Quick Reference Guide

748 PowerConnect B-Series TI24X Configuration Guide
53-1002269-02
Overview of BGP4
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7. If the routes have the same origin type, prefer the route with the lowest MED. For a definition of
MED, refer to “Configuring the Layer 3 Switch to always compare Multi-Exit Discriminators
(MEDs)” on page 779.
BGP4 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
Layer 3 Switch 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 Layer 3 Switch favoring the route paths
that are missing their MEDs. You can use the med-missing-as-worst command to make the
Layer 3 Switch regard a BGP route with a missing MED attribute as the least favorable route,
when comparing the MEDs of the routes.
NOTE
MED comparison is not performed for internal routes originated within the local AS or
confederation.
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 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, prefer the route that comes from the BGP4 router with the lowest router
ID.
NOTE
Layer 3 Switches support BGP4 load sharing among multiple equal-cost paths. BGP4 load
sharing enables the Layer 3 Switch 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.
BGP4 message types
BGP4 routers communicate with their neighbors (other BGP4 routers) using the following types of
messages:
• OPEN
• UPDATE
• KEEPALIVE
• NOTIFICATION