There are multiple ways to build a highly available network. Let’s assume you require a 100% concurrently maintainable network, which ensures when any one connection is lost you can maintain network connectivity.
Internet Service Provider
If you maintain multiple connections that end users must access 24×7, you can achieve connectivity through DNS and BGP.
Adjust Domain Name Servers (DNS)
Consider what happens if you have two separate carriers providing diverse IP subnets and your end users are accessing them via DNS and your primary carrier fails? Your users will attempt to access unavailable services.
To avoid those situations, set the time-to-live on DNS entries to five minutes ahead of time, which will allow you to change the IP addressing. As the updated DNS propagates, it will allow your users to slowly adjust.
There are automated DNS services such as UltraDNS, which monitor external services, detect a failure and automatically adjust DNS entries. The advantage of an automated system is that it performs failover at any time and you can expect a failover of roughly 5 minutes.
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Border Gateway Protocol (BGP)
BGP is a dynamic routing protocol that controls the flow of information on the Internet. BGP allows you to influence the flow of traffic both in and out of your network and is preferable to DNS management.
An Autonomous System Number (ASN) is assigned via a local registry. This ASN allows you to peer with upstream providers. You can advertise your registry-assigned IP addresses with your primary and secondary ISPs. If one ISPfails, the identical addresses are available via your secondary ISP.
Multiple connections should be hosted on multiple routers to maintain connectivity should a single router fail. Border routers can then be connected to redundant firewalls. From the firewall, you should have connectivity into a pair of core switches for layer 2 aggregation.
Can you see the pattern? For max redundancy, host connectivity on multiple devices from beginning to end. Often this configuration can be collapsed where multiple functions are performed via a single piece of equipment. Your required uptime should be balanced with cost.