Information Technology Reference
In-Depth Information
MED is evaluated. The path with the lowest MED wins. By default, this comparison
is done only if the first (neighboring) AS is the same in the two paths; any confedera-
tion subautonomous systems are ignored. In other words, MEDs are compared only if
the first AS in the AS_SEQUENCE is the same for multiple paths. Any preceding
AS_CONFED_SEQUENCE is ignored. If
bgp always-compare-med
is enabled,
MEDs are compared for all paths, regardless of whether they come from the same AS.
If you enable this option, you should do so over the entire AS to avoid routing loops.
Note the following MED modification options:
•
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With
bgp deterministic-med
, the result of a MED comparison is consistent
regardless of the order in which prefixes are being received. With this con-
figuration, all paths are grouped based on AS_PATH. Within each group of
the AS_PATH, paths are sorted by MED. A path with the lowest MED is
selected for each group. The final best path is the path with the lowest MED
among all the selected paths. This is the recommended configuration if
MED is present.
•
If
bgp bestpath med-confed
is enabled, MEDs are compared for all paths that
consist only of AS_CONFED_SEQUENCE—that is, paths originated within the
local confederation. Note that if a path consists of any external autonomous
systems, this path is not considered in the comparison, and its MED is passed
unchanged inside the confederation.
•
Paths received with no MED are assigned a metric of 0, unless
bgp bestpath
missing-as-worst
is enabled, in which case they are assigned a value of
4,294,967,294 (the maximum). This is for compatibility with the old standard.
External (eBGP) paths are preferred over internal (iBGP) paths. Paths containing
AS_CONFED_SEQUENCE are local to the confederation and therefore are treated
as internal paths. There is no distinction between Confederation External and Confed-
eration Internal in path selection.
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BGP prefers the path with the lowest IGP metric to the BGP next hop. This step allows
the local topology to be taken into consideration.
8
If
maximum-paths
[
ibgp
]
n
is enabled, where
n
is between 2 and 6, and multiple
equal-cost paths exist (the results from Steps 1 through 6 are the same among the
paths, and AS_PATH is identical), BGP inserts up to
n
received paths in the IP routing
table. This allows BGP multipath load sharing. Without the optional keyword
ibgp
,
multipath applies only to eBGP or confederation external paths from the same
neighboring AS or sub-AS. The default value of
n
, when this option is not enabled,
is 1.
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When both paths are external, BGP prefers the path that was received first (the oldest
one). This step minimizes route flap, because a newer path does not replace an
older one, even if it is the preferred route based on additional decision criteria,
as described in steps 11, 12, and 13.
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