You have 99 routes in your dynamic BGP propagated route table and you wish to add 2 more: 10.1.0.0 and 10.3.0.0. You cannot modify or remove routes that have already been announced. What should you do?
A.
Summarize the two routes to combine them into one and advertise it.
B.
Just advertise them, the 100 route limit is a "soft limit" and will be expanded automatically.
Suggested Answer:A🗳️
You cannot add these routes. If you try to summarize them, that would create a 10.0.0.0/14, which is too low of a CIDR to advertise to AWS. AWS has a minimum of /16. You cannot have the 100 route limit modified in any way. It is a hard 100 route limit.
Why though? Why couldnt it be A ie. whats wrong with summarising the 2 routes into a 10.0.0.0/14 with a static route to null and then advertising the single summary route only?
Answer is A. It is possible to summarize.
When you associate a virtual private gateway with a Direct Connect gateway, you specify a list of Amazon VPC prefixes to advertise to the Direct Connect gateway. The prefix list acts as a filter that allows the same CIDRs, or smaller CIDRs to be advertised to the Direct Connect gateway. You must set the Allowed prefixes to a range that is the same or wider than the VPC CIDR because we provision entire VPC CIDR on the virtual private gateway.
Consider the case where the VPC CIDR is 10.0.0.0/16. You can set the Allowed prefixes to 10.0.0.0/16 (the VPC CIDR value), or 10.0.0.0/15 ( a value that is wider than the VPC CIDR).
https://docs.aws.amazon.com/directconnect/latest/UserGuide/multi-account-associate-vgw.html
The CIDR limitation is only applicable when VPC is created (see reference below), but this question is about prefix summarizing into AWS routing table. Nothing prevents from having your on-prem network advertising 10.x.x.x/8 subnet - I'd say answer is A
When you create a VPC, you must specify an IPv4 CIDR block for the VPC. The allowed block size is between a /16 netmask (65,536 IP addresses) and /28 netmask (16 IP addresses).
https://docs.aws.amazon.com/vpc/latest/userguide/VPC_Subnets.html#VPC_Sizing
Indeed, Ispent over an hour searching, could not find any link specifying the maximum CIDR to advertise to aws vpc.
A
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