If you try, for example, to pump 4-gigabit traffic though the switch with a 3-gigabit backplane, you end up with a dramatic loss of the traffic. Understanding network switch architecture is out of the scope of this document; however, when considering the capacity of a network switch, pay attention to the following aspects:. Congestion on the switch inband path can result in a spanning tree loop or other types of instability on the network.
The inband port on any Cisco switch is a virtual port that provides interface for management traffic traffic such as Cisco Discovery Protocol and Port Aggregation Protocol [PAgP] to the management processor. The inband port is considered virtual because, in some architectures, the user cannot see it, and the inband functions are combined with the normal port operation. If the inband port gets overloaded due to a misconfigured application or user traffic , it may result in instability of any protocols for which the protocol state stability is based on regular messages or "hellos" received.
This state can result in temporary loops, interfaces flapping, and other issues, causing this type of slowness. It is difficult to cause congestion of the inband port on the switch, though maliciously formed denial of service DoS attacks may succeed. There is no way to rate-limit or reduce traffic on the inband port. A solution requires switch administrator intervention and investigation. Inband ports generally have a high tolerance for congestion.
Rarely does the inband port malfunction or get stuck in the Rx or Tx direction. This would mean severe hardware outage and would affect the whole switch. This condition is difficult to recognize and is usually diagnosed by Cisco Technical Support engineers. The symptoms are that a switch suddenly becomes "deaf" and stops seeing control traffic such as Cisco Discovery Protocol neighbor updates. This indicates an Rx inband problem. If, however, just one Cisco Discovery Protocol neighbor is seen, you can be confident that inband is working.
Correspondingly, if all the connected switches lose Cisco Discovery Protocol from a single switch as well as all other management protocols , it indicates Tx problems from the inband interface of that switch. If an inband path is overloaded, it can cause a switch to experience high CPU conditions; and, as the CPU processes all that unnecessary traffic, the situation worsens.
If high CPU utilization is caused by an overloaded inband path or an alternate issue, it can affect management protocols as described in the Congestion on Switch Inband Path section, above. In general, consider the management CPU to be a vulnerable point of any switch. A correctly configured switch reduces the risk of problems caused by high CPU utilization. Keep in mind the following:. If an inband port is overloaded, it causes any new path to be dropped.
This results in packet lost silent discard and slowness in higher-layer protocols when traffic is switched between ports. Refer to the section Congestion on Switch Inband Path , above. Also, the MAC address learning process even if it is completely implemented in hardware is a relatively slow process, compared to a switching process.
If there is a continuously high rate of MAC address relearning, then the cause must be found and eliminated. A spanning tree loop on the network can cause this type of MAC address relearning. In turn, this may lead to a high rate of MAC address relearning and may degrade performance. Cut-through ingress error packet propagation is related to Slow Collision Domain Connectivity , but because the error packets are transferred to another segment, the problem appears to be switching between segments.
So, while the packet is being switched between ingress and egress ports, the beginning of the packet is already forwarded out of the egress port, while the rest of the packet is still being received by the ingress port.
What happens if the ingress segment is not healthy and generates a cyclic redundancy check CRC error or a runt? The switch recognizes this only when it receives the end of the frame and, by that time, most of the frame is transferred out of the egress port. Since it makes no sense to transfer the rest of the erroneous frame, the rest is dropped, the egress port increments the "underrun" error, and the ingress port increments the corresponding error counter.
If multiple ingress ports are unhealthy and their server resides on the egress port, it appears that the server segment is having the problem, even though it is not. For cut-through L3 switches, watch for underrruns and, when you see them, check all ingress ports for errors. Misconfiguration may cause a VLAN to be slow. These negative effects may result from a VLAN being oversubscribed or overloaded, but most often, they result from a bad design or overlooked configurations.
For example, a segment VLAN can be easily overwhelmed by multicast traffic for example, video or audio stream if multicast traffic constraining techniques are not properly configured on that VLAN. Such multicast traffic may affect data transfer, causing packet loss on an entire VLAN for all the users and flooding the segments of users who did not intend to receive the multicast streams. Software bugs and hardware problems are difficult to identify because they cause deviation, which is hard to troubleshoot.
If you believe that the problem is caused by a software bug or hardware problem, contact the Cisco Technical Support engineers to have them investigate the problem.
Most of the time, slow interVLAN connectivity is caused by user misconfiguration. To avoid misconfiguration and to troubleshoot efficiently when necessary, you should understand the mechanism used by your L3 forwarding device. The security of your privacy is the top priority.
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