Types of Routers

Routers intended for ISP and major enterprise connectivity will almost invariably exchange routing information with the Border Gateway Protocol (BGP).

RFC 4098 defines several types of BGP-speaking routers:

• Edge Router: Placed at the edge of an ISP network, it speaks external BGP
  
  (EBGP) to a BGP speaker in another provider or large enterprise
  
  Autonomous System (AS).
  
• Subscriber Edge Router: Located at the edge of the subscribers network, it speaks EBGP to its providers AS(s). It belongs to an end user (enterprise) organization.
• Inter-provider Border Router: Interconnecting ISPs, this is a BGP speaking router that maintains BGP sessions with other BGP speaking routers in other providers.
• Core router: A router that resides within the middle or backbone of the LAN network rather than at its periphery.
• Within an ISP: Internal to the provider's AS, such a router speaks internal BGP (IBGP) to that provider's edge routers, other intra-provider core routers, or the provider's inter-provider border routers.


• Internet backbone: The Internet does not have a clearly identifiable backbone, as did its predecessors. Nevertheless, it is the major ISPs routers that make up what many would consider the core. These ISPs operate all four types of the BGP-speaking routers described here.  In ISP usage, a core router is internal to an ISP, and used to interconnect its edge and border routers. Core routers may also have specialized functions in virtual private networks based on a combination of BGP and Multi-Protocol Label Switching (MPLS).

Routers are also used for port forwarding for private servers. Office Home Office (SOHO) connectivity Residential gateways (often called routers) are frequently used in homes to connect to a broadband service, such as IP over cable or DSL.  Such a router may also include an internal DSL modem. Residential gateways and SOHO routers typically provide network address translation and port address translation
in addition to routing. Instead of directly presenting the IP addresses of local computers to the remote network, such a residential gateway makes multiple local computers appear to be a single computer.  SOHO routers may also support Virtual Private Network tunnel functionality to provide connectivity to an enterprise network.

A three-layer model is in common use, not all of which need be present in smaller networks.

Access routers, including SOHO routers, are located at customer sites such as branch offices that do not need hierarchical routing of their own. Typically, they are optimized for low cost.

Distribution routers aggregate traffic from multiple access routers, either at the same site, or to collect the data streams from multiple sites to a major enterprise location. Distribution routers often are responsible for enforcing quality of service across a WAN, so they may have considerable memory, multiple WAN interfaces, and substantial processing intelligence.

They may also provide connectivity to groups of servers or to external networks. In the latter application, the router's functionality must be carefully considered as part of the overall security architecture. Separate from the router may be a firewall or VPN concentrator, or the router may include these and other security functions.

When an enterprise is primarily on one campus, there may not be a distinct distribution tier, other than perhaps off-campus access. In such cases, the access routers, connected to LANs, interconnect via core routers.

In enterprises, a core router may provide collapsed backbone" interconnecting the distribution tier routers from multiple buildings of a campus, or large enterprise locations.  They tend to be optimized for high bandwidth.

When an enterprise is widely distributed with no central location(s), the function of core routing may be subsumed by the WAN service to which the enterprise subscribes, and the distribution routers become the highest tier.

Definition: Wireless Router.
A network device that combines a router, switch and access point (base station) in one box. Wireless routers provide a convenient way to connect a small number of wired and any number of wireless computers to the Internet.  

Definition of: Router
A network device that forwards packets from one network to another. Based on internal routing tables, routers read each incoming packet and decide how to forward it. The destination address in the packets determines which line (interface) outgoing packets are directed to.  In large-scale enterprise routers, the current traffic load, congestion, line costs and other factors determine which line to forward to.

Definition of Hub
A common connection point for devices in a network. Hubs are commonly used to connect segments of a LAN (at layer 1) of the OSI (open system interconnection) reference Model.  A hub contains multiple ports. When a packet arrives at one port, it is copied to the other ports so that all segments of the LAN can see all packets.

Definition of Switch
In networks, a device that filters and forwards packets between LAN segments. Switches operate at the data link layer (layer 2) and sometimes the network layer (layer 3) of the OSI Reference Model and therefore support any packet protocol. LANs that use switches to join segments are called switched LANs or in the case of Ethernet networks, switched Ethernet LANs.

Hubs vs. Switches

The primary reason for choosing a switch over a hub is bandwidth needs.
Switches can greatly improve network performance because switches do not broadcast the packets they receive.

Hubs broadcast the packets they receive to all available ports on the hub, thereby increase network traffic.

Both switches and hubs support the same protocols.  Hubs are cheaper than switches but can result in higher cost over time when you consider the potential for issues with lower bandwidth.

Both switches and hubs support different types of nodes.

Definition of Bridge
Bridges allows you to divide networks into segments while keeping the appearance of 1 segment to the upper layer protocols.

Bridges works at the OSI data link layer and can be used to connect similar network but not dissimilar networks.

A bridge is a Layer 2 device that connects network segments. A bridge is more efficient that a repeater because it can analyze traffic.

Definition of Gateway
(1) Gateways are used to connect networks that use different protocols.

(2) A device that converts one protocol or format to another. A network gateway converts packets from one protocol to another. An application gateway converts commands and/or data from one format to another.  An e-mail gateway converts messages from one mail format to another. In addition, there are also IP gateways, proxy server, cable/DSL gateways, default gateways, media gateways, NAS gateways, wireless gateways, messaging gateways, signaling gateways, WAP gateways, switches, EGP, BGP and IGP. (3) A device that acts as a go-between two or more networks that use the same protocols.  In this case, the gateway functions as an entry/exit point to the network.  Transport protocol conversion may not be required, but some form of processing is typically performed.

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