Black Box 5000 series Manuale utente

Series 5000 Bridge/Router

Reference Manual

LR5100A-T, LR5200A-R2

5500148-10 equivalent to 5500072-11

Section 1 — Introduction

This product provides IP and IPX routing combined with a protocol transparent bridge. This bridge/router combination is

often the best solution to linking remotely located LANs where most of the traffic is IP or IPX with smaller amounts of

traffic from other protocols such as NetBIOS or DEC LAT.

The P1700 series routers support the widely implemented Routing Information Protocol, otherwise known as RIP. RIP

support allows the router to interoperate with other vendors’ routers.

The router will operate as delivered, providing increased LAN performance directly out of the box without the need for

complex pre-configuration. However, in those situations where specific customization is required, an easy-to-use “hotkey”

menuing Bridge/Router Manager console provides access to LAN and Link statistical information, and control of the

network configuration.

With increased LAN and Link management capability, you will be able to detect LAN and Link problems, determine

utilization patterns, and plan for future expansion that will optimize your existing data-communication resources.

The router can be thought of as a group of discrete functions combined in a single box. The first functional module is the

LAN interface, which receives all LAN traffic and then decides where individual frames should be sent: to the IP router, to

the IPX router, to the bridge, to the management system, or discarded altogether. After the LAN interface there are several

functional units including the IP router, the IPX router, the bridge, and the management system. Any traffic that these

modules need sent across a link is then forwarded to the link modules, which control data coming and going on the WAN

ports. The following figure illustrates the relationships between the various component modules in a router.

LAN Interface

Module Bridge

Module

IPX Router

Module

Management

Module

Link #1

Module

Link #2

Module

IP Router

Module

Figure 1 - 1Router Block Diagram

The router menu system provides a method to control whether IP & IPX traffic is routed through the router modules, or

bridged through the bridge module along with all other bridged data.

IP Routing

The router may be used to route only between subnets within the same network, or between different networks.

Network broadcasts sent within a subnet-routed environment will not be forwarded to the other subnets in the network.

The procedure for establishing an IP connection through an IP router is explained on the next few pages.

Introduction

ARP—Address Resolution Protocol

A protocol called ARP (Address Resolution Protocol) is used to determine the MAC address of a particular IP address. The

MAC (Medium Access Control) address is unique predefined number for each device on the LAN. The manufacturer of the

device assigns MAC addresses. The IP address for each device is assigned by the network administrator according to the

network structure.

If the originating station does not know the MAC address of the destination station, a MAC broadcast will be transmitted

onto the LAN asking “Who has IP address 170.22.10.4?” This MAC broadcast is called an ARP request. Because the ARP

request is a MAC broadcast, every device on the LAN will see the frame. The device that has the IP address 170.22.10.4 will

respond with a frame to the originating station. The ARP reply frame will include the MAC address of the destination device.

Now when the two devices wish to send data across the LAN to each other, they will both use the MAC and IP address of

the other device.

Each device on the LAN maintains a table for MAC addresses and IP addresses called the ARP cache. The ARP cache

contains a list of IP addresses and their corresponding MAC addresses.

Proxy ARP

Each time an originating station does not know the MAC address of a destination station, the originating station sends out an

ARP request. If the destination station is on a different network, the router connected to the originating network will see

from the IP address that the frame is to be routed to another network. If the router has an entry for the destination address,

the router will generate an ARP reply to send back to the originating station. The ARP reply will specify the MAC address of

the router as the MAC address to send frames to for the IP address of the destination station.

The Complete IP Connection

The following are the steps that a frame of data will take when being transmitted from an originating station on an IP network

to a destination station on a different IP network. In this example, the two networks are separated by a third network with

two router hops between the originating network and the destination network.

•Originating station will send an ARP request if it does not have the MAC address of the destination station.

•Local router will see ARP request and send an ARP reply to the originating station with the MAC address of the local

router port.

•Originating station will send the data frame addressed to the IP address of the destination station, and the MAC

address of the local router port.

Introduction

•Local router will receive the data frame and strip off the MAC portion. The resulting IP frame will be examined to

determine the destination IP address.

•Local router will look in its routing table to find the IP address of the router to send the IP frame to next. The local

router will see that the destination router is the next router.

•Local router will look in its ARP cache to find the MAC address of the destination router as determined by the IP

address in the routing table.

•Local router will rebuild the complete frame with a new MAC header indicating the MAC address of the destination

router. The local router does not alter the destination IP address, so the destination IP address will still be the IP

address of the destination station.

•Destination router will receive the data frame and strip off the MAC portion. The resulting IP frame will be

examined to determine the destination IP address.

•Destination router will look in its routing table to find the IP address of the router to send the IP frame to next. The

destination router will see that the destination IP address is on a locally connected network.

•Destination router will look in its ARP cache to see if it has a MAC address for the destination IP address. If it does

not have an entry, the destination router will generate an ARP request. The destination station will send an ARP

reply.

•Destination router will rebuild the complete frame with a new MAC header indicating the MAC address of the

destination station. The destination IP address once again will be unchanged and remain as the destination station IP

address.

•Destination station will receive the data frame and process it.

If the destination station wishes to send a frame back to the originating station, the process will happen in the reverse

direction.

If the path from the originating station to the destination station causes the frame to pass through more than two routers, the

above process will simply be extended to include the interaction between the intermediate routers.

Introduction

IP Header Details

Every IP header has common fields of information. The layout of the information is always the same. Refer to the following

diagram for a representation of the IP header.

Figure 1 - 2IP Header

Protocol

The protocol section is used to indicate the protocol being used by the transport layer. This could be TCP, UDP, or

something else.

Time to live

The time to live section is used to prevent a frame from traversing the network forever. This field contains a number

(maximum 255) that is set when the frame is originally generated. Each time the frame is passed through the

bridge/router, the bridge/router will decrement the time to live by two. When the time to live reaches zero, the

frame is discarded.

Header Checksum

The header checksum is used to verify the data in the IP header. The IP header is recalculated each time a frame is

passed through a router. The recalculation is necessary because the time to live field is changed.

Fragmentation

Fragmentation occurs when an IP frame must be split up into smaller IP frames. When the originating device

generates the IP frame, the device is not aware of all the paths the frame must traverse to get to the destination

device. If the IP frame is to pass through a network that has small packet capabilities, the IP frame must be split up

and reassembled at the destination device. Each of the fragments is assigned a fragment offset value, which

determines where the fragment fits into the original IP frame.

The router will accept fragmented frames directed to itself and reassemble them, but it will not fragment frames.

Introduction

Options

There are various options that may be set for any IP frame.

Source Routing

Source routing is used to predetermine the path that the IP frame must travel through the network. There are two

types of source routing: strict source routing and loose source routing.

Strict source routing will contain a list of IP addresses of routers that must be used when the IP frame is sent through

the network. Strict source routing is used mainly to provide some type of data security. Once the IP frame has

reached the destination station, the destination station will take the list of IP addresses from the options field, reverse

them, and use them for a strict route back to the originating station.

Loose source routing will also contain a list of IP address of routers to be used on the path to the destination station.

However, the IP frame may pass through other intermediate routers to get to the next IP address in the loose source

routing list.

Route Recording

Route recording simply keeps a list of all the IP addresses of the routers that the IP frame has passed through on its

way to the destination station.

Time Stamps

The time stamp option is used to record the time at which the IP frame passed through each router on its way to the

destination station.

ICMP Messages

Internet Control Message Protocol (ICMP) messages are used to perform station and router protocol participation. ICMP

messages are passed between routers, or between routers and stations. There are several different messages, as discussed

below.

Unreachable

The “unreachable” message is sent back to the originating station when the path to the destination network has

disappeared. A destination network may be unreachable because of a broken link, a downed router, a downed station,

or other reasons.

Redirect

The “redirect” message is sent to the originating station when there is a better router to use to reach the destination

network. Because the routers share routing tables, each router has the ability to determine whether it is the best

router to use for network traffic. Once a station receives a redirect, all future IP frames destined for the particular

destination network will be sent to the new router.

Quench

The “quench” message is sent to the originating station when the path to the destination network has become

congested. The originating station will slow down the rate of transmission of frames for an internally (to the station)

predetermined period of time upon receiving a quench message.

Introduction

Ping

The “ping” message is actually a query status message that may be sent to devices on the LAN to determine their

operation status. The LAN device will reply with a message if it is active.

Time and Mask server

Two other ICMP messages are used to query the time and/or subnet mask from a particular LAN device. A message

is sent to a LAN device asking for the time or mask, and the device replies appropriately.

Introduction

RIP—Routing Information Protocol

The most important function of the IP protocol is routing. IP routers constantly exchange information keeping their routing

tables up to date. A method of communication is required to ensure compatibility between all IP routers in the network. RIP

is the portion of the IP protocol that is used for router communication.

Route Tables

Each router will maintain a table of network addresses and the appropriate action to take with an IP frame it receives.

A routing table entry will usually consist of the following items:

•Network or sub-network address

•IP address of the next hop router

•Network interface to use to get to the next hop router

•Subnet mask for this network interface

•Number of hops to reach the destination network

•Number of seconds since this route was updated

When a router receives an IP frame, the router will examine it to determine the destination network address. The

router will then look in the routing table, determine the next router to send the IP frame to, and send the frame to

that router.

The selection of the best route path is based solely on the number of hops to the destination network.

Update Mechanism

In order to ensure that the routing tables of all routers in the network are kept up to date, each router will broadcast

its routing table onto each of its locally connected networks. The broadcast of the routing tables occurs every 30

seconds.

The process of updating a routing table with current information, and deciding which router to use to reach a

destination network, creates a ripple effect of changes through the network. When a router goes down and an

adjacent router determines that the path has disappeared, the remaining adjacent routers on that network must

determine the next path to use to reach the destination network. Each router will now broadcast its new routing table

with the updated information. The updated information will propagate through the network until all routing tables

have been brought up to date. This process is called convergence.

The broadcast of the routing tables is also used as a method of determining whether a router is still alive or has been

removed from the network. If a router has not heard from an adjacent router in 180 seconds, the local router will

mark the adjacent router as unreachable and start to adjust the routing table, if necessary.

Introduction

IPX Routing

The router may be used to route between IPX networks.

Novell Netware uses a suite of protocols for LAN communications. The Novell protocols include IPX, SPX, RIP, SAP, plus

others, and operate at layers 3 and above. These protocols, their relationship with each other, and the general operation of a

Novell network are discussed in this section.

The Netware Network Operating System implements the concept of “Client-Server” computing. In this system, there are

various Servers, such as File Servers, Print Servers, and Fax Servers, to name a few. The Client stations, where the users work,

connect to these servers to retrieve files, get application software, or submit print jobs. Most of the interaction between the

Clients and Servers is invisible to the users. These operations rely on the transfer of packets between Clients and Servers using

the IPX/SPX protocols.

IPX Addressing

The IPX protocol is based on the Xerox XNS protocol. The IPX header contains all the IPX addressing information, and

not much else.

Network Layer Addressing vs. MAC Addressing

An Ethernet frame has at least two levels of addressing. The MAC addresses for both the source and destination are

contained in the MAC header. The MAC addresses are essentially physical port addresses, and are globally unique.

Hardware vendors encode the port MAC address as part of the manufacturing process. All Ethernet devices have the

same MAC address format. The MAC address is used to communicate frames between LAN ports regardless of

protocol.

The Network layer addressing is assigned by the network administrator, in a format prescribed by the layer 3

protocol, for example IPX. The network address is used to structure the network system and for communications

between ports operating the same protocol.

Note that it is possible for a single network port to have several different network addresses, but it can have one and

only one MAC address. An example of this is a computer acting as an IPX File Server, an IPX Router, and an IP

Router. In this case the port would have a MAC address, an IPX address for its IPX functions, and an IP address for

the IP Routing functions.

IPX Address Format

The IPX Address is made up of three components: the Network Number, the Node Number, and the Socket

Number. These components are fixed length (unlike the IP addressing) and function.

Figure 1 - 3IPX Address Format

Network Addresses

The Network Number addresses the network. All stations on the same “network” will have the same Network

Number. Note that a network could be a single segment, or multiple segments joined by either bridges or repeaters.

In IPX internetworks, routers must be used to join different networks together.

Introduction

Node Addresses

The Node Number identifies the individual stations in a Network. In IPX devices, this address is assigned

automatically and is identical to the MAC address. This means that the Node Number is self-configuring, and will be

unique within the Network because the MAC address that was copied is (supposed to be) unique.

The use of the MAC address as the Node Number allows IPX stations to be self-configuring. This makes the initial

configuration of a station much simpler, but there are factors to consider before doing this. The Node Numbers

cannot be assigned in a way that you might desire; for example with groups of stations having consecutive addresses.

Instead, the network is forced to use whatever MAC address is assigned to the LAN port.

Socket Addresses

The Socket Number identifies the process within the source/destination that is communicating. Common Sockets

include File Servers (Socket Number 0451), SAP (Socket Number 0452), and RIP (Socket 0453). The Socket

Number can be thought of as the address of the upper layer using the IPX communication.

The Socket Numbers are assigned by Novell and do not change from LAN to LAN. In other words, all

communications with File Servers use Socket Number 0451. When a software vendor uses IPX to communicate

across a Netware network, the vendor will apply to Novell to receive a Socket Number for the application. As an

example, if Acme Schedule Company made a groupware scheduling program for Netware, they would get a Socket

assigned for their use. No other communications on the LAN would use the Acme Scheduler Socket.

Other IPX Header Information

The IPX header contains some other information besides the source and destination addresses.

Figure 1 - 4IPX Header

The checksum is a hold-over field from the XNS model used by Novell. The checksum is no longer used and this

field is now permanently set to FFFF.

The length field indicates the total length of the IPX packet. Note that the data portion can be any length up to 546

bytes, so the length field is needed in the header.

The Transport Control field is used for counting the number of routers the frame has traversed. In other words, it is

a hop count. This operation uses only 4 of the 8 bits; the remaining 4 bits are reserved (by Novell) for future use so

we could see additional information contained in the Transport Control field if Novell decides to use the excess

capacity.

The Packet Type indicates what type of service is using the packet. Some common packet types include type 1, RIP;

type 2, Echo; type 4, IPX; and type 17, Netware Core Protocol.

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