BGP Auto-Summary Best Practices
BGP auto-summary is used to automatically advertise the classful network if you have the classful network in your BGP table or a subnet of a classful network. By doing so, BGP will advertise fewer prefixes to its neighbors. It reduces the size of the routing table of your neighbors.
This of course has advantages and disadvantages, and they must be weighed carefully before using this command.
- The advantages include a simplification of the routing table, reducing its size, improving efficiency, and simplifying management.
- Disadvantages include a loss of granularity, and the potential for unintended route advertisements, especially in networks with discontiguous subnets.
So should auto-summary be enabled? Well, it is disabled by default on Cisco IOS routers in versions after 12.2(8)T, so that is an indication of what you should ideally configure. So, if it can be avoided, don't use it. It should only be used if routing tables get too large and CPU and memory of BGP routers are being overly taxed.