Dell PowerConnect B-FCXs Configuration Manual page 1026

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Overview of BGP4
3. If the weights are the same, prefer the route with the largest local preference.
4. If the routes have the same local preference, prefer the route that was originated locally (by
5. If the local preferences are the same, prefer the route with the shortest AS-path. An AS-SET
6. If the AS-path lengths are the same, prefer the route with the lowest origin type. From low to
7.
8. Prefer routes in the following order:
9. If all the comparisons above are equal, prefer the route with the lowest IGP metric to the BGP4
10. If the internal paths also are the same and BGP4 load sharing is enabled, load share among
984
this BGP4 Layer 3 Switch).
counts as 1. A confederation path length, if present, is not counted as part of the path length.
high, route origin types are valued as follows:
•
IGP is lowest
•
EGP is higher than IGP but lower than INCOMPLETE
•
INCOMPLETE is highest
If the routes have the same origin type, prefer the route with the lowest MED. For a definition of
MED, refer to "Configuring the Layer 3 Switch to always compare Multi-Exit Discriminators
(MEDs)" on page 1016.
BGP4 compares the MEDs of two otherwise equivalent paths if and only if the routes were
learned from the same neighboring AS. This behavior is called deterministic MED.
Deterministic MED is always enabled and cannot be disabled. In addition, you can enable the
Layer 3 Switch to always compare the MEDs, regardless of the AS information in the paths. To
enable this comparison, enter the always-compare-med command at the BGP4 configuration
level of the CLI. This option is disabled by default.
NOTE
By default, value 0 (most favorable) is used in MED comparison when the MED attribute is not
present. The default MED comparison results in the Layer 3 Switch favoring the route paths
that are missing their MEDs. You can use the med-missing-as-worst command to make the
Layer 3 Switch regard a BGP route with a missing MED attribute as the least favorable route,
when comparing the MEDs of the routes.
NOTE
MED comparison is not performed for internal routes originated within the local AS or
confederation.
•
Routes received through EBGP from a BGP4 neighbor outside of the confederation
•
Routes received through EBGP from a BGP4 router within the confederation
•
Routes received through IBGP
next hop. This is the closest internal path inside the AS to reach the destination.
the paths. Otherwise, prefer the route that comes from the BGP4 router with the lowest router
ID.
PowerConnect B-Series FCX Configuration Guide
53-1002266-01