Multi homed BGP describes the practice of connecting a single network to multiple Internet Service Providers using the Border Gateway Protocol. This architecture provides immediate redundancy, allowing traffic to reroute instantly if one upstream fails. Unlike single homed connections, it ensures continuous availability for critical enterprise operations. The design also opens paths for sophisticated traffic engineering, influencing how data crosses global networks.
Architectural Models and Implementation
At the core of a multi homed deployment lies the interaction between the Customer Edge router and the Provider Edge devices. Network engineers typically choose between distinct options such as active/passive or active/active designs. The active/passive setup utilizes one primary link for traffic while the secondary link remains on standby, ready to activate during a failure. An active/active configuration, however, leverages both connections simultaneously, balancing the load and maximizing the utilization of the available bandwidth.
Configuring Path Selection and Administrative Attributes
Effective path selection is the most critical function in a multi homed environment, and it is primarily governed by the manipulation of BGP attributes. The LOCAL_PREF attribute allows engineers to prefer one exit point over another for outbound traffic, effectively setting a policy for egress. Conversely, the AS_PATH attribute, which lists the transit AS numbers, is often used to influence inbound selection, as routers typically prefer shorter paths. These adjustments ensure that traffic follows the desired routes based on business requirements rather than solely on hop count.
Operational Benefits and Traffic Engineering
Reliability is the most obvious advantage, as the redundant connectivity eliminates the single point of failure that exists in a single homed topology. This resilience is essential for businesses where minutes of downtime result in significant financial loss or reputational damage. Beyond uptime, multi homing allows for intelligent traffic engineering, where specific prefixes are steered towards specific providers. An organization might prioritize a low latency path for real-time applications while sending bulk data transfers over a higher capacity, cost effective link.
Routing Instability and the BGP Flapping Issue
However, the implementation is not without its challenges, as multi homed BGP networks are susceptible to a phenomenon known as route flapping. If the network does not carefully control the advertisement of its prefixes, it can inadvertently signal to the upstreams that it is a better path for specific destinations. This oscillation creates instability in the global routing table, causing routes to churn as different providers continuously withdraw and re-advertise paths. Proper outbound filtering and the strategic use of the ATOMIC_AGGREGATE attribute are essential to mitigate this risk and maintain global routing integrity.
Security Considerations and Best Practices
Security is a paramount concern when interconnecting with multiple independent networks, as the attack surface is effectively broadened. The use of prefix filtering, whether through manual route maps or automated systems like IRR or RPSL, is vital to ensure that only authorized prefixes are accepted from peers. Implementing BGPsec offers a robust cryptographic solution to validate the authenticity of the path, preventing sophisticated hijacking attacks. These practices transform the multi homed edge from a simple connectivity point into a secure and trusted network boundary.
