Brocade BigIron RX Series Configuration Guide - Manual (Page 895)

BigIron RX Series Configuration Guide 817
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Overview of BGP4
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• 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.
In addition, you can 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 BGP 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 router within the confederation
• 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.
NOTE
The BigIron RX supports BGP4 load sharing among multiple equal-cost paths. BGP4 load
sharing enables the device to balance the traffic across the multiple paths instead of choosing
just one path based on router 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 ASs are
not compared, unless multipath multi-as is enabled.
11. Prefer the path that comes from the BGP4 router with the lowest router ID, if compare-router ID
is enabled. If a path contains originator ID attributes, then originator ID is substituted for the
ROUTER ID in the decision process.
12. Prefer the path with the minimum cluster list length.
13. If the route is a BGP VRF instance, prefer the route with the smallest RD value.
BGP4 message types
BGP4 routers communicate with their neighbors (other BGP4 routers) using the following types of
messages:
• OPEN
• UPDATE