Brocade Multi-Service IronWare Routing Configuration Guide (Supporting R05.6.00) User Manual
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Multi-Service IronWare Routing Configuration Guide
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Overview of BGP4
4. Prefer the route that was originated locally (by this device).
5. If the local preferences are the same, prefer the path with the shortest AS-path. An AS-SET
counts as 1. A confederation path length, if present, is not counted as part of the path length.
NOTE
This step can be skipped if BGP4-as-path-ignore is configured.
6. If the AS-path lengths are the same, prefer the path with the lowest origin type. From low to
high, route origin types are valued as follows:
•
IGP is lowest.
•
EGP is higher than IGP but lower than INCOMPLETE.
•
INCOMPLETE is highest.
7. If the paths have the same origin type, prefer the path with the lowest MED. For a definition of
MED, refer to
“Configuring the device to always compare Multi-Exit Discriminators”
•
The device 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.
You can also enable the device 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 device favoring the route paths that are
missing their MEDs. You can use the med-missing-as-worst command to make the device
regard a BGP4 route with a missing MED attribute as the least favorable path, when comparing
the MEDs of the route paths.
NOTE
MED comparison is not performed for internal routes originated within the local AS or
confederation unless the compare-med-empty-aspath command is configured.
8. Prefer paths in the following order:
•
Routes received through EBGP from a BGP4 neighbor outside of the confederation
•
Routes received through EBGP from a BGP4 device within the confederation OR Routes
received through IBGP.
9. If all the comparisons above are equal, prefer the route with the lowest IGP metric to the BGP4
next hop. This is the closest internal path inside the AS to reach the destination.
10. If the internal paths also are the same and BGP4 load sharing is enabled, load share among
the paths. Otherwise go to step 11.