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Q: What is the recommended way to connect multiple VLANs between a routing switch and a layer 2 switch?
The diagram below illustrates the question.
The following HP switches provide VLANs and have a single MAC/Ethernet address (filtering) table: Switch 800t, 2000, 1600m, 2400m, 2424m, 2512, 2524, 4000m, 8000m. In the diagram above we show a Switch 2500, but the following discussion applies to all of the switches listed in the previous sentence. The ProCurve Networking by HP Routing Switch 9304m, 9308m, or 6308m-SX, as a default gateway, has a single MAC address (for all of its VLANs) if using virtual Ethernet interfaces. In the diagram above we show a 9304m, but this could be a 9308m or 6308m-SX as well.
Let's consider PC "A" attempting to send an IP packet to PC "B". PC "A" will send the 2500 a packet with the 9304m's MAC address in the destination field. If the 2500 has not yet learned this MAC address, the 2500 will flood the packet out all of its VLAN1 ports, including the VLAN1 link to the 9304m. The 9304m will then route the packet toward PC "B" via its link with the 2500's VLAN2 connection. The 2500 will enter the 9304m's MAC address into its MAC address table as located in VLAN2. The 2500 will also forward the packet to PC "B".
Let's consider a second packet that PC "A" sends to PC "B". PC "A" sends the packet, again addressed to the 9304m's MAC address, to the 2500. The 2500 will check its address table and find that the 9304m appears to be located on VLAN2. Since the 2500 believes that this MAC address is not located on VLAN1, the switch will discard the packet.
Later, when the 9304m transmits a packet to the 2500 via the VLAN1 link, the 2500 will update its address table to indicate that the 9304m's MAC address is located in VLAN1 instead of VLAN2. As you can see, the 2500's location information for the 9304m's MAC address will vary over time between VLAN1 and VLAN2. For this reason, some packets directed through the 2500 for the 9304m's MAC address will be discarded. Performance may appear to be poor or connectivity may appear to be broken.
To avoid this issue, simply use one cable between the 2500 and the 9304m instead of two, making sure that the two VLANs use tags on that link, as shown below.
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Q: What should I do if my switch (which has switched ports to the desktop) reports many Spanning Tree Protocol (STP) topology changes, even if I am not having connectivity problems?
STP was developed to manage switch-to-switch links, or bridge-to-bridge links as they were initially called. For most of STP's lifetime, a topology change was a serious event as it indicated a change in the LAN topology. It also indicated a likelihood that STP had to bring (at least) a portion of the LAN down for awhile in order to rebuild a new working topology. Of course, in a LAN we expect the topology to change only rarely.
Over the past few years, the cost of switch ports has dropped dramatically. Users can now afford to dedicate switch ports to end nodes.
Once an STP topology is stable, establishing an Ethernet link on a switch port results in a topology change. This happens every time a user powers up their PC (assuming that the PC is directly connected to a switch port), resets the PC, or brings up the PC's network stack. The establishment of this link causes the topology change count to increment in:
- the switch to which the end node is directly attached; and
- "upstream" switches. That is, the root switch and the switches between the root switch and the switch to which the end node is directly connected.
This scenario does not result in any topology issues or changes. It does not result in any loss of connectivity in the LAN. If the incrementing of your topology change count is due to this type of scenario, you do not need to take any action.
The above discussion applies to Hewlett-Packard switches in general and ProCurve Networking by HP switches in particular when configured in STP Normal mode. When an ProCurve Networking by HP switch's port is configured in STP Fast mode, the switch will not increment the topology change count as a result of link changes on that port. Please see the switch's Management and Configuration Guide for details on normal and fast modes.
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Q: Is 10/100Mbps auto-negotiation the same as Plug-n-Play?
No. The following configuration will cause severe network problems:
The hub, switch, or router will correctly sense (not auto-negotiate) the 10Mbps or 100Mbps speed. Since the end node was configured for a specific speed and duplex state and therefore does not negotiate, the hub, switch, or router will choose the communication mode specified by the 802.3u standard, namely half-duplex.
With one device running at half-duplex and the device on the other end of the connection at full-duplex, the connection will work reasonably well at low levels of traffic. At high levels of traffic the full-duplex device (end node, in this case) will experience an abnormally high level of CRC or alignment errors. The end users usually describe this situation as, "Performance seems to be approximately 1Mbps!" Often, end nodes will drop connections to their servers.
In this same situation, the half-duplex device will experience an abnormally high level of late collisions.
The network administrator must take care to verify the configuration of each network device during installation. Also, check the operational mode of each network device. That is, check both how you configured it and also that it comes up as you expect, for example, at 10Mbps/half-duplex.
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Q: Is Gigabit Ethernet auto-negotiation the same as Plug-n-Play?
