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Multiple Exit Discriminator

A multiple exit discriminator (MED) is an arbitrary metric assigned to a route to determine the exit point to a destination when all other factors are equal. By default, MED metrics are compared only for routes to the same peer AS, but you can also configure routing table path selection options for different ways of comparing MEDs.

Default MED Usage

Because the AS path rather than the number of hops between hosts is the primary criterion for BGP route selection, an AS with multiple connections to a peer AS can have multiple equivalent AS paths. When the routing table contains two routes to the same host in a neighboring AS, a multiple exit discriminator (MED) metric assigned to each route can determine which to include in the forwarding table. The MED metric you assign can force traffic through a particular exit point in an AS.

Figure 60 illustrates how MED metrics are used to determine route selection.

Figure 60: Default MED Example

Image g015017.gif

Figure 60 shows AS 1 and AS 2 connected by two separate BGP links to Routers C and D. Host E in AS 1 is located nearer Router C. Host F, also in AS 1, is located nearer Router D. Because the AS paths are equivalent, two routes exist for each host, one through Router C and one through Router D. To force all traffic destined for Host E through Router C, network administrator for AS 2 assigns an MED metric for each router to Host E at its exit point. An MED metric of 10 is assigned to the route to Host E through Router C, and an MED metric of 20 is assigned to the route to Host E through Router D. BGP routers in AS 2 then select the route with the lower MED metric for the forwarding table.

Additional MED Options for Path Selection

By default, only the MEDs of routes that have the same peer ASs are compared. However, you can configure the routing table path selection options listed in Table 99 to compare MEDs in different ways. The MED options are not mutually exclusive and can be configured in combination or independently. For the MED options to take effect, you must configure them uniformly all through your network. The MED option or options you configure determine the route selected. Thus we recommend that you carefully evaluate your network for preferred routes before configuring the MED options. For information about configuring the MED options, see the JUNOS Routing Protocols Configuration Guide.

Table 99: MED Options for Routing Table Path Selection

Option (Name)	Function	Use
Always comparing MEDs (always-compare-med)	Ensures that the MEDs for paths from peers in different ASs are always compared in the route selection process	Useful when all enterprises participating in a network agree on a uniform policy for setting MEDs. For example, in a network shared by two ISPs, both must agree that a certain path is the better path to configure the MED values correctly.
Adding IGP cost to MED (med-plus-igp)	Before comparing MED values for path selection, adds to the MED the cost of the IGP route to the BGP next-hop destination. This option replaces the MED value for the router, but does not affect the IGP metric comparison. As a result, when multiple routes have the same value after the MED-plus-IPG comparison, and route selection continues, the IGP route metric is also compared, even though it was added to the MED value and compared earlier in the selection process.	Useful when the downstream AS requires the complete cost of a certain route that is received across multiple ASs.
Applying Cisco IOS nondeterministic behavior (cisco-non-deterministic)	Specifies the nondeterministic behavior of the Cisco IOS software: The active path is always first. All nonactive but eligible paths follow the active path and are maintained in the order in which they were received. Ineligible paths remain at the end of the list. When a new path is added to the routing table, path comparisons are made among all routes, including those paths that must never be selected because they lose the MED tie-breaking rule.	We recommend that you do not configure this option, because the nondeterministic behavior sometimes prevents the system from properly comparing the MEDs between paths.

Option (Name)

Function

Use

Always comparing MEDs (always-compare-med)

Ensures that the MEDs for paths from peers in different ASs are always compared in the route selection process

Useful when all enterprises participating in a network agree on a uniform policy for setting MEDs. For example, in a network shared by two ISPs, both must agree that a certain path is the better path to configure the MED values correctly.

Adding IGP cost to MED (med-plus-igp)

Before comparing MED values for path selection, adds to the MED the cost of the IGP route to the BGP next-hop destination.

This option replaces the MED value for the router, but does not affect the IGP metric comparison. As a result, when multiple routes have the same value after the MED-plus-IPG comparison, and route selection continues, the IGP route metric is also compared, even though it was added to the MED value and compared earlier in the selection process.

Useful when the downstream AS requires the complete cost of a certain route that is received across multiple ASs.

Applying Cisco IOS nondeterministic behavior (cisco-non-deterministic)

Specifies the nondeterministic behavior of the Cisco IOS software:

The active path is always first. All nonactive but eligible paths follow the active path and are maintained in the order in which they were received. Ineligible paths remain at the end of the list.
When a new path is added to the routing table, path comparisons are made among all routes, including those paths that must never be selected because they lose the MED tie-breaking rule.

We recommend that you do not configure this option, because the nondeterministic behavior sometimes prevents the system from properly comparing the MEDs between paths.

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