Discussion

Selecting Active Routes
For each destination (prefix) in the routing table, RPD selects the best route, called
the active route, and installs it into the forwarding table. The algorithm that RPD
uses to select the active route is fairly involved, but there will be times when you will
be analyzing the flow of traffic through your network and you will need to understand
how and why RPD has chosen a particular path. The following is the JUNOS
algorithm for selecting the active route:
1. Choose the route with the lowest preference value.
2. For BGP routes, prefer the one with the higher local preference value. Otherwise,
choose the path with the lowest preference2 value. (This is a secondary
preference you can set for some protocols to use as a tiebreaker when the primary
preferences are identical.)
3. If the route includes an AS path:
a. Prefer the route with a shorter AS path. (Confederation sequences are
assigned a path length of 0, and AS and confederation sets have a path
length of 1.)
b. Prefer the route with the lower origin code. Routes learned from an IGP
have a lower origin code than those learned from an EGP, and both have
lower origin codes than incomplete routes (routes whose origin is unknown).
c. If you are not using BGP nondeterministic routing-table path selection
behavior, for paths with the same neighboring AS numbers at the beginning
of the AS path, prefer the path with the lowest multiple exit discriminator
(MED) metric. Confederation AS numbers are not considered when deciding
what the neighbor AS number is.
If you are using nondeterministic routing-table path selection behavior, prefer
the path with the lowest MED metric.
In both cases, confederations are not considered when determining neighboring
ASs, and a missing metric is treated as a MED of 0.
4. Prefer strictly internal routes, which include IGP routes and locally generated
routes (such as static and direct).
5. Prefer strictly EBGP routes over external paths learned through IBGP.
6. For BGP, prefer the route whose next hop is resolved through the IGP route with
the lowest metric.
7. For BGP, prefer the route with the greatest number of BGP next hops.
8. For BGP, prefer the route with the shortest route reflection cluster list. Routes
without a cluster list are considered to have a cluster list of length 0.
9. For BGP, prefer the route with the lowest IP address value for the BGP router ID.
10. Prefer the path that was learned from the neighbor with the lowest peer IP
address.
250 | Chapter 8: IP Routing
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