The routing protocol

Open Shortest Path First (OSPF) is a robust link-state interior gateway protocol (IGP). People use OSPF when they discover that Routing Information Protocol (RIP) just isn't going to work for their larger network, or when they need very fast convergence

OSPF is a link-state routing protocol, as we said. Think of this as a distributed map of the network. To get this information distributed, OSPF does three things.

Short for "interior gateway protocol", a generic term for a routing protocol that is used to exchange routing information among routers in an autonomous network, such as a enterprise LAN. IGPs typically support confined geographical areas.

RIP, an interior gateway protocol defined by RFC 1058 that specifies how routers exchange routing table information. With RIP, routers periodically exchange entire tables. Because this is inefficient, RIP is gradually being replaced by a newer protocol called Open Shortest Path First (OSPF).

In some cases, routing protocols can themselves run over routed protocols: for example, BGP runs over TCP which runs over IP; care is taken in the implementation of such systems not to create a circular dependency between the routing and routed protocols. That a routing protocol runs over particular transport mechanism does not mean that the routing protocol is of layer (N+1) if the transport mechanism is of layer (N). Routing protocols, according to the OSI Routing framework, are layer management protocols for the network layer, regardless of their transport mechanism:

  • IS-IS runs over the data link layer
  • OSPF, IGRP, and EIGRP run directly over IP; OSPF and EIGRP have their own reliable transmission mechanism while IGRP assumed an unreliable transport
  • RIP runs over UDP
  • BGP runs over TCP
  • IGRP (Interior Gateway Routing Protocol)
  • EIGRP (Enhanced Interior Gateway Routing Protocol)
  • OSPF (Open Shortest Path First)
  • RIP (Routing Information Protocol)
  • IS-IS (Intermediate System to Intermediate System)

Router Layer

A Router is a device, which works in the Network Layer (Layer three) of the OSI Model. Its main function is to route packets that arrive at its ports. Router takes a routing decision for each of the packet that arrives at its port, on the basis of Routing Tables.

Firewall Layer

Firewalls operate at different layers to use different criteria to restrict traffic. The lowest layer at which a firewall can work is layer three. In the OSI model this is the network layer. In TCP/IP it is the Internet Protocol layer. This layer is concerned with routing packets to their destination. At this layer a firewall can determine whether a packet is from a trusted source, but cannot be concerned with what it contains or what other packets it is associated with. Firewalls that operate at the transport layer know a little more about a packet, and are able to grant or deny access depending on more sophisticated criteria. At the application level, firewalls know a great deal about what is going on and can be very selective in granting access.

MPLS based IP VPN: MPLS-based Layer 3 VPNs uses MPLS labeling algorithms and signaling protocols to encapsulate IP packets and distribute VPN-related information. MPLS based IP VPN can seamlessly interface with traditional VPN technologies such as ATM, Frame Relay and TDM etc. It can be an alternative or a complementary VPN solution to the legacy deployment. A primary advantage of MPLS is that it provides the scalability to support both small and very large-scale VPN deployments. It can support end-to-end QoS, rapid faultcorrection of link and node failure, bandwidth protection, and a foundation for deploying additional value-added services. MPLS technology also simplifies configuration, management, and provisioning, helping service providers to deliver highly scalable, differentiated, end-to-end IP based services. The service provider can offer SLAs by enabling MPLS traffic engineering and fast reroute capabilities in the core network. MPLS based IP VNP is a network based VPN technology for site-to-site VPN communications only.

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