BGP Configuration Guide - Juniper Networks

BGP Configuration Guide
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12. Prefer the path from the peer with the lowest router ID. For any path with an originator
ID attribute, substitute the originator ID for the router ID during router ID comparison.
13. Prefer the path with the shortest cluster list length. The length is 0 for no list.
14. Prefer the path from the peer with the lowest peer IP address.
By default, only the multiple exit discriminators (MEDs) of routes that have the same
peer autonomous systems (ASs) are compared. You can configure routing table path
selection options to obtain different behaviors.
The third step of the algorithm, by default, evaluates the length of the AS path and
determines the active path. You can configure an option that enables Junos OS to skip
this third step of the algorithm by including the as-path-ignore option.
NOTE: The as-path-ignore option is not supported for routing instances.
To configure routing table path selection behavior, include the path-selection statement:
path-selection {
(always-compare-med | cisco-non-deterministic | external-router-id);
as-path-ignore;
med-plus-igp {
igp-multiplier number;
med-multiplier number;
}
}
For a list of hierarchy levels at which you can include this statement, see the statement
summary section for this statement.
Routing table path selection can be configured in one of the following ways:
• Using the same nondeterministic behavior as does the Cisco IOS software
(cisco-non-deterministic). This behavior has two effects:
• The active path is always first. All nonactive but eligible paths follow the active path
and are maintained in the order in which they were received, with the most recent
path first. Ineligible paths remain at the end of the list.
• When a new path is added to the routing table, path comparisons are made without
removing from consideration those paths that should never be selected because
those paths lose the MED tie-breaking rule.
NOTE: The result of these two effects is that the system only sometimes
compares the MED values between paths that it should otherwise compare.
Because of this, we recommend that you not configure nondeterministic
behavior.
• Always comparing MEDs whether or not the peer ASs of the compared routes are the
same (always-compare-med).
Copyright © 2012, Juniper Networks, Inc.