An administrator is configuring Border Gateway Protocol (BGP) routing on a Tier-0 Gateway to optimize north--south traffic flow between the NSX environment and multiple upstream physical routers…

Question

Options

• ABFD
• BCost
• CAS-Path Prepend
• DMED

• A
  17% (8)
• B
  11% (5)
• C
  72% (33)

Explanation

In a VMware Cloud Foundation (VCF) architecture, the Tier-0 Gateway is the primary point of integration between the virtualized network and the physical world. When dealing with multiple upstream routers (multi-homing), administrators must influence the BGP path selection process to ensure traffic follows the desired path and avoids suboptimal routing or asymmetric flows. AS-Path Prepend is a common technique used to influence inbound traffic (traffic coming from the physical network into the NSX environment). By repeating its own Autonomous System (AS) number multiple times in the BGP advertisement, the Tier-0 Gateway makes a specific path look "longer" and therefore less desirable to the upstream physical routers. Since BGP prefers the shortest AS-Path, the routers will favor the alternate link that does not have the prepended AS numbers. This is a critical tool in VCF designs to ensure that a primary link is utilized unless a MED (Multi-Exit Discriminator) is an attribute that suggests to an adjacent external AS which path to take among multiple entry points to the same AS. Like AS-Path Prepend, it influences inbound traffic. A lower MED value is preferred over a higher one. In a VCF environment with multiple Edge Nodes or multiple Tier-0 uplinks, setting different MED values allows the administrator to prioritize specific entry points for traffic entering the SDDC. BFD (Bidirectional Forwarding Detection) is not a BGP attribute; it is a detection protocol used to provide fast failure detection of the link between BGP neighbors. While it triggers faster convergence, it does not influence path selection based on attributes. Cost is an OSPF attribute, not a native BGP attribute. Therefore, in the context of NSX Tier-0 BGP configuration, AS-Path Prepend and MED are the verified methods for path manipulation.

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