Hello,
I manage BGP for our /24 network using multiple carriers. I have a client to whom I would like to allocate a /29 subnet within our /24 IP address space. I’m currently trying to determine if this is feasible and what modifications are necessary to ensure that the remaining IPs continue to function with our /24 subnet mask. Any guidance would be appreciated.
Lets say you have a x.x.x.x/24 route in your routing table now. Then you carve a /29 out of that /24 and add a x.x.x.x/29 route into your routing table. The /24 route will apply for everything in that /24 except for the /29 since its more specific.
Depending on your network …
If you have a network that is a /24 ( example 192.168.1.0/24 defined on an ethernet interface example: 192.168.1.1/24 ) then you can not route a /29 from that network and route it somewhere else.
You can do something like this however.
Ethernet1 - 192.168.1.1/25
Ethernet2 - 192.168.1.128/26
Where all of the IPs in a block all route to the same place - in this case above all IPs in the /25 go one place and all of the IPs in the /26 go somewhere also and you have a free /26 left over to work with for future networks that can be then broken into /27 or /28 or /29 or /30 as needed.
Do a google search for subnet mask calculator.
note - the more specific the block is ( /30 ) is where the route will go.
note - with a /24 , it is possible to route it to a null default route , then carve out smaller blocks to be routed to other networks.
I think it should be possible with proxy-arp turned on on the interface that has 192.168.1.1/24?
Yes, it absolutely is.
There is no problem with overlapping a longer prefix on top of a shorter, covering one in general. If you assign the whole short prefix to a broadcast-medium-capable interface like Ethernet, though, then you will need something like proxy-ARP to mediate the exchange, otherwise the hosts who think they are part of a /24 network will just try to talk directly to the other hosts in the longer prefix, and naturally get nowhere.
If all you’re doing is adding two routes to your table, one pointing the /24 in one direction and the /29 in the other, there is no problem at all. Though if the /24 itself ever gets added directly to an Ether-like interface as its final destination, then you will need to turn proxy-ARP on on the gateway that is serving those hosts (on the interface), regardless of whether the same gateway is also handling the /29 on a different one of its interfaces, or if that /29 is several hops away on another part of your network.
In addition to what the other said here use /31 to your advantage specially if you use v7.18 /30 still using 4 address and use private IP as a point to point mechanism inside your network that’s how we conserved V4 address
MikroTik supports /32 addressing (does require a small script to work with DHCP leases) so you don’t even need to do that. Infact you can even re-use /32 addresses so if i.e. you had a router connected directly to multiple customers all requiring a public IP. You could i.e. have your router as 1.0.0.0/32 on every one of those interfaces and the customers as 1.0.0.1/32, 1.0.0.2/32 etc
The customer router however also needs to support /32 and not all of them do
Though usually you’d want to use a CGNAT address on your side, so you can allocate the full 256 public IP’s to 256 customers with zero wastage no matter where they are in your network
For the customers who have routers that don’t support /32, they can use PPPoE
You’re right but this is really case to case basis depends also in the implementor, anyway the OP opt to choose to route /30 with proxy arp 
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manage BGP for our /24 network using multiple carriers. I have a client to whom I would like to allocate a /29 subnet within our /24 IP address space. I’m currently trying to determine if this is feasible
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I’m sorry to say that that won’t be possible.
first, where did you get your /24. each carrier has their own address allocation for their routing table and their bgp policies.
second, route leaking. unless you are going as transit network ie. becoming ix or multihoming isp - see point 1 above.
third, /24 is the smallest subnet that can be accepted by peers.
a fraction from your /24 network is your decision - but any peers probably only accepting /24.
you can do a full /30 route to the customers - but announce only /24 to peers.
Question is a bit vague but 'm going to take the assumption that you (the OP) have a /24 and you’re simply wanting to give a number of addresses (in this case /29) to some other company that is using you (and only you) for transit with those IP addresses, right?
As already mentioned the minimum BGP advertisement is /24 so they cannot advertise a /29 to another BGP transit provider
However if they are exclusively using you for transit then you can either use eBGP with them, or just do a static route to them. Use private/CGNAT addresses between you and them so as to not waste any public IP’s. They can then assign up those /29 however they want internally
You don’t want to do anything differently in your BGP advertisements to your existing peers, you still just advertise your /24 as normal