Explanation/Reference:
Explanation:
Common Causes of Slow IntraVLAN and InterVLAN Connectivity The symptoms of slow connectivity on a
VLAN can be caused by multiple factors on different network layers. Commonly the network speed issue
may be occurring on a lower level, but symptoms can be observed on a higher level as the problem masks
itself under the term "slow VLAN". To clarify, this document defines the following new terms: "slow collision
domain", "slow broadcast domain" (in other words, slow VLAN), and "slow interVLAN forwarding". These
are defined in the section Three Categories of Causes, below.
In the following scenario (illustrated in the network diagram below), there is a Layer 3 (L3) switch
performing interVLAN routing between the server and client VLANs. In this failure scenario, one server is
connected to a switch, and the port duplex mode is configured half-duplex on the server side and full-
duplex on the switch side. This misconfiguration results in a packet loss and slowness, with increased
packet loss when higher traffic rates occur on the link where the server is connected. For the clients who
communicate with this server, the problem looks like slow interVLAN forwarding because they do not have
a problem communicating to other devices or clients on the same VLAN. The problem occurs only when
communicating to the server on a different VLAN.Thus, the problem occurred on a single collision domain,
but is seen as slow interVLAN forwarding.

Three Categories of Causes
The causes of slowness can be divided into three categories, as follows:
Slow Collision Domain Connectivity
Collision domain is defined as connected devices configured in a half-duplex port configuration, connected
to each other or a hub. If a device is connected to a switch port and full-duplex mode is configured, such a
point-to-point connection is collisionless. Slowness on such a segment still can occur for different reasons.
Slow Broadcast Domain Connectivity (Slow VLAN)
Slow broadcast domain connectivity occurs when the whole VLAN (that is, all devices on the same VLAN)
experiences slowness.
Slow InterVLAN Connectivity (Slow Forwarding Between VLANs) Slow interVLAN connectivity (slow
forwarding between VLANs) occurs when there is no slowness on the local VLAN, but traffic needs to be
forwarded to an alternate VLAN, and it is not forwarded at the expected rate.
Causes for Network Slowness
Packet Loss
In most cases, a network is considered slow when higher-layer protocols (applications) require extended
time to complete an operation that typically runs faster. That slowness is caused by the loss of some
packets on the network, which causes higher-level protocols like TCP or applications to time out and
initiate retransmission.
Hardware Forwarding Issues
With another type of slowness, caused by network equipment, forwarding (whether Layer 2 [L2] or L3) is
performed slowly. This is due to a deviation from normal (designed) operation and switching to slow path
forwarding. An example of this is when Multilayer Switching (MLS) on the switch forwards L3 packets
between VLANs in the hardware, but due to misconfiguration, MLS is not functioning properly and
forwarding is done by the router in the software (which drops the interVLAN forwarding rate significantly).