Reducing the size of the internet routing table and static routes

Much ink has been spilled about the 512K routes and various network equipment that has trouble with that limit. You may consider either reducing the number of routes (say not take in the /24s after getting the default route sent to you by the ISP) and not load balance as well as you would like to just do the default route only anyway.

The issue is you may have VPNs, or NATs or other things with static routes that assume that you have the internet routing table. If you have redistributed those static routes into the IGP, when you get rid of your eBGP internet routes you find youself with a routing loop.

Net of this is when you are making a change to your internet routing audit your devices for static routes that are being redistributed and ensure that you do not create a routing loop when you reduce your internet routing table size.

anycast and bgp may not mix

Anycast is a common way of doing distributed services. The idea is to have duplicate IP addresses and thus have duplicate routes. What then happens is a user picks the nearest service instance based on the routing table. When you are using BGP, you have to be aware of ebgp behavior. If data center A and data center B want to use a common anycast address, remember that only 1 of the edge routers will have an ebgp route the other data center will not. What happens is data center A send ebgp router, data center B receives it. Data center B puts ebgp route in the routing table. Since there is not an IGP route in the routing table data center B will not inject its own ebgp route. A distance command can fix this you just set the distance for any received ebgp anycast prefix to less than that of ospf

router bgp ####
distance 0.0.0.0 255.255.255.255 120 <acl>

Point is that when you are using BGP you have to design around where the anycast addresses will be injected and ensure that you have IGP prefixes where they need to be