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BGP MED Attribute

In networking, the BGP MED attribute is an important part of the routing process. But what is it, and why is it important? This post will explain the purpose of the MED attribute, and we'll also take a look at some common use cases for this powerful feature.

So if you're looking to learn more about BGP MED, you've come to the right place!

What is BGP MED Attribute?

The BGP MED attribute, also known as the Multi-Exit Discriminator, is a metric used by Border Gateway Protocol (BGP) to determine the best path for routing traffic between autonomous systems.

The value of the MED attribute can be set by network administrators, allowing for the customization of routing decisions. In addition to manually setting a value for the MED attribute, it can also be automatically propagated from one autonomous system to another. This allows for greater flexibility and adaptability in BGP routing decisions. Overall, the BGP MED attribute plays an important role in optimizing network traffic flow across multiple autonomous systems.

How is a MED Metric Advertised?

There are several rules to consider when a MED metric is advertised:

  • First and foremost, a more specific metric will cancel out a less specific one. For example, if you have two metrics of different groups, the group-specific metric takes precedence over the global BGP metric. Likewise, if you have peer-specific and other global metrics, the peer-specific metrics override both others.
  • A routing policy's metric definition takes priority over a metric-out statement.
  • Any defined metric will take precedence over a metric received from a route.
  • If a MED metric is not associated with a received route, and if a metric value is not configured, no metric is advertised. If you do not set a metric value, it will be automatically set to zero when advertising an active route.

BGP MED vs Local Preference

As we mentioned before, BGP MED, or Multi-exit Discriminator, is a BGP attribute that allows a network to inform its BGP neighbors about preferred entry points into the network.

Local Preference, on the other hand, is an attribute used to influence routes within a single BGP autonomous system. In other words, BGP MED influences which BGP neighbor a network will use to reach a destination, while Local Preference influences the choice of exit point within a particular network.

BGP MED can also be considered a more globally significant attribute, as it can influence routing decisions for all BGP neighbors, while Local Preference only affects choices made within one autonomous system. Both attributes are important in determining BGP routing decisions, but they serve different purposes and should not be confused with one another.

What are some common uses for BGP MED?

One common use case for BGP MED is when different ISPs connect to a provider through different points of presence with varying levels of available bandwidth. In this scenario, the provider could assign a higher BGP MED value to the slower PoPs to influence external peers to primarily use the higher bandwidth connections.

Another potential use case is for directing traffic towards specific data centers in cases where there are multiple options available within a geographical region. By assigning a lower BGP MED value to the preferred data center, external peers may be more likely to route traffic toward it. BGP MED should be used carefully, as it can lead to unbalanced traffic distribution and potential network congestion if not implemented properly.

Final Words

The BGP MED attribute is a useful tool for networking. By setting the MED value on your border gateway protocol (BGP) advertisements, you can tell other networks how highly you think they should route traffic to your network.

This could be helpful if you have multiple connections to different providers and want to make sure that traffic from one provider is routed through another provider’s network as efficiently as possible.

Have you tried using the BGP MED attribute in your own networking?

Before, you must to check our CCIE Enterprise Infrastructure course about BGP MED.

Created by
Stanley Arvey

I am a certified network engineer with over 10 years of experience in the field. I have a deep understanding of networking and IT security, and I am always looking for new challenges.

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