Multi-Exit Discriminator (Med); Choosing Best Path - Siemens S223 User Manual

User Manual
SURPASS hiD 6615 S223/S323 R1.5
10.1.2.3
i
10.1.2.4
A50010-Y3-C150-2-7619

Multi-Exit Discriminator (MED)

During the best-path selection process, the switch compares weight, local preference and
as-path in turn among the similar parameters of BGP routers. Then, the MED is consid-
ered when selecting the best path among many alternative paths.
The hiD 6615 S323, MED comparison is configured only among all paths from the
autonomous system. You can configure the comparison of MEDs among all BGP routers
within autonomous system. In addition, MED is used when comparing of routes from the
neighboring routers placed within different AS.
To find the best route by comparing MED values, use the following command.
Command
bgp always-compare-med
no bgp always-compare-med
Meanwhile, when the best-path is selected among the neighbor routers within same
Autonomous System, it doesn't compare MED values of them. However, in case the
paths have same AS-path information, it does compare MED values. If there are two
paths with different AS-path each other, the comparison of MED is unnecessary work.
Other parameter's path information can be used to find the best path.
To compare MED values in order to choose the best path among lots of alternative paths
included same AS-path value, use the following command.
Command
bgp deterministic-med
no bgp deterministic-med
During the best-path selection process, use the bgp always-compare-med command in
case of comparing MED values regardless of AS-path. Otherwise, use the bgp
deterministic-med command if it compares MED values of lots of paths contained same
AS-path information.

Choosing Best Path

There are a lot of path parameters BGP protocol, which are IP address, AS, MED value
and router ID. Even if two paths look same under the condition of IP address, they are ac-
tually different when other parameters are compared with each other.
Mode
Configures the router to consider the comparison of
MEDs in choosing the best path from among paths.
Router
Chooses the best path regardless of the comparison of
MEDs.
Mode
Configures the router to compare MEDs in choosing
the best path when paths have same AS-path informa-
tion.
Router
Configures the router not to compare MEDs even if the
paths have same AS-path.
UMN:CLI
Description
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