No. By the time the IEEE issued the 802.3z specification, they knew about the 10/100Mbps auto-negotiation problem (see the FAQ "Is 10/100Mbps auto-negotiation the same as Plug-n-Play?"). To prevent it, 802.3z auto-negotiation requires that, if one side of a connection is configured to auto-negotiate, the other side must also auto-negotiate if the connection is to come up. In other words, if a switch is configured to auto-negotiate and its attached end node is configured to, say, 1000Mbps/full-duplex, the 803.2z spec requires that the switch NOT allow the link to come up.
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Q: Can I use category 3 cables with the HP ProCurve Switch 100/1000Base-T Transceiver?
No, only category 5 100-ohm UTP or STP cables are supported. In fact, for the most robust connections you should use cabling that complies with the Category 5E specifications, as described in Addendum 5 to the TIA-568-A standard (ANSI/TIA/EIA-568-A-5).
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Q: What is the maximum length for cables used with the HP ProCurve Switch 100/1000Base-T Transceiver?
The maximum length is 100 meters using category 5, 100-ohm UTP or STP cable. This distance is correct for the IEEE 802.3ab specification.
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Q: What are the differences between the HP ProCurve Switch 2512 (product number J4812A) and the HP ProCurve Switch 212m (product number J3298A)?
The HP ProCurve Switch 2512 has 12-fixed 10/100Base-T RJ-45 ports with two transceiver slots supporting Gigabit-SX, Gigabit-LX, 100/1000Base-T, 100-FX SC and Gigabit Stacking transceivers. The HP ProCurve Switch 212m has 12-fixed 10 Mbps RJ-45 ports with one transceiver slot supporting 10/100Base-T or 100Base-FX connectivity and one fixed 10/100Base-T RJ-45 port.
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Q: What are the differences between the HP ProCurve Switch 2524 (product number J4813A) and the HP ProCurve Switch 2424m (product number J4093A)?
The HP ProCurve Switch 2524 has 24-fixed 10/100Base-T RJ-45 ports with two transceiver slots supporting Gigabit-SX, Gigabit-LX, 100/1000Base-T, 100-FX SC and Gigabit Stacking transceivers. The HP ProCurve Switch 2424m has 24-fixed 10/100Base-T RJ-45 ports with one module slot supporting Gigabit-SX, Gigabit-LX, 100/1000Base-T, 100-FX SC and Gigabit Stacking transceivers.
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Q: What are the differences between the HP ProCurve Switch 2324 (product number J4818A) and the HP ProCurve Switch 2224 (product number J4095A)?
The HP ProCurve Switch 2324 is an unmanaged switch that has 24-fixed 10/100Base-T RJ-45 ports with two transceiver slots supporting Gigabit-SX, Gigabit-LX, 100/1000Base-T, 100-FX SC and Gigabit Stacking transceivers. The HP ProCurve Switch 2324 also provides a "download" port for future software upgrades. The HP ProCurve Switch 2224 is an unmanaged switch that has 24-fixed 10/100Base-T RJ-45 ports with one transceiver slot supporting 100Base-FX connectivity.
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Q: What transceiver modules are supported on the ProCurve Networking by HP Series 2300 and 2500 switches? Are the supported transceiver modules "hot-swappable?"
The following transceiver modules are supported for the Series 2300 and 2500 switches:
- ProCurve Networking by HP Gigabit-SX Transceiver (J4131A)
- ProCurve Networking by HP Gigabit-LX Transceiver (J4132A)
- ProCurve Networking by HP 100-FX SC Transceiver (J4853A)
- ProCurve Networking by HP 100/1000-T Transceiver (J4834A)
- The transceivers that are part of the ProCurve Networking by HP Gigabit Stacking Kit (J4116A)
Transceiver modules can be inserted into the switch while it is powered on. However, the switch must be rebooted to activate the transceiver. For more information, refer to chapter 2 of the ProCurve Networking by HP Series 2300 and 2500 Switches Installation and Getting Started Guide.
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Q: How does the HP Auto-MDIX feature work? What products support this feature?
HP Auto-MDIX is a feature on all 10/100Base-TX ports of the ProCurve Networking by HP Series 2300 and 2500 switches. When configured in the default configuration "AUTO", the switch automatically detects the signaling on the cable from the connected device and operates as either an MDI or MDIX port. As a result, a "straight-through" twisted-pair cable can be used; you no longer have to use "crossover" cables, although "crossover" cables can also be used for any of the connections. The following devices support the HP Auto-MDIX feature:
- Fixed 10/100Base-T ports of the Switch 2512 and 2524 (J4812A and J4813A)
- Fixed 10/100Base-T ports of the Switch 2312 and 2324 (J4817A and J4818A)
- 100/1000-T Transceiver module (J4834A)
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Q: By default, VLAN support on the HP ProCurve Series 2500 switches is enabled. Can you disable VLAN support like you can on the HP ProCurve Switch 4000m?
No. VLAN support cannot be disabled on the HP ProCurve Series 2500 switches. By default, all ports are configured in the default VLAN (DEFAULT_VLAN). The following table shows the differences between the HP ProCurve Switch 4000m, 8000m, 1600m, 2424m and 2400m and the HP ProCurve Series 2500 switches with respect to VLAN support:
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