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BGP
| Term | Definition |
|---|---|
| IX | Internet Exchange |
| Transit | Accessing a backbone which interconnects other networks |
| Peering | Providing an interconnect between two or more networks |
| Tier 1 ISP | Only peers with other ISPs, connecting to all tier 1 ISPs guarantees connectivity to the entire internet |
| Tier 2 ISP | An ISP which uses a combination of peering and transit from Tier 1 ISPs |
| Tier 3 ISP | A smaller ISP with a minimal network, very little peering, receiving transit from a tier 1 or 2 ISP |
| IGP | Interior Gateway Protocol, a type of routing protocol used on smaller-scale, multi-homed networks |
| BGP | Border Gateway Protocol, a routing protocol used for large-scale, internetworking applications |
| RIP | Routing Information Protocol, a distance-vector routing protocol |
| OSPF | Open Shortest Path First, a link-state routing protocol |
Typically, routers store a table of the possible routes for an IP packet to take. These tables are usually a combination of directly connected routes and routes that have been statically set to point at another gateway which is directly connected. An example routing table is below for reference:
| Route | Next Hop | Cost | | 10.0.0.0/16 | Directly Connected on interface eth0 | 0 | | 10.1.0.0/16 | Directly Connected on interface eth1 | 0 | | 10.2.0.0/16 | 10.1.25.4 | 10 |
In this scenario, a packet from 10.0.2.45 destined for 10.2.55.4 would be forwarded to the router at 10.1.25.4, which is directly available to the current router.
On larger and more complex networks, manually maintaining these routing tables becomes tedious, especially in complex network setups with multiple possible routes of varying cost. The solution to this problem is to use routing protocols, which automatically distribute routes and calculate cost to optimize routing.
The most basic of routing protocols is the distance-vector Routing Information Protocol, which periodically broadcasts the routes onto the specified broadcast domain, listening for other routers to share their routes, and forwarding received routes onto its other broadcast domains (in the process incrementing the cost). A more powerful and complex alternative is Open Shortest Path First (OSPF). Unlike RIP and its newer alternatives, OSPF only sends updates when there is a change in the network, maintaining an internal topology map, and distributing limited updates to confirm that the router is still online. OSPF is therefore a link-state protocol, which calculates its cost metric based no just on the hop count but also the capacity of a particular link.