Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 387)

4. Prefer the route that was originated locally (by this BGP4 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.
If the routes were learned from the same neighboring AS, BGP4 compares the MEDs of two
otherwise equivalent paths. 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 routes 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 prefer the route that comes from the BGP4 device with the lowest device ID.
NOTE
Brocade devices support BGP4 load sharing among multiple equal-cost paths. BGP4 load sharing
enables the device to balance traffic across the multiple paths instead of choosing just one path
based on device ID. For EBGP routes, load sharing applies only when the paths are from neighbors
within the same remote AS. EBGP paths from neighbors in different autonomous systems are not
compared, unless multipath multi-as is enabled.
11.If the compare-router ID is enabled, prefer the path that comes from the BGP4 device with the
lowest device ID. If a path contains originator ID attributes, then originator ID is substituted for the
ROUTER ID in the decision.
12.Prefer the path with the minimum cluster list length.
Configuring BGP4 (IPv4)
FastIron Ethernet Switch Layer 3 Routing Configuration Guide 387
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