D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch

33

Figure 4- 9. Static Router Ports Settings window

The following parameters can be set:

Parameter

Description

VID (VLAN ID)

This is the VLAN ID that, along with the VLAN name, identifies the VLAN

where the multicast router is attached.

VLAN Name

This is the name of the VLAN where the multicast router is attached.

Slot

This is the Unit ID of the switch in a switch stack for which you are creating

an entry into the switch’s static router port table.

Member Ports

There are the ports on the switch that will have a multicast router attached

to them.

Configuring Spanning Tree

The switch supports 802.1d Spanning Tree Protocol (STP) and 802.1w Rapid Spanning Tree

Protocol (RSTP). 802.1d STP will be familiar to most networking professionals. However

since 802.1w RSTP has been recently introduced to D-Link managed Ethernet switches, a

brief introduction to the technology is provided below followed by a description of how to set

up 802.1 d STP and 802.1w RSTP.

802.1w Rapid Spanning Tree

The Switch implements two versions of the Spanning Tree Protocol, the Rapid Spanning Tree

Protocol (RSTP) as defined by the IEEE 802.1w specification and a version compatible with

the IEEE 802.1d STP. RSTP can operate with legacy equipment implementing IEEE 802.1d,

however the advantages of using RSTP will be lost.

The IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) evolved from the 802.1d STP

standard. RSTP was developed in order to overcome some limitations of STP that impede the

function of some recent switching innovations, in particular, certain Layer 3 function that are

D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch

34

increasingly handled by Ethernet switches. The basic function and much of the terminology is

the same as STP. Most of the settings configured for STP are also used for RSTP. This section

introduces some new Spanning Tree concepts and illustrates the main differences between the

two protocols.

Port Transition States

An essential difference between the two protocols is in the way ports transition to a

forwarding state and the in the way this transition relates to the role of the port (forwarding or

not forwarding) in the topology. RSTP combines the transition states disabled, blocking and

listening used in 802.1d and creates a single state

Discarding

. In either case, ports do not

forward packets; in the STP port transition states disabled, blocking or listening or in the

RSTP port state discarding there is no functional difference, the port is not active in the

network topology. Table 5-7 below compares how the two protocols differ regarding the port

state transition.

Both protocols calculate a stable topology in the same way. Every segment will have a single

path to the root bridge. All bridges listen for BPDU packets. However, BPDU packets are sent

more frequently – with every Hello packet. BPDU packets are sent even if a BPDU packet

was not received. Therefore, each link between bridges are sensitive to the status of the link.

Ultimately this difference results faster detection of failed links, and thus faster topology

adjustment. A drawback of 802.1d is this absence of immediate feedback from adjacent

bridges.

802.1d STP

802.1w

RSTP

Forwarding?

Learning?

Disabled

Discarding

No

No

Blocking

Discarding

No

No

Listening

Discarding

No

No

Learning

Learning

No

Yes

Forwarding

Forwarding

Yes

Yes

Comparing Port States

RSTP is capable of more rapid transition to a forwarding state – it no longer relies on timer

configurations – RSTP compliant bridges are sensitive to feedback from other RSTP

compliant bridge links. Ports do not need to wait for the topology to stabilize before

transitioning to a forwarding state.

In order to allow this rapid transition, the protocol

introduces two new variables: the edge port and the point-to-point (P2P) port.

Edge Port

The edge port is a configurable designation used for a port that is directly connected to a

segment where a loop cannot be created. An example would be a port connected directly to a

single workstation. Ports that are designated as edge ports transition to a forwarding state

immediately without going through the listening and learning states. An edge port loses its

status if it receives a BPDU packet, immediately becoming a normal spanning tree port.

D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch

35

P2P Port

A P2P port is also capable of rapid transition. P2P ports may be used to connect to other

bridges. Under RSTP, all ports operating in full-duplex mode are considered to be P2P ports,

unless manually overridden through configuration.

