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This appendix provides information about the formats of the following types of packets, frames, and cells:
Figure A-1 shows the format of a Token Ring frame.
The Starting Delimiter field indicates the arrival of a frame or token. This field includes bits that are set to intentionally violate the Differential Manchester Code to distinguish this field as a delimiter.
The Access Control field contains the following bits:
The Frame Control field indicates the frame type and contains the following:
The Destination Address field indicates the address of the device or devices for which the frame is intended. The destination address can be one of the following:
The Source Address field identifies the station that sent the frame. In the source address the first bit (bit 0) is called the routing information indicator (RII) bit. When this bit is set to one it indicates that the frame contains routing information. If the bit is set to zero then no routing information is included.
Used only in SRB, the Routing Information field indicates the route the frame is to take through the network. The routing information field consists of the following:
The Information field contains the data that is being sent to upper layers.
The Frame Check Sequence field contains the cyclic redundancy check (CRC) value for all bits from the Frame Control field through the Frame Check Sequence field. The Frame Check Sequence value is checked by a receiving station to determine if errors occurred in transmission.
The Ending Delimiter field indicates the end of the frame or token. It also contains bits to indicate if a frame is damaged or if the frame is the last in a logical sequence.
The Frame Status field indicates to the transmitting station whether this frame has been copied by the destination station.
The CDP packet contains information about the Cisco devices in the network. It consists of a header, followed by a set of variable-length fields consisting of type/length/value triplets.
Figure A-2 shows the format of a CDP packet.
The Version field indicates the version of CDP being used. The value is always 0x01.
The Time-to-Live field indicates the amount of time, in seconds, that a receiver should retain the information contained in this packet.
The Checksum field indicates the standard IP checksum.
The Type field indicates the type/length/value type. The possible CDP type/length/value types are as follows:
The Length field indicates the total length, in bytes, of the type, length, and value fields.
The Value field contains the type/length/value value, which depends on the type/length/value type as described below:
| Field | Description |
|---|---|
Protocol | Protocol type. It can be one of the following values:
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Length | Length of the protocol field. For protocol type 1, the length is 1. For protocol type 2, the length is either 3 or 8, depending on whether SNAP is used. |
Protocol | One of the following values:
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Address length | Length of the address field in bytes. |
Address | Address of the interface, or the address of the system if addresses are not assigned to the interface. |
| Bit | Description |
|---|---|
0x01 | Performs level 3 routing for at least one network layer protocol. |
0x02 | Performs level 2 transparent bridging. |
0x04 | Performs level 2 source-route bridging. A source-route bridge would set both this bit and bit 0x02. |
0x08 | Performs level 2 switching. The difference between this bit and bit 0x02 is that a switch does not run the Spanning-Tree Protocol. This device is assumed to be deployed in a physical loop-free topology. |
0x10 | Sends and receives packets for at least one network layer protocol. If the device is routing the protocol, this bit should not be set. |
0x20 | The bridge or switch does not forward IGMP Report packets on nonrouter ports. |
0x40 | Provides level 1 functionality. |
The DRiP frame contains information about the VLANs configured in the management domain. It consists of some header information followed by one or more VLAN information fields.
Figure A-4 shows the format of a DRiP frame.
The Version field identifies the version of DRiP being used.
The Code field indicates whether this message is an advertisement that indicates a change (0x01) or no change (0xFF).
The VLAN Information Count field indicates the number of VLAN information fields contained in this advertisement.
The Header Length field indicates the size in bytes of the header for this type of advertisement. The header includes all fields from the version up to the first VLAN information field.
The Configuration Revision Number field indicates the revision number of the configuration information. A configuration revision number starts at zero and increments by one with each modification until it reaches the value 4294947295, at which point it wraps back to zero and starts incrementing again.
The Last Changed Revision field indicates the revision number of the last change associated with the originating MAC address. Switches in the domain compare the value in this field to their current configuration number to determine whether the advertisement contains new information.
The MAC Address field contains the MAC address, in canonical format, of the device that is sending the DRiP advertisement.
The VLAN Information fields contain information for each active or configured TrCRF on the switch. A TrCRF is considered active if a port associated with the TrCRF is open on the ring. If a TrCRF ceases to be included in the VLAN information field, it indicates there are no longer ports active or configured on the TrCRF. The TrCRF should then be removed from the database.
If a periodic timer triggers an advertisement, regardless of whether there has been configuration revision change, all VLAN information for the device is included.
Figure A-5 shows the format of the VLAN information fields.
The Length field indicates the length, in bytes, of the VLAN information field (including this length field). This length will be a multiple of 4.
The Status field indicates that the status of the TrCRF has changed. The TrCRF now either has an active port or the last active port that was on the TrCRF has become inactive, leaving the TrCRF with no active ports. Possible values are the following:
If the message is triggered by a periodic timer, then the status will indicate the current status of the VLAN.
The ISL VLAN ID indicates the VLAN ID of this VLAN on ISL trunks. Possible values are 0 through 1023.
There are three types of VTP frames: Advert-Request, Summary-Advert, and Subset-Advert.
An Advertisement Request (Advert-Request) is a request for configuration information. Figure A-6 shows the format of an Advert-Request frame.
The Version field indicates the VTP version number. This value is always 0x01.
The Code field indicates the message type. Possible values are:
The Management Domain Length field indicates the length of the name of the management domain.
The Start Value field indicates the VLAN ID of the first VLAN for which information is requested. Any response to the request should contain information for all VLANs having an ISL VLAN ID greater than or equal to this value. For example, in a request for information on all VLANs, this value is 0.
