Dell BigIron RX Series, Configuration Manual

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BigIron RX Series Configuration Guide 741
53-1002253-01
Overview of BGP4
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1. Is the next hop accessible though an Interior Gateway Protocol (IGP) route? If not, ignore the
path.
NOTE
By default, the device does not use the default route to resolve BGP4 next hop. Also refer to
“Enabling next-hop recursion” on page 783 and “Using the IP default route as a valid next hop
for a BGP4 route” on page 782
2. Use the path with the largest weight.
3. If the weights are the same, prefer the path with the largest local preference.
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 bgp-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.
•
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