Bgp what is med

The reason for the path selection is listed as MED From operational mode, enter the show route advertising-protocol bgp This example shows how to configure a policy that uses route filters to modify the multiple exit discriminator MED metric to advertise in BGP update messages.

To configure a route-filter policy that modifies the advertised MED metric in BGP update messages, include the metric statement in the policy action. Figure 3 shows a typical network with internal peer sessions and multiple exit points to a neighboring autonomous system AS. For From configuration mode, confirm your configuration by entering the show interfaces , show protocols , show policy-options , and show routing-options commands.

The output shows that the preferred path to the routes advertised by Device R4 is through Device R2 for all routes except From operational mode, enter the show route advertising-protocol bgp command.

Set the multiple exit discriminator MED metric to 20 for all routes from a particular community. This example shows how to associate the multiple exit discriminator MED path attribute with the interior gateway protocol IGP metric, and configure a timer to delay update of the MED attribute. The IGP metric enables internal routing to follow the shortest path according to the administrative setup.

Zero is the value advertised. The IGP metric is a nonzero value when a BGP peer sends third-party next hops that require the local system to perform next-hop resolution—IBGP configurations, configurations within confederation peers, or EBGP configurations that include the multihop statement.

In these scenarios, it might make sense to associate the MED value with the IGP metric by including the metric-out minimum-igp or metric-out igp option. Configuring a delay for the MED update provides a mechanism to reduce route advertisements in such scenarios.

The approach uses a timer to periodically advertise MED updates. When the timer expires, the MED attribute for routes with metric-out igp delay-updates configured is updated to the current IGP metric of the next hop.

The time for advertising MED updates is set to 10 minutes by default. You can increase the interval up to minutes by including the med-igp-update-interval statement in the routing-options configuration.

If you have nonstop active routing NSR enabled and a switchover occurs, the delayed MED updates might be advertised as soon as the switchover occurs. The router monitors and remembers the lowest IGP cost until the routing process rpd is restarted. The BGP peer sends an update only if the MED is lower than the previously advertised value or another attribute associated with the route has changed, or if the BGP peer is responding to a refresh route request.

The OSPF metric can range from 1 through 65, The default for the MED update is 10 minutes when you include the delay-med-update option.

The metric statement is used here to demonstrate what happens when the IGP metric changes. Repeat the configuration steps on the other devices in the topology, as needed for your network. After waiting 12 minutes the configured delay period , enter the show route advertising-protocol bgp command from operational mode.

Your account is fully activated, you now have access to all content. Close Welcome back! You've successfully signed in. Close Nearly there! To activate your account, please click the link in the email we just sent you. Finally R2 is compared with R5. Additionally we do not have bgp bestpath compare-routerid enabled which leads the R1 to select the oldest advertisement. Since R5 is listed below R2 we know that it is older and in turn wins out due to being the older advertisement and is installed as the best path to reach the This is where the bgp deterministic-med router configuration command is useful.

When this command is enabled the router will group all paths from the same AS and compare them together before comparing them to paths from different ASes. Lets enable the command on R1. We should see that R2 is selected as the preferred path between R2, R3 and R4 but this will mean that once R2 is compared to R5, R5 will be installed since it is an older advertisement. Once the neighbor relationship with R5 comes back up and R5 advertised the Of course to always ensure R2 is selected in our network as the best path we could also use the bgp always-compare-med command to compare MED between different ASes but this command is normally not used in the real world unless MED policies are standardized between neighboring ASes.

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When the local preference and AS path length are the same for two or more routes towards a certain prefix, the Multi Exit Discriminator MED attribute comes into play.

So normally, the MED is only considered when two or more routes are received from the same neighboring AS. However, routers can be configured with bgp always-compare-med and then, well, the MED is always compared if multiple routes to a destination have the same local preference and AS path length.

However, there may still be routers out there that exhibit the old behavior, and there are certainly routers that have been configured with bgp bestpath med missing-as-worst. AS 30 has two routers in both locations so the Denver router sees two paths through each location. Which of the routes BGP prefers now depends on the always-compare-med and med missing-as-worst settings. When both always-compare-med and med missing-as-worst are in effect, then the best route is the one through AS 20 by way of New York City, which has a metric of 2.