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, compare it with Local Preference and BGP Weight, 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.
MED is used to convey the preferred path for traffic entering an autonomous system when multiple entry points exist. MED values are exchanged between autonomous systems, acting as a suggestion rather than a directive: the lower the MED value, the more preferable the path. This makes MED highly effective in multi-homed network environments where differential routing is necessary between various external links.
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.
BGP MED vs BGP Weight
Another attribute that is often compared with MED is BGP Weight. BGP Weight is a Cisco proprietary attribute that plays a pivotal role in route selection within a single router. It is locally significant, meaning its value doesn't get advertised to other routers, even within the same autonomous system (AS). The higher the weight, the higher the priority of the route: if multiple paths are available to the same destination, the path with the highest weight will be chosen.
One of the main advantages of using BGP Weight is its simplicity and local effect, which ensures that changes don't ripple through the network and affect other routers. This makes it an ideal tool for scenarios where local preferences need urgent adjustments without impacting the broader network.
The key differences between BGP Weight and MED can be summarized as follows:
- Scope: Weight is local to the router, while MED is exchanged between autonomous systems.
- Standardization: Weight is Cisco proprietary, while MED is standardized in BGP.
- Value impact: With Weight, the higher value is preferred; with MED, the lower value is preferred.
- Preference control: Weight controls route selection within a router, while MED is used to influence incoming traffic from other autonomous systems.
- Usability: Weight is suitable for intra-AS route management, while MED is ideal for multi-homed network scenarios across autonomous systems.
Despite their differences, BGP Weight and MED share some common ground. Both are key in making strategic routing decisions and achieving optimal pathway selections—crucial for enhancing network reliability and performance. Additionally, they are both configurable attributes within BGP, allowing network administrators to tweak routing policies according to specific requirements and objectives. Whereas BGP Weight is oriented towards managing internal router decisions without influencing neighbor routers, MED shines in scenarios where the interaction between different autonomous systems forms a critical part of the network strategy.
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.
In multi-homed scenarios, a corporation may also use MED to control how traffic from various global regions enters its network through different ISP links. By selectively setting MED values, the corporation can reduce latency and manage traffic loads more effectively during peak usage times. This topic is covered in depth in our self-paced BGP training.
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.
Understanding attributes like MED, Local Preference, and Weight thoroughly provides a robust toolkit for managing and manipulating network routes. Each attribute serves unique scenarios, and knowing when to apply each one can significantly enhance the operational efficiency of a network.
Have you tried using the BGP MED attribute in your own networking?
Before you do, make sure to check our CCIE Enterprise Infrastructure course about BGP MED.
