VNswitch Router Packet Counters

Introduction

This document provides an overview of the VNswitch counters and the effect of packets on counters as packets flow through the router.

Packet Counter Overview

The VNswitch contains packet counters that allow you to observe the amount and types of traffic being processed. The counters keep track of sent and received traffic, in categories that indicate how many packets have reached various outcomes (terminated, dropped, bridged, routed, flooded, fragmented, and so on).

Packet counters exist at four internal layers to help you trace packets as they flow within the VNswitch:

  • Logical interfaces
  • Bridge ports
  • VLAN interfaces
  • IP Router

When a packet is received by an entity within a layer, the packet is either dropped, processed, or passed on to one or more entities within the next layer. Dropped packets cause an error counter within that layer to increment. In some situations, such as bridge ports and interfaces, entities are tightly coupled and dropped packets can increment error counters in two layers. The effects of the packets are not seen in counters at any layers it does not reach. Packets that are successfully processed at a layer increment non-error counters within that layer. Packets sent to the next layer increment non-error counters in that subsequent layer as well.

Figure 1 illustrates the four layers and shows the relationships between logical interfaces, bridge ports, VIs, and the IP router. The figure shows which counters are incremented for a typical path a packet can take within a VNswitch.

Figure 1: Packet Flow

Packet Flow

Packets arriving at the VNswitch enter the physical interface and can then travel through each of the four layers. Physical interfaces are the connection jacks for cables, and have a one-to-one or one-to-many (in the case of an ATM physical interface) relationship with interfaces. Logical interfaces, shown as circles in Figure 1, are the lowest layer where counters are used. All packets received and sent are counted by logical interface counters. Error counters for interfaces can catch some basic types of errors appropriate to the level of decoding the packet has undergone at this point (for example, a bad FCS) or other errors that are not necessarily associated with a specific higher-level protocol (for example, buffer overflow). If such an error is detected on an interface, the packet being sent or received is discarded, and the appropriate interface error counter is incremented. Otherwise, it is passed to a bridge port (where bridging runs on all interfaces).

Packets arriving at a bridge port (dark triangle in Figure 1) are first subject to the effects of bridging. They may be dropped for numerous reasons (destination address filtering, STP port state, and so on), each causing a single bridge error or a dropped packet counter to increment for that port. If a packet is not dropped, its destination address determines whether it is unicast to another port, flooded out all ports, terminated, and/or delivered to routing. If the packet is bridged out other ports, the bridge attempts to translate and enqueue the packet for sending, if necessary. A failure in this process causes a packet to be dropped and the error counter to be incremented for the received port. A success means that the packet is sent out other ports and counted by them as well. If a received packet is not dropped or sent out by bridging, it is terminated (such as an STP BPDU) and/or submitted to routing.

VLAN interfaces (VIs) receive all packets destined to routing. VIs are paired one-to-one with VLANs, which are groups of bridge ports. VIs submit packets for routing on behalf of any ports within their VLAN. VI receive counters keep track of the total number of packets submitted to routing from their VLAN. Outbound packets sent by routing also go through a VI for transmission on a VLAN. VI transmit counters increment once for each packet sent by routing, although multiple packets may be sent on one or more ports (whose counters are incremented as well). Packets sent or received on VIs cannot be dropped by the VI. All errors, overflows, and so on, are detected and counted in other layers.

Packets reaching the router may be terminated and are counted by routing. The VNswitch IP counters count transmitted, received, and error packets across all VIs and do not display this information on a per-VI basis.

 

Router Packet Overview

The VNswitch V3.0 offers two primary services: bridging and routing. Routing is layered on top of bridging and packets destined for routing are subject to the effects of bridging. For example, routed packets can be dropped due to a user-defined protocol filter on a bridge port and these packets are counted as being received by bridging, but not by routing.

In addition to the packets sent to the router, the router can generate packets to send out. For example, the router is capable of generating a Ping request for another node on the network. In this situation, the Ping packets are counted by IP counters, VI transmit counters, interface transmit counters, and bridge port transmit counters.

Packets destined to the router are terminated by the router. You can filter packets by enabling a filter to suppress packets destined to a network or subnet, or by disabling a protocol type. In this situation, only the interface and bridge port receive packet counters are affected. 

 

Supported Counters

 

This section provides a brief description of interface counters, bridge port counters and the relationship between both counter types.

For a complete definition of interface counters and bridge port counters, refer to the DIGITAL VNswitch 900 Series Switch Management guide.

Interface Counters

The following table describes each interface counter (IC) associated with the VNswitch. Interface counters IC1A, IC2A, IC3A, and IC4A are identical to IC1, IC2, IC3, and IC4, except the A counters represent VI counters.

Interface Counter Number Interface Counter Name
IC1, IC1A Unicast packets received
IC2, IC2A Multicast packets received
IC3, IC3A Unicast packets transmitted
IC4, IC4A Multicast packets transmitted
IC5 Input overflow drops
IC6 Input error drops
IC7 Input unknown protocol drops
IC8 Input congestion control drops
IC9 Output overflow drops
IC10 Output error drops

Bridge Port Counters

The following table describes each VNswitch bridge port counter (BC):

Counter Number Counter Name
BC1 Port restarts
BC2 Total frames received by interface
BC3 IP frames fragmented
BC4 IP frames not fragmented
BC5 Frames submitted to bridging
BC6 Frames submitted to routing
BC7 Frames with unknown destination address
BC8 Frames causing learning transactions
BC9 Source address filter drops
BC10 Destination address filter drops
BC11 Protocol filter drops
BC12 Address rate limiting drops
BC13 Protocol rate limiting drops
BC14 Input buffer overflow drops
BC15 Input queue overflow drops
BC16 Source or destination port blocked drops
BC17 Terminating queue overflows
BC18 Fragmentation queue overflows
BC19 Translate flood queue overflows
BC20 Translation failures
BC21 Frames sent by bridging
BC22 Transmit queue overflows
BC23 Transmit errors
BC24 Too big to send on port drops

Counter Relationships

Some simple relationships exist between interface counters, bridge port counters, and VI counters. For a given packet in Figure 1, the following relationships exist. Refer to the Interface Counters and Bridge Port Counters sections for a complete list of the counters.

Receive Relationships

Example 1

The sum of unicast and multicast packets received on an interface is the total of all received packets. It is expressed as:

IC1 + IC2 = BC2
(Unicast packets received + Multicast packets received = Total frames received by the interface)

Example 2

Some packets received on an interface may be dropped before being submitted to bridging. It is expressed as:

BC2 ³ BC 5
(Total frames received by interface ³ Frames submitted to bridging.)

Example 3

Some packets submitted to bridging will be submitted to routing. The rest are either dropped or bridged. It is expressed as:

BC5 ³ BC6
(Frames submitted to bridging ³ Frames submitted to routing)

Example 4

The packets submitted to routing by a port’s VI may represent only a portion of the packets received by that VI since the VI’s VLAN may contain other ports. It is expressed as:

BC6 £ IC1A + IC2A
(Frames submitted to routing £ Unicast packets received + Multicast packets received)

 

Transmit Relationships

Example 1

Some packets sent by bridging may be dropped at the interface layer. It is expressed as:

IC3 + IC4 £ BC20
(Unicast packets transmitted + Multicast packets transmitted £ Translation failures)