Small site multihoming
These requirements are viable for central sites of enterprise networks or high-availability e-commerce solutions, but completely unrealistic if you want to multi-home a small site (or numerous remote sites of a single enterprise network).
This tutorial describes how to design and deploy a multihomed remote site. It covers several scenarios with varying levels of redundancy.
Basic small-site multihoming
Connecting a small site to multiple service providers can be extremely easy – you get two upstream links and two provider-assigned (PA) IP addresses (either static or dynamically assigned). Since each ISP will give you only a single IP address, you have to use private IP addresses on the LAN side of the router and perform Network Address Translation (NAT) on the gateway router.
As most ISPs will not be willing to run a dynamic routing protocol with small sites, you have to configure static default routing on your end. Reliable static routing (static routes tied to IP SLA measurements) should be used to ensure failure detection of any component between your router and the core of the Service Provider network.
Redundant Small Site Multi-Homing
A redundant remote site is even simpler to implement. The addressing and routing requirements do not change (NAT from private to PA address space is performed on both gateway routers), but the routing becomes simpler: each gateway router has a single reliable static route and redistributes it into an intra-site dynamic routing protocol.
OSPF should be used as the intra-site routing protocol as its default route origination mechanisms require no route redistribution.
Servers in Small Site Multi-Homing
If you want to deploy high-availability public servers within your network, you should implement proper multi-homing solution including BGP routing with the Service Providers. In most other cases, it’s better to use a decent hosting service.
However, if you want to deploy local mail server within your LAN or you have a special application that simply cannot be hosted anywhere else and you’re willing to accept less-than-perfect reliability and complex design, it’s possible to deploy servers in a small-site multihoming environment.