Transit Gateway (TGW) is a service that connects multiple VPCs and on premises networks using a single gateway. Before Transit Gateways if you had multiple VPCs that needed to talk to each other, you had to create multiple peering connections. Each connection required manual routing, was non-transitive and hard to scale. TGW solves this by acting as a regional layer 3 router which provides transitive connectivity between attached VPCs, centralised control over routing and propagation, and can scale to thousands of VPCs.
Attachments
TGW Attachments are the connections between the TGW and the VPCs or VPNs. Each attachment represents a link between a specific VPC or VPN and the Transit Gateway, allowing for efficient traffic routing. Without attachments the TGW doesn’t even know those VPCs exist.
Route Tables
A TGW by default has one route table and all other attachments use this for table for routing decisions. Routes are propagated from the attachments.
A route table can contain static routes, propagated routes automatically learned from attachments that you enabled to propagate into this table and blackhole route with “blackhole = true” to drop matching traffic and prevent specific paths.
TGW makes a routing decision based on the route table associated with the ingress attachment. This means that traffic entering from Attachment A will look at A’s associated route table to decide where to send the packet next. Return traffic will look at the returner’s associated table to avoid asymmetric routing.
Associations
An association binds one TGW route table to an attachment and determines which route table a connection (attachment) uses. That table governs egress from that attachment and you can re-associate an attachment to a different TGW route table at any time.
Propagations
Propagation lets an attachment advertise its network prefixes into a TGW route table automatically so other attachments associated with that table can reach it without manual static routes. You turn propagation on per attachment per TGW route table.
Top comments (0)