Router

A router is a physical device that performs routing. Physically, a router is usually a special-purpose computer, although it is possible to program a personal computer to route. Minimally, it contains a control plane and a forwarding plane, but also some network management facilities for configuration and error handling.

The network management facilities may have no direct human interface (i.e., any human control is from a network management server), or the physical router provide a text, windowing, or Web-like (i.e., HTTP) interface.

Routers are network elements that, even within the Internet end-to-end assumption, must maintain state of the routes they service. If the router supports quality of service, it also needs to maintain state on the performance it guarantees, whether this is an essentially automatic mechanism such as internal buffer management in the forwarding plane, or explicit agreements such as those made with the Resource Reservation Protocol (RSVP).

Resource reservation, although it is built up from per-hop behavior, is an end-to-end function, so this is one way in which the router breaks the end-to-end assumption. Other common violations include firewall and network address translation functions.

Routers by application and cost-performance

BGP-speaking

Routers that run the Border Gateway Protocol usually are either part of the Internet service provider infrastructure, or interfaces to the Internet. BGP may also be used in complex enterprise networks. While there are many informal definitions, several definitions used in categorizing performance are listed below.

Provider Edge Router

A provider edge router is a router at the edge of a provider's network that speaks exterior eBGP to a BGP speaker in another AS. The traffic that transits this router may be destined to or may originate from non-adjacent autonomous systems. In particular, the multi-exit discriminator values used in the Provider Edge Router would not be visible in the non-adjacent autonomous systems. Such a router will always speak eBGP and may speak iBGP.

Subscriber Edge Router

A subscriber edge router is router at the edge of the subscriber's network that speaks eBGP to its provider's AS(s). The router belongs to an end user organization that may be multi-homed, and that carries traffic only to and from that end user AS. Such a router will always speak eBGP and may speak iBGP.

This definition of an enterprise border router (which is what most Subscriber Edge Routers are) is practical rather than rigorous. It is meant to draw attention to the reality that many enterprises may need a BGP speaker that advertises their own routes and accepts either default alone or partial routes.

Inter-provider Border Router

An inter-provider border router is a BGP speaking router that maintains BGP sessions with other BGP speaking routers in other providers' ASes. Traffic transiting this router may be originated in or destined for another AS that has no direct connectivity with this provider's AS. Such a router will always speak eBGP and may speak iBGP.

Core Router

A core router is a provider router internal to the provider's net, speaking iBGP to that provider's edge routers, other intra-provider core routers, or the provider's inter-provider border routers. Such a router will always speak iBGP and may speak eBGP. It will often run Multiprotocol Label Switching (MPLS).