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I don't understand why would an organization aside from an ISP even use routing protocols.

Let's say there is a big company with 5000 employees, and they want to have a network throughout their building. Aren't switches enough for their network? Why would they install many routers and configure OSPF for example?

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  • Imagine a broadcast storm with 5000 hosts on a single broadcast domain switched network. It would be a nightmare to try to resolve. Even with chassis switches that have 240 host interfaces that is over 20 switches, and you need to isolate each switch until you find the one with the problem. That takes a lot of downtime from the storm and trying to resolve it, and businesses can lose millions of dollars per minute of downtime. Routers will break up the broadcast domains so such problems are much smaller and easier to resolve, not affecting the entire company. – Ron Maupin Sep 22 '20 at 13:58
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer. – Ron Maupin Dec 17 '20 at 21:32
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A good, if basic question.

A single LAN or layer 2 domain can be defined as having the following characteristics:

  1. All hosts can directly address each other, using their layer 2 protocol address (In the case of Ethernet, it's called the MAC address).
  2. Any host can send a broadcast to all other hosts on the network.

As the number of hosts increases, this begins to cause problems.

  1. Switches need to keep track of all the MAC addresses on the network, so they know which port to send the dat out of.
  2. If a switch doesn't know the MAC address of the destination, it finds it by broadcasting the data frame to every host on the network.
  3. Every time a host moves, disconnects, or connects to the network, all the switches need to update their information about the host.
  4. A network problem, like a loop or misbehaving host, affects all the other hosts on the network. A host in one building will affect every other building in the network. This is why a LAN is sometimes called a "failure domain."

To increase reliability, reduce the amount of broadcast traffic (that affects every host), and to reduce the amount of memory and processing power a switch needs, organizations will break up their network into smaller networks, and use routers to interconnect them.

When there are multiple routers, they use a routing protocol to learn which networks each others are connected to.

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  • But this broadcast problem is caused by ARP, right? Are there any other ways to fill up the MAC table without using ARP? – Noob_Guy Sep 19 '20 at 3:44
  • @Noob_Guy no there's many protocol that use broadcasts, DHCP, for example, by its very nature. – JFL Sep 22 '20 at 11:13
  • There's also a huge security dimension to the network segmentation. – JFL Sep 22 '20 at 11:16
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To build secure , optimise. , reliable network we should use routers , switches and firewalls devices by connecting accordinglly to our design and as per business requirement we can secure our networks from threat hackers and moreover we can fullfill our business requirements as required

ISP will provide internet connectivity feasibility .But its our responsibility to distribute this ISP among 5000 employees in building for establishing this requirement we requires layer2 switches where as each employee workstation is connected with each layer2 switch port. Assume for 5000 employee 5000 switch ports are used . Layer3 device router is used to interconnect all layer2 switches . Routing protocols are configured in router for forward outbound and inbound traffic from and towards ISP. we can choose any routing protocols like static , RIP, EIGRP ,OSPF as per our business requirements .. Nating is enable in router to hide private subnet of LAN and natting and access-list is used to translated public to private ip address and to control traffic for all this configuration router is must .

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A routing protocol is a protocol between routers to exchange their routing information/tables. OSPF is one of those protocols that's very popular for internal route exchange (an interior gateway protocol IGP). Running a routing protocol is only necessary on a larger network where it becomes impractical to manually configure routes.

ISPs need to exchange routes on another level and with more control. They use an exterior routing protocol EGP, must prominently BGP.

Routers use no special protocol for their forwarding - routing is a mechanism in IP.

Switches create segments on the data link layer. All nodes share a common broadcast domain and everyone can directly talk to anyone else. Usually, you'd want some control over your network: Put IP phones in one network, normal users in another and security-sensitive devices in yet another. Traffic between those networks - across a router - can be controlled by policies.

Also, a single, giant broadcast domain doesn't scale well. It's very common to limit each segment to roughly 200 devices (a /24 IP subnet). Anyone building a network for 5000 users (across numerous buildings and probably several geographic locations) with just a single segment seriously needs to be fired.

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