Router Architecture Overview:)
Two Key Router Functions:)
- Run routing algorithms/protocol( RIP(routing information protocol),OSPF(open shortest path first ),BGP(border GateWay Protocol) )
- Forwarding datagrams from incoming to outgoing link
Input Port Functions:)
Pysical layer: bit-level reception
Data link layer: Ethernet
Decentralized switching:
- datagram destination lookup output port using forwarding table in input port memory
- Goal: complete input port processing ar line speed.
- Queuing: If datagrams arrive faster than forwarding rate into the switch fabric
Three Types of switching fabrics: 1.Memory 2.Bus 3.Crossbar
What's inside a router?
- We have yet to consider the switching function of a router - the actual transfer of datagrams from a router's incoming links to the appropriate outgoing links.
- The real work (that is, the reason the network layer exists in the first place) is the forwarding of datagrams.
- A key component in this forwarding process is the transfer of a datagram from a router's incoming link to an outgoing link.
- This section we study how this is accomplished
- A high-level view of a generic router architecture is shown in above Figure.
Four components of a router:)
Input ports. The input port performs several functions:
- It performs the physical layer functionality of terminating an incoming physical link to a router.
- It performs the data link layer functionality needed to interoperate with the data link layer functionality on the other side of the incoming link.
- It also performs a lookup and forwarding function so that a datagram forwarded into the switching fabric of the router emerges at the appropriate output port.
- Control packets (e.g., packets carrying routing protocol information such as RIP, OSPF, or IGMP(Internet Group Management Protocol)) is forwarded from the input port to the routing processor. In practice, multiple ports are often gathered together on a single line card within a router.
Switiching Fabric:
The switching fabric connects the router's input ports to its output ports. This switching fabric is completely contained with the router - a network inside of a network router!
Output Ports:
An output port stores the datagrams that have been forwarded to it through the switching fabric and then transmits the datagrams on the outgoing link. The output port thus performs the reverse data link and physical layer functionality as the input port.
Routing processor:
The routing processor executes the routing protocols maintains the routing tables, and performs network management functions, within the router.
