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Zorluk: ZorAWS VPN, Direct Connect, and Transit Gateway Management

A company connects its multi-VPC environment using an AWS Transit Gateway. The primary connection to the corporate datacenter is a 1 Gbps1\text{ Gbps} AWS Direct Connect (DX) connection using a Transit Virtual Interface (Transit VIF) through a Direct Connect Gateway (DXGW). A Site-to-Site VPN is configured directly to the Transit Gateway as a backup path. The on-premises database subnet resides in the CIDR block `172.16.100.0/24172.16.100.0/24`, which belongs to the larger corporate network range of `172.16.0.0/12172.16.0.0/12`. During a routine audit, a SysOps administrator notices that database-bound traffic is routing over the VPN tunnel instead of the DX connection, even though the DX connection is fully operational and has low utilization. In the Transit Gateway route table, the administrator identifies a propagated route for `172.16.0.0/12172.16.0.0/12` pointing to the DXGW and a propagated route for `172.16.100.0/24172.16.100.0/24` pointing to the VPN attachment.

Which two actions should the SysOps administrator take to ensure that traffic to the database subnet primarily uses the Direct Connect connection and only fails over to the VPN if the Direct Connect connection becomes unavailable? (Select TWO.)

  1. Advertise the `172.16.100.0/24172.16.100.0/24` prefix over the Direct Connect BGP session from the on-premises router.Cevap
  2. Configure the customer gateway to only advertise the summarized `172.16.0.0/12172.16.0.0/12` prefix over the Site-to-Site VPN BGP session, removing the `172.16.100.0/24172.16.100.0/24` prefix advertisement.Cevap
  3. C
    Configure AS Path prepending on the Site-to-Site VPN BGP session from the customer gateway to deprioritize the `172.16.100.0/24172.16.100.0/24` route.
  4. D
    Add a static route for `172.16.100.0/24172.16.100.0/24` pointing to the Transit Gateway attachment in all VPC subnet route tables.
  5. E
    Configure a static route for `172.16.0.0/12172.16.0.0/12` pointing to the Direct Connect Gateway attachment in the Transit Gateway route table.

Cevap

To direct database traffic over the Direct Connect connection as the primary path, the administrator must either advertise the specific database subnet prefix (172.16.100.0/24172.16.100.0/24) over the Direct Connect BGP session, or stop advertising that specific prefix over the VPN BGP session so that both paths only advertise the summarized range (172.16.0.0/12172.16.0.0/12).
AWS Transit Gateway uses Longest Prefix Match (LPM) to route traffic. Because the VPN was advertising a more specific prefix (172.16.100.0/24172.16.100.0/24) than the Direct Connect (172.16.0.0/12172.16.0.0/12), traffic preferred the VPN. To resolve this, the prefix lengths must be equal on both paths. This is achieved by either advertising the specific prefix (172.16.100.0/24172.16.100.0/24) over the Direct Connect BGP session or by only advertising the summary prefix (172.16.0.0/12172.16.0.0/12) over the VPN. Once prefix lengths are equal, the Transit Gateway applies its default preference order, which prioritizes Direct Connect Gateway over VPN.

Adım Adım Çözüm

1
Analyze the route advertisements and determine that Longest Prefix Match (LPM) is taking precedence because the VPN is advertising a more specific CIDR (172.16.100.0/24172.16.100.0/24) than the Direct Connect path (172.16.0.0/12172.16.0.0/12).
Confirmed that the mismatch in prefix length is bypassing the default preference for Direct Connect.
AWS Transit Gateway evaluates prefix length first before evaluating attachment type preferences.
2
Align the prefix lengths by either advertising the specific subnet (172.16.100.0/24172.16.100.0/24) over Direct Connect, or removing the specific advertisement from the VPN so both paths use the summary (172.16.0.0/12172.16.0.0/12).
Prefix lengths are matched on both the Direct Connect and VPN paths.
Matching the prefix lengths forces Transit Gateway to use the attachment type hierarchy for routing decisions.
3
Verify that Transit Gateway defaults to prioritizing the Direct Connect Gateway (DXGW) attachment over the VPN attachment for identical prefixes.
Traffic correctly routes over the Direct Connect connection and fails over to the VPN only if the Direct Connect connection goes down.
Direct Connect Gateway has a higher default priority than Site-to-Site VPN in Transit Gateway route tables.

Anahtar Kavram

AWS Transit Gateway path selection logic which prioritizes Longest Prefix Match (LPM) over BGP attributes and attachment preferences, followed by a preference for Direct Connect Gateway over VPN for equal prefixes.
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