D-Link DES-6500 Router Manual PDF (Setup & Configuration Guide)

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118

For two DNS servers to communicate across different subnets, the

DNS Relay

of the Switch

must be used. The DNS servers are identified by IP addresses.

Mapping Domain Names to Addresses

Name-to-address translation is performed by a program called a Name server. The client

program is called a Name resolver. A Name resolver may need to contact several Name

servers to translate a name to an address.

The Domain Name System (DNS) servers are organized in a somewhat hierarchical fashion.

A single server often holds names for a single network, which is connected to a root DNS

server – usually maintained by an ISP.

Domain Name Resolution

The domain name system can be used by contacting the name servers one at a time, or by

asking the domain name system to do the complete name translation. The client makes a

query containing the name, the type of answer required, and a code specifying whether the

domain name system should do the entire name translation, or simply return the address of the

next DNS server if the server receiving the query cannot resolve the name.

When a DNS server receives a query, it checks to see if the name is in its subdomain. If it is,

the server translates the name and appends the answer to the query, and sends it back to the

client. If the DNS server cannot translate the name, it determines what type of name resolution

the client requested.

A complete translation is called recursive resolution and requires the

server to contact other DNS servers until the name is resolved. Iterative resolution specifies

that if the DNS server cannot supply an answer, it returns the address of the next DNS server

the client should contact.

Each client must be able to contact at least one DNS server, and each DNS server must be

able to contact at least one root server.

The address of the machine that supplies domain name service is often supplied by a DHCP or

BOOTP server, or can be entered manually and configured into the operating system at

startup.

Configuring DNS Relay Information

To configure the DNS function on the Switch, open the

Configuration

folder and click the

DNS Relay

folder. In this folder, click the

DNS Relay Information

link to open the

following window.

Figure 4- 74. DNS Relay Information window

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The following fields can be set:

Parameter

Description

DNS Relay Status

<

Disabled

>

This field can be toggled between

Disabled

and

Enabled

using the pull-

down menu, and is used to enable or disable the DNS Relay service on the

switch.

Primary Name

Server <

0.0.0.0

>

Allows the entry of the IP address of a primary domain name server (DNS).

Secondary Name

Server (2) <

0.0.0.0

>

Allows the entry of the IP address of a secondary domain name server

(DNS).

DNSR Cache Status

<

Disabled

>

This can be toggled between

Disabled

and

Enabled.

This determines if a

DNS cache will be enabled on the switch.

DNS Static Table

Status <

Disabled

>

This field can be toggled using the pull-down menu between

Disabled

and

Enabled.

This determines if the static DNS table will be used or not.

DNS Relay Static Table

To view the

DNS Relay Static Table

, open the

DNS Relay

folder in the

Configuration

folder and click the

DNS Relay Static Table

link, which will open the following window.

Figure 4- 75. DNS Relay Static Table

To add an entry into the DNS Relay Static Table, simply enter a Domain Name with its corresponding IP address

and click

Apply

. A successful entry will be presented in the table below, as shown in the example above. To

erase an entry from the table, click the

corresponding to the entry you wish to delete.

IP Multicasting

The functions supporting IP multicasting are added under the

IP Multicasting

folder, from

the

Layer 3 IP Networking

folder.

IGMP

,

DVMRP

, and

PIM-DM

can be

enabled

or

disabled

on the switch without changing

the individual protocol’s configuration.

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IGMP Interface Configuration

The Internet Group Multicasting Protocol (IGMP) can be configured on the switch on a per-IP

interface basis. To view the

IGMP Interface Table

, open the

IP Multicasting

folder under

Configuration

and click

IGMP Interface Configuration

Each IP interface configured on the

switch is displayed in the below

IGMP Interface Table

dialog box.

To configure IGMP for

a particular interface, click the corresponding hyperlink for that IP interface.

This will open

another

IGMP Interface Configuration

window:

Figure 4- 76. IGMP Interface Configuration Table

Figure 4- 77. IGMP Interface Configuration window

This window allows the configuration of IGMP for each IP interface configured on the switch.

