Bgp4 message types – Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) User Manual
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Brocade Virtual ADX Switch and Router Guide
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Overview of BGP4
8
•
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”
•
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 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
•
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 device supports 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 ASs are not
compared, unless multipath multi-as is enabled.
11. If 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 route that comes from the lowest BGP4 neighbor address.
BGP4 message types
BGP4 devices communicate with neighbors (other BGP4 devices) using the following types of
messages: