Does the peering between two transit gateways is a AWS managed connection ?

Question

we have a big mesh of transit gateways that is distributed on different regions and some of them even exists in the same region connecting different accounts.

my question is that, does the peering between two transit gateways is AWS managed connection so it will not be go down eventually or I need to make a redundant path to connect VPCs behind these two connecting transit gateways

I am asking this question because the peering between transit gateways does not ask for speed or bandwidth for example when create the peering.

Answer

In the sense of redundancy for the peering between transit gateways, Yes, this is managed by AWS and you don't need to worry about redundant link for the sake of availability. 
For bandwidth, its up to 100 Gbps per availability zone, see TGW quotas here https://docs.aws.amazon.com/vpc/latest/tgw/transit-gateway-quotas.html#bandwidth-quotas

Does the peering between two transit gateways is a AWS managed connection ?

Contenido relevante