802.1d/802.1w Compatibility

RSTP can interoperate with legacy equipment and is capable of automatically adjusting

BPDU packets to 802.1d format when necessary. However, any segment using 802.1 STP will

not benefit from the rapid transition and rapid topology change detection of RSTP. The

protocol also provides for a variable used for migration in the event that legacy equipment on

a segment is updated to use RSTP.

STP Switch Settings

The Spanning Tree Protocol (STP) operates on two levels: on the switch level, the settings are

globally implemented.

On the port level, the settings are implemented on a per user-defined

Group of ports basis. To open the following window, open the S

panning Tree

folder in the

Configuration

menu and click the

STP Switch Settings

link.

Figure 4- 10.

STP Switch Settings

D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch

36

Configure the following parameters and click the

Apply

button to implement them:

Parameter

Description

Spanning Tree

Protocol

<

Disabled

>

This field can be toggled between

Enabled

and

Disabled

using the pull-

down menu.

This will enable or disable the Spanning Tree Protocol (STP),

globally, for the switch.

Max Age: (6 - 40 sec)

<

20

>

The Max. Age can be set from 6 to 40 seconds. At the end of the Max.

Age, if a BPDU has still not been received from the Root Bridge, your

Switch will start sending its own BPDU to all other Switches for permission

to become the Root Bridge. If it turns out that your Switch has the lowest

Bridge Identifier, it will become the Root Bridge.

Hello Time: (1 - 10

sec)

<

2

>

The Hello Time can be set from 1 to 10 seconds. This is the interval

between two transmissions of BPDU packets sent by the Root Bridge to

tell all other Switches that it is indeed the Root Bridge.

Forward Delay: (4 -

30 sec)

<

15

>

The Forward Delay can be from 4 to 30 seconds. This is the time any port

on the Switch spends in the listening state while moving from the blocking

state to the forwarding state.

Priority: (0 - 61440)

<

32768

>

A Priority for the switch can be set from 0 to 61440.

This number is used

in the voting process between switches on the network to determine which

switch will be the root switch. A low number indicates a high priority, and a

high probability that this switch will be elected as the root switch.

STP Version

<

RSTP

>

Choose RSTP (default) or STP Compatibility. Both versions use STP

parameters in the same way. RSTP is fully compatible with IEEE 802.1d

STP and will function with legacy equipment.

Tx Hold Count

<

3

>

This is the maximum number of Hello packets transmitted per interval. The

count can be specified from 1 to 10. Default value = 3.

Forwarding BPDU

<

Enabled

>

This field can enabled or disabled. When it is enabled it allows the

forwarding of STP BPDU packets from other network devices when STP is

disabled on the switch. The default is enabled.

Note

: The Hello Time cannot be longer than the Max. Age.

Otherwise, a configuration error will occur.

Observe the following formulas when setting the above

parameters:

Max. Age

≤

2 x (Forward Delay - 1 second)

Max. Age

≥

2 x (Hello Time + 1 second)

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37

STP Port Settings

For stacked switch installations, first select the Unit to be configured.

Figure 4- 11.

STP Port Settings

In addition to setting Spanning Tree parameters for use on the switch level, the switch allows

for the configuration of groups of ports, each port-group of which will have its own spanning

tree, and will require some of its own configuration settings. An STP Group will use the

switch-level parameters entered above, with the addition of Port Priority and Port Cost.

An STP Group spanning tree works in the same way as the switch-level spanning tree, but the

root bridge concept is replaced with a root port concept. A root port is a port of the group that

is elected on the basis of port priority and port cost, to be the connection to the network for the

group. Redundant links will be blocked, just as redundant links are blocked on the switch

level.

The STP on the switch level blocks redundant links between switches (and similar network

devices).

The port level STP will block redundant links within an STP Group.

It is advisable to define an STP Group to correspond to a VLAN group of ports.

The following fields can be set:

Parameter

Description

Unit

This is the Unit ID of a switch in a switch stack.

15 indicates a DES-6500

switch in standalone mode.

From/To <

Port 1

>

A consecutive group of ports may be configured starting with the selected

port.