Currently we are using ISP A given /26 public ip address, and we couldn't get /24 public from Internet Registry for some reason. Is it possible to setup BGP multihoming using ISP A /26 public ip address and then subscribe another link with ISP B (backup link)? In case ISP A link down, the /26 public ip address still can reach through ISP B link?
if you haven't your own public IP address space you will need a second address block from ISP B.
>> In case ISP A link down, the /26 public ip address still can reach through ISP B link?
Generally this is not possible unless ISP A and ISP B are involved and agree on this.
More likely ISP B will give you a separate address block of its own,
You need to configure NAT in such a way that when ISP A is down you will use the address block provided by ISP B.
There was a nice design guide on the cisco site for this scenario.
Hope to help
the place to look is here
Unfortunately that specific whitepaper is not here.
Hope to help
Are these provider independent allocations if so they are not tied to any local internet registry they follow you around to the ISP you want to peer with.
If I understand you correctly I think what you are asking you cannot done, the subnet you have been assigned cannot be split to peer with two independent ISP’s at the same time, but you can peer with your PI subnet and also obtain a PA subnet from ISP B and obtain resiliency that way.