Choose Route

Forwarding Honesty Boundary

This is exact static lookup arithmetic, not a full router.

highlighted = computed this step

What this lookup shows

This toy table shows one static forwarding lookup for one destination.

dest=192.168.1.130\text{dest}=192.168.1.130
Forwarding honesty boundaryThe winner is recomputed from all matching prefixes.Longest prefix match - destination 192.168.1.130, winner 192.168.1.128/25 -> labprefixmaskdest & maskmatchnext hopwinner192.168.0.0/16255.255.0.0192.168.0.0yescoreno192.168.1.0/24255.255.255.0192.168.1.0yesfloor-1no192.168.1.128/25255.255.255.128192.168.1.128yeslabyes0.0.0.0/00.0.0.00.0.0.0yesdefaultno

Checked values

The trust boundary recomputes every mask, every masked destination, every match, and the winning prefix length 25.

winner length=25\text{winner length}=25
Forwarding honesty boundaryThe winner is recomputed from all matching prefixes.Longest prefix match - destination 192.168.1.130, winner 192.168.1.128/25 -> labprefixmaskdest & maskmatchnext hopwinner192.168.0.0/16255.255.0.0192.168.0.0yescoreno192.168.1.0/24255.255.255.0192.168.1.0yesfloor-1no192.168.1.128/25255.255.255.128192.168.1.128yeslabyes0.0.0.0/00.0.0.00.0.0.0yesdefaultno

What this does not model

NOTE: toy static forwarding table only; not BGP, ECMP, policy routing, administrative distance, metrics, route cache, ACLs, or real router behavior.

route rows=4\text{route rows}=4
Forwarding honesty boundaryThe winner is recomputed from all matching prefixes.Longest prefix match - destination 192.168.1.130, winner 192.168.1.128/25 -> labprefixmaskdest & maskmatchnext hopwinner192.168.0.0/16255.255.0.0192.168.0.0yescoreno192.168.1.0/24255.255.255.0192.168.1.0yesfloor-1no192.168.1.128/25255.255.255.128192.168.1.128yeslabyes0.0.0.0/00.0.0.00.0.0.0yesdefaultno

Summary

Mask every route, keep the matching rows, then choose the longest prefix.

winner prefix=25\text{winner prefix}=25
Forwarding honesty boundaryThe winner is recomputed from all matching prefixes.Longest prefix match - destination 192.168.1.130, winner 192.168.1.128/25 -> labprefixmaskdest & maskmatchnext hopwinner192.168.0.0/16255.255.0.0192.168.0.0yescoreno192.168.1.0/24255.255.255.0192.168.1.0yesfloor-1no192.168.1.128/25255.255.255.128192.168.1.128yeslabyes0.0.0.0/00.0.0.00.0.0.0yesdefaultno