Siemens S223, S323 Multi-Exit Discriminator MED, Choosing Best Path, Bgp deterministic-med

User Manual	UMN:CLI
SURPASS hiD 6615 S223/S323 R1.5

10.1.2.3Multi-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	Mode	Description
bgp always-compare-med		Configures the router to consider the comparison of
		MEDs in choosing the best path from among paths.
	Router
no bgp always-compare-med		Chooses the best path regardless of the comparison of
		MEDs.

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	Mode	Description
		Configures the router to compare MEDs in choosing
bgp deterministic-med		the best path when paths have same AS-path informa-
	Router	tion.
no bgp deterministic-med		Configures the router not to compare MEDs even if the
		paths have same AS-path.

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During the best-path selection process, use the bgp always-compare-medcommand in case of comparing MED values regardless of AS-path. Otherwise, use the bgp deterministic-medcommand if it compares MED values of lots of paths contained same AS-path information.

10.1.2.4Choosing 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.

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