so I see.

Qualified next-hop is therefore only applicable where recursive lookups are being performed?.

And where lets say we have two static routes, both pointing via an interface (st0.0 & st0.1 respectively), then using 'preference' or even 'metric' with do the job?

Thanks

Hi Ajaz,

Not exactly - think of it more as here's my destination, but here are some additional qualifiers for this route (eg: the metric is higher or lower).

Regarding your example - yes - if you wanted to route a subnet down st0.0 unless it was down in which case it shold favour st0.1, then create your route with a next-hop of st0.0 and a qualified-next-hop of st0.1 with a metric of 50.

Hope this clears things up!