Repeaters, Switches and Routers

Repeaters, Switches and Routers
This presentation will focus on network hardware used to support Ethernet networks

A Physical Layer device that regenerates digital signals, bringing them back to standard form, and transferring each bit from one LAN segment to another.

Cisco Fasthub 100 series Repeater

This is the least complex and least expensive of the three devices It operates on Layer 1 of the OSI model Typically no configuration required It deals with one bit at a time It knows nothing about addresses

Any signal coming in on one port is sent out on all other ports It has the lowest latency of any of these three devices It has the least memory and processing power of any of these three devices

Usually people say Repeater when they mean Multiport Repeater When someone refers to a Hub, they are usually referring to a Multiport Repeater Although you can repeat any digital signal, these days people usually mean a UTP Ethernet repeater

Repeaters require no configuration, but can often be configured to support protocols like SNMP. A true repeater can regenerate a signal from any media to any media, but it can’t change the data rate of the signal between ports

Devices on a network can’t detect when a repeater is being used on a network Repeaters allow collisions to pass, therefore all devices connected to a repeater or series of repeaters are on the same collision domain.

A repeater was originally installed when we wanted to extend the network beyond the maximum cable length allowed by A repeater regenerated the signal allowing for a longer network.

Repeaters are rarely used in commercial installations any more. Switch prices have come down to be very competitive in price with repeaters, and they provide considerably improved functionality Most repeaters that are sold are now used in a home network.

A typical Bridge: It is a Layer 2 device that understands Frames Has only two ports Learns the MAC address of each host by looking at the transmitted frames No configuration is required A bridge breaks up collision domains

Bridge: It will pass a broadcast, but will isolate unicast traffic, allowing separate simultaneous conversations on each side of the bridge Bridges are often installed to improve the performance of a network with too high of a collision rate

Bridge: Bridges do more processing than a multiport repeater because they assemble Frames and do error detection and correction The value of bridges can be defeated by a broadcast intensive Network Operating System

Bridge: Bridges were commonly selected to provide a transition from one media type to another, e.g. from coax to UTP Repeaters can also provide that transition However, with bridges, ports can run at differing data rates

Bridge:

A Multiport bridge is called a switch It performs the same functions as a bridge It is typically used within a LAN and requires no configuration. It requires a lot of memory to buffer data as it comes in on each port

It has more latency than a repeater because of the data buffering requirement The amount of RAM required is related to the number of hosts supported per port The amount of RAM is also related to the switching method

Switches typically use one of three selectable switching methods: Cut Through Fragment Free Store and Forward

Cut Through: A frame begins coming in through a port. As soon as the address of a frame is received and before the entire frame is received, the destination address is looked up in it’s address table and the frame is sent out.

Cut Through: The frame is typically not yet completely received, nor is error detection and correction completed. It provides for extremely fast switching but often increases the number of errors, especially on a busy network Errors take a heavy toll on the network

Fragment Free: This allows the first 64 Bytes of the frame to be received. Since 64 bytes is the minimum size of an Ethernet packet, enough time has passed for all devices on the network to recognize there is traffic out there and a collision that would cause a fragment is less likely.

Fragment Free: Error detection is only done on minimum size frames since the FCS field is not received by the time the other frames are switched. This technique is still a pretty low latency method of switching and it provides a significant improvement in numbers of errors over cut-through because many packets are only 64 bytes long.

Store-and-forward: With this method the entire frame is received before it is forwarded on to the destination port. Error checking is completed, so errors are not forwarded. Latency, though, goes way up. This is the best choice for a very busy network. Can you understand why?

Early on people recognized that switches could provide different data rates for different ports, effectively giving priority to one port over another.

Cisco 2950 series Switches

Cisco 4500 series Switches

We migrated from bridging LAN segments to putting a single host on each port, which is the preferred configuration today. Repeaters, bridges and routers are almost absent from LANs now because of modern switches

Spanning Tree Protocol: This allows a network engineer to include a redundant link, so if one link fails, another is already queued up to carry the data. Without some way of dealing with redundant links (sometimes called loops), broadcast storms occur

A broadcast storm occurs when a broadcast goes around a loop and enters a single switch from two different segments. In that case, switches keep forwarding frames out more than one port, so they are forwarding frames to themselves.

Spanning Tree is a protocol that is used strictly between switches that allow them to negotiate a root bridge, and then temporarily turn off (block) one or more port to break up the loop. If a network link failure occurs, the blocked port is reactivated.

The organizer of this action is the “root” switch, which stays in communication with all other switches. The “root” switch is elected by comparing the priority settings of each switch. Conflicts are resolved by comparing MAC addresses. The lowest number wins.

In the event of multiple connections to the root switch, ports are assigned a cost figure. (See table 2.1). The lowest cost is associated with the highest data rate, and it becomes the port used. Blocking and port selection decisions are made from the perspective of the root switch.

Many switch manufacturers recognize the problems a loop can cause but are unwilling to implement STP because of its complexity and delay (50 seconds for convergence). Instead they isolate links that cause a loop.

Layer 3 devices Routers are put at the edge of a network They define the end of a network They are the most common devices used to provide access to a WAN

They can help you logically segment your network They block broadcasts Routers are typically much more expensive than repeaters or switches Routers typically cause much higher latencies than switches

They buffer the complete packet before making a forwarding decision Error detection occurs on the header Traditionally used to interconnect two dissimilar LANs (eg ATM and Token Ring). More recently have lost this role to Switches.

A complex routing protocol is employed to accommodate multiple links between routers They are much more software based than switches Routers can be configured to block or pass specific protocols, provide NAT, and other functions

These devices require a much greater processing capability You typically (Cisco) pay for each protocol that is included Each router manufacturer has its own unique operating system

SoHo Router

Cisco 2600 series Routers

Cisco series Routers

Typically must be manually configured It works harder and transfers data more slowly than either repeaters or switches Routers used to be employed within a LAN but are now rarely used that way because of the impact on traffic

Wireless

Where are the Bus cables? Coaxial bus cables are rarely used in modern LAN installations All of the high speed LAN standards are designed for UTP and Fiber, not on bus cables. Star and Tree structures are used almost exclusively in LANs

Where are the fiber interfaces? Where are the UTP interfaces? Where are the WAN interfaces? Where are the console interfaces?

Modern hardware can be purchased with whatever interface you choose to pay for. Repeaters and switches come with UTP interfaces on most ports Some hardware has modular ports, allowing for some flexibility, but it comes at a price

Routers are most likely to have the most flexibility with regard to interface. They will have multiple LANs and WANs Switches often have configurations that provide for a higher speed uplink port.

VLAN: Virtual Local Area Network Defines Broadcast Domains Dynamically configured By protocol By IP address By authenticated login Port configured

Repeaters, Switches and Routers

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