D-Link 3250TG Product Manual - Page 24

Packet Forwarding, 1w Rapid Spanning Tree

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D-Link DES-3250TG Standalone Layer 2 Switch Packet Forwarding The Switch enters the relationship between destination MAC addresses and the Ethernet port the destination resides on into its forwarding table. This information is then used to forward packets. This reduces broadcast storms on the network, because packets, instead of being transmitted to all ports, are transmitted to the destination port only. Example: if Port 1 receives a packet destined for a station on Port 2, the Switch transmits that packet through Port 2 only, and transmits nothing through the other ports. This process is referred to as 'learning' the network topology. MAC Address Aging Time The Aging Time affects the learning process of the Switch. Dynamic forwarding table entries, which are made up of the source MAC addresses and their associated port numbers, are deleted from the table if they are not accessed within the aging time. The aging time can be from 10 to 1,000,000 seconds with a default value of 300 seconds. A very long aging time can result in dynamic forwarding table entries that are out-of-date or no longer exist. This may cause incorrect packet forwarding decisions by the switch. If the Aging Time is too short however, many entries may be aged out too soon. This will result in a high percentage of received packets whose destination addresses cannot be found in the forwarding table, in which case the switch will broadcast the packet to all ports, negating many of the benefits of having a switch. Static forwarding entries are not affected by the aging time. 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 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 5-1 below compares how the two protocols differ regarding the port state transition. 16

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D-Link DES-3250TG Standalone Layer 2 Switch
Packet Forwarding
The Switch enters the relationship between destination MAC addresses and the Ethernet port the
destination resides on into its forwarding table. This information is then used to forward packets. This
reduces broadcast storms on the network, because packets, instead of being transmitted to all ports,
are transmitted to the destination port only. Example: if Port 1 receives a packet destined for a station
on Port 2, the Switch transmits that packet through Port 2 only, and transmits nothing through the
other ports. This process is referred to as ‘learning’ the network topology.
MAC Address Aging Time
The Aging Time affects the learning process of the Switch.
Dynamic forwarding table entries, which are
made up of the source MAC addresses and their associated port numbers, are deleted from the table if
they are not accessed within the aging time.
The aging time can be from 10 to 1,000,000 seconds with a default value of 300 seconds. A very long
aging time can result in dynamic forwarding table entries that are out-of-date or no longer exist. This
may cause incorrect packet forwarding decisions by the switch.
If the Aging Time is too short however, many entries may be aged out too soon. This will result in a high
percentage of received packets whose destination addresses cannot be found in the forwarding table, in
which case the switch will broadcast the packet to all ports, negating many of the benefits of having a
switch.
Static forwarding entries are not affected by the aging time.
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 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 5-1 below compares how the two
protocols differ regarding the port state transition.
16