Milo - I saw what you read in the manual as well, and it doesn't make sense. I'll investigate this and open a documentation bug if needed.
For each prefix in the routing table, the routing protocol process selects a single best path, called the active path.
The algorithm for determining the active path is as follows:
Choose the path with the lowest preference value (routing protocol process preference). Routes that are not eligible to be used for forwarding (for example, because they were rejected by routing policy or because a next hop is inaccessible) have a preference of -1 and are never chosen. For BGP, prefer the path with higher local preference. For non-BGP paths, choose the path with the lowest preference2 value. If the path includes an AS path: Prefer the route with a shorter AS path. Confederation sequences are considered to have a path length of 0, and AS and confederation sets are considered to have a path length of 1. 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 these have lower origin codes that incomplete routes (routes whose origin is unknown). Depending on whether nondeterministic routing table path selection behavior is configured, there are two possible cases: If nondeterministic routing table path selection behavior is not configured (that is, if the path-selection cisco-nondeterministic statement is not included in the BGP configuration), for paths with the same neighboring AS numbers at the front 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. When you display the routes in the routing table using the show route command, they generally appear in order from most preferred to least preferred. Routes that share the same neighbor AS are grouped together in the command output. Within a group, the best route is listed first and the other routes are marked with the NotBest flag in the State field of the show route detail command. If nondeterministic routing table path selection behavior is configured (that is, the path-selection cisco-nondeterministic statement is included in the BGP configuration), prefer the path with the lowest multiple exit discriminator (MED) metric. When you display the routes in the routing table using the show route command, they generally appear in order from most preferred to least preferred and are ordered with the best route first, followed by all other routes in order from newest to oldest. In both cases, confederations are not considered when determining neighboring ASs. Also, in both cases, a missing metric is treated as if a MED were present but zero. Prefer strictly internal paths, which include IGP routes and locally generated routes (static, direct, local, and so forth). Prefer strictly external (EBGP) paths over external paths learned through interior sessions (IBGP). For BGP, prefer the path whose next hop is resolved through the IGP route with the lowest metric. For BGP, prefer the route with the lowest IP address value for the BGP router ID. Prefer the path that was learned from the neighbor with the lowest peer IP address. Multiple Active Routes The interior gateway protocols (IGPs) compute equal-cost multipath next hops, and internal BGP (IBGP) picks up these next hops. When there are multiple, equal-cost next hops associated with a route, the routing protocol process installs only one of the next hops in the forwarding path with each route, randomly selecting which next hop to install. For example, if there are three equal-cost paths to an exit router and 900 routes leaving through that router, each of the paths ends up with about 300 routes pointing at it. This mechanism provides load distribution among the paths while maintaining packet ordering per destination. Default Route Preference Values The JUNOS software routing protocol process assigns a default preference value to each route that the routing table receives. The default value depends on the source of the route. The preference is a value from 0 through 255, with a lower value indicating a more preferred route. In general, the narrower the scope of the statement, the higher precedence its preference value is given, but the smaller the set of routes it affects. To modify the default preference value for routes learned by routing protocols, you generally apply routing policy when configuring the individual routing protocols.
It's a mouthful, but that pretty much covers it.
Thanks,
Bob O'Hara
Systems Engineer
Juniper Networks
Northeast Sales Region
.........................................
. email: rohara@juniper.net .
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-----Original Message-----
From: mark forest [mailto:milo279@yahoo.com]
Sent: Saturday, November 03, 2001 3:51 PM
To: puck
Subject: [j-nsp] route selection
List,
I have been reading through some info regarding path
selction as listed on page 7 of the routing and
routing protocol documentation. In the list of how
JUNOS selects the route to make active, number 8 says
"Prefer paths with larger number of next hops".
If someone can lend some interpretation to this I
would appreciate it. Each time I attempt to clarify it
myself, I hit the brick wall and find that I am human
and bleed.
thanks in advance for your replys.
milo
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