The Summary Advertisement (Summary-Advert) contains information about the sending device and summary information about the advertisement, including the number of subset advertisements to follow. The maximum size of a Summary-Advert is 1492 bytes. Figure A-7 shows the format of a Summary-Advert frame.
The Version field indicates the VTP version number. This value is always 0x01.
The Code field indicates the message type. Possible values are:
The Followers field indicates the number of Subset-Advert messages that follow this Summary-Advert.
The Management Domain Length field indicates the length of the name of the management domain.
The Management Domain Name field indicates the name of the management domain.
The Configuration Revision Number field indicates the revision number of the configuration information. As with CDP configuration revision numbers, a configuration revision number starts at zero and increments by one with each modification until it reaches the value 4294947295, at which point it wraps back to zero and starts incrementing again.
The Updater Identity field indicates the IP address of the device that received the command that caused the configuration revision number to have its current value.
The Update Timestamp field indicates the time at which the configuration revision number was most increased to its current value. The timestamp is in the format "yymmddhhmmss", where yymmdd represents the year, month, and day and hhmmss represents the hours, minutes, and seconds.
MD5 digest value over the secret value and all VLAN information
The Subset Advertisement (Subset-Advert) contains information about the VLANs being advertised. Figure A-8 shows the format of a Subset-Advert frame.
The Version field indicates the VTP version number. This value is always 0x01.
The Code field indicates the message type. Possible values are:
The Sequence Number field indicates the order of this Subset-Advert frame within the series of Subset-Advert frames that follow a Summary-Advert. For the first Subset-Advert frame following a Summary-Advert frame the sequence number is 1.
The Management Domain Length field indicates the length of the name of the management domain.
The Management Domain Name field indicates the name of the management domain.
The Configuration Revision field indicates the revision number of the configuration information. As with CDP configuration revision numbers, a configuration revision number starts at zero and increments by one with each modification until it reaches the value 4294947295, at which point it wraps back to zero and starts incrementing again.
Each VLAN Information field contains information for a different VLAN, starting with the VLAN with the lowest ISL VLAN IDs. Figure A-9 shows the format of the VLAN information field.
The VLAN Information Length field indicates the length, in bytes, of the VLAN information field for this VLAN in this advertisement. The length is a multiple of 4.
The Status field indicates the status of this VLAN. Possible values are:
The VLAN Type field indicates the type of VLAN. Possible values are:
The VLAN Name Length field indicates the length, in bytes, of the VLAN name for this VLAN.
The ISL VLAN ID field indicates the ID of this VLAN on ISL trunks. Possible values are 0 through 1023.
The MTU Size field indicates the maximum transmission unit (MTU) for this VLAN. Possible values are 1500 through 18190.
The 802.10 Index field indicates the 802.10 security association identifier (SAID) value for this VLAN.
The VLAN Name field indicates the VLAN name for this VLAN. The name can be between 1 and 32 bytes in length and is padded with zeros.
The VLAN type/length/value fields are variable in length and contain the a type, length, and value. Figure A-10 shows the format of the VTP variable-length fields.
| Field | Description |
|---|---|
Type | The Type field indicates the type/length/value type. Possible values are:
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Length | The Length field indicates the length of this VLAN type/length/value. |
Value | The Value field contains the type/length/value value, which depends on the type/length/value type as described below:
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The format of a Spanning-Tree Protocol BPDU frame varies depending on the type of protocol used.
Figure A-11 shows the format of an IEEE 802.1d Spanning-Tree Protocol BPDU frame.
Figure A-12 shows the format of an IBM Spanning-Tree Protocol BPDU frame.
Figure A-13 shows the format of a Cisco Spanning-Tree Protocol BPDU frame.
The Frame Control field is always 01.
The Destination Address field indicates the destination address as specified in the Bridge Group Address table. For IEEE Spanning-Tree Protocol BPDU frames, the address is 0x800143000000. For IBM Spanning-Tree Protocol BPDU frames, the address is 0xC00000000100. For Cisco Spanning-Tree Protocol BPDU frames, the address is 0x800778020200.
The Source Address field indicates the base MAC address used by the switch. For Cisco Spanning-Tree Protocol BPDU frames, the multicast bit is set to indicate the presence of a RIF in the header.
Applicable only to Cisco Spanning-Tree Protocol BPDU frames, the Routing Information field must be set to 0x0200.
For all three types of Spanning-Tree Protocol BPDU frames, this field is set to 0x424203.
Figure A-14 shows the format of the fields inside a BPDU.
The Protocol Identifier Field indicates the type of protocol. This field contains the value zero.
The Version field indicates the version of the protocol. This field contains the value zero.
The Message Type field indicates the type of message. This field contains the value zero.
The Flags field includes one of the following:
The Root ID field indicates the root bridge by listing its 2-byte priority followed by its 6-byte ID.
The Root Path Cost field indicates the cost of the path from the bridge sending the configuration message to the root bridge.
The Bridge ID field indicates the priority and ID of the bridge sending the message.
The Port ID field indicates the port number (IEEE or Cisco Spanning-Tree Protocol BPDU) or the ring and bridge number (IBM Spanning-Tree Protocol BPDU) from which the configuration message was sent. This field allows loops created by multiple attached bridges to be detected and corrected.
The Message Age field indicates the amount of time that has elapsed since the root sent the configuration message on which the current configuration message is based.
The Maximum Age field indicates when the current configuration message should be deleted.
The Hello Time field indicates the time between root bridge configuration messages.
The Forward Delay field indicates the length of time that bridges should wait before transitioning to a new state after a topology change. If a bridge