IGMP can be configured as Version 1 or 2 by toggling the

Version

field using the pull-down

menu. The length of time between queries can be varied by entering a value between 1 and

65,500 seconds in the

Query Interval

field.

The maximum length of time between the

receipt of a query and the sending of an IGMP response report can be varied by entering a

value in the

Max Response Time

field.

The

Robustness Variable

field allows IGMP to be ‘tuned’ for sub-networks that are expected

to lose a lot of packets. A high value (max. 255) for the robustness variable will help

compensate for ‘lossy’ sub-networks. A low value (min. 2) should be used for less ‘lossy’

sub-networks.

The following fields can be set:

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Parameter

Description

Interface Name

<

System

>

Displays the name of the IP interface that is to be configured for IGMP.

This must be a previously configured IP interface.

IP Address

Displays the IP address corresponding to the IP interface name above.

Version <

2

>

Enter the IGMP version (1 or 2) that will be used to interpret IGMP queries

on the interface.

Query Interval <

125

>

Allows the entry of a value between

1

and

65535

seconds, with a default of

125 seconds. This specifies the length of time between sending IGMP

queries.

Max Response Time

<

10

>

Sets the maximum amount of time allowed before sending an IGMP

response report.

A value between 1 and 25 seconds can be entered, with

a default of 10 seconds.

Robustness Variable

<

2

>

A tuning variable to allow for subnetworks that are expected to lose a large

number of packets. A value between 2 and 255 can be entered, with larger

values being specified for subnetworks that are expected to lose larger

numbers of packets.

State <

Disabled

>

This field can be toggled between

Enabled

and

Disabled

and enables or

disables IGMP for the IP interface. The default is

Disabled

.

Configuring DVMRP

The Distance Vector Multicast Routing Protocol (DVMRP) is a hop-based method of building

multicast delivery trees from multicast sources to all nodes of a network. Because the delivery

trees are ‘pruned’ and ‘shortest path’, DVMRP is relatively efficient. Because multicast group

membership information is forwarded by a distance-vector algorithm, propagation is slow.

DVMRP is optimized for high delay (high latency) relatively low bandwidth networks, and

can be considered as a ‘best-effort’ multicasting protocol.

DVMRP resembles the Routing Information Protocol (RIP), but is extended for multicast

delivery. It relies upon a metric analogous to RIP hop counts to calculate ‘shortest paths’ back

to the source of a multicast message, but defines a ‘route cost’ to calculate which branches of

a multicast delivery tree should be ‘pruned’ – once the delivery tree is established.

When a sender initiates a multicast, DVMRP initially assumes that all users on the network

will want to receive the multicast message. When an adjacent router receives the message, it

checks its unicast routing table to determine the interface that gives the shortest path (lowest

cost) back to the source. If the multicast was received over the shortest path, then the adjacent

router enters the information into its tables and forwards the message. If the message is not

received on the shortest path back to the source, the message is dropped.

Route cost is a relative number that is used by DVMRP to calculate which branches of a

multicast delivery tree should be ‘pruned’. The ‘cost’ is relative to other costs assigned to

other DVMRP routes throughout the network.

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The higher the route cost, the lower the probability that the current route will be chosen to be

an active branch of the multicast delivery tree (not ‘pruned’) – if there is an alternative route.

Enabling DVMRP

DVMRP can be

Enabled

or

Disabled

, globally on the switch, using the

DVMRP

Configuration

link to open the

DVMRP Global Setting

page, as shown below.

Figure 4- 78. DVMRP Global Setting Page

Select

Enabled

or

Disabled

, as appropriate.

DVMRP Interface Configuration

To view the

DVMRP Interface Table

, open the

IP Multicasting

folder under

Configuration

and click

DVMRP Interface Configuration

. This menu allows the

Distance-Vector

Multicast Routing Protocol (DVMRP)

to be configured for each IP interface defined on the

switch. Each IP interface configured on the switch is displayed in the below

DVMRP

Interface Configuration

dialog box.

To configure DVMRP for a particular interface, click

the corresponding hyperlink for that IP interface.

This will open the

DVMRP Interface

Configuration

window:

Figure 4- 79. DVMRP Interface Table

Figure 4- 80. DVMRP Interface Configuration

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