Managing Outbound Routing Decisions in BGP
Q: In a multi-homed environment using BGP, how do you manage outbound routing decisions?
- Static and Dynamic Routing
- Senior level question
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In a multi-homed environment using Border Gateway Protocol (BGP), managing outbound routing decisions involves strategically configuring BGP attributes and leveraging route policies. The main goal is to control which path outbound traffic takes when there are multiple upstream providers.
Firstly, we use BGP attributes such as AS Path, Local Preference, and MED (Multi-Exit Discriminator) to influence outbound routing.
1. Local Preference: This attribute is one of the most influential tools for outbound routing decisions. By assigning higher local preference values to routes learned from specific upstream providers, we can prefer these paths for outgoing traffic. For example, if we have two upstream providers, we might set a local preference of 200 for routes from the primary provider and 100 for the secondary provider. This ensures that traffic prefers the primary provider unless it becomes unavailable.
2. AS Path Manipulation: We can prepend our AS number to routes advertised to our peers. This makes routes from a specific provider appear less favorable to other networks due to the longer AS path length and can influence incoming traffic but can also affect outbound traffic since some routers may prefer shorter AS paths for return traffic.
3. MED: When advertising routes to multiple neighboring ASes, we can use the MED to hint at preferred routes. A lower MED value is preferred, so if we have a MED set lower for our primary upstream provider compared to a secondary one, this will influence how neighboring routers route traffic back to us.
4. Route Filtering and Policy-based Routing: We can implement filtering to select which prefixes to announce based on specific criteria or employ route maps and policies to direct traffic through certain upstream providers based on attributes like source IP or application type.
As an example, if our primary upstream provider offers lower latency for VoIP traffic, we can create a route map that sets a higher local preference for VoIP-related IPs, ensuring that this specific traffic is directed through that provider even though we have multiple connections.
In summary, by effectively using BGP attributes and route policies, we can influence outbound routing decisions in a multi-homed environment to optimize traffic flow and enhance network performance.
Firstly, we use BGP attributes such as AS Path, Local Preference, and MED (Multi-Exit Discriminator) to influence outbound routing.
1. Local Preference: This attribute is one of the most influential tools for outbound routing decisions. By assigning higher local preference values to routes learned from specific upstream providers, we can prefer these paths for outgoing traffic. For example, if we have two upstream providers, we might set a local preference of 200 for routes from the primary provider and 100 for the secondary provider. This ensures that traffic prefers the primary provider unless it becomes unavailable.
2. AS Path Manipulation: We can prepend our AS number to routes advertised to our peers. This makes routes from a specific provider appear less favorable to other networks due to the longer AS path length and can influence incoming traffic but can also affect outbound traffic since some routers may prefer shorter AS paths for return traffic.
3. MED: When advertising routes to multiple neighboring ASes, we can use the MED to hint at preferred routes. A lower MED value is preferred, so if we have a MED set lower for our primary upstream provider compared to a secondary one, this will influence how neighboring routers route traffic back to us.
4. Route Filtering and Policy-based Routing: We can implement filtering to select which prefixes to announce based on specific criteria or employ route maps and policies to direct traffic through certain upstream providers based on attributes like source IP or application type.
As an example, if our primary upstream provider offers lower latency for VoIP traffic, we can create a route map that sets a higher local preference for VoIP-related IPs, ensuring that this specific traffic is directed through that provider even though we have multiple connections.
In summary, by effectively using BGP attributes and route policies, we can influence outbound routing decisions in a multi-homed environment to optimize traffic flow and enhance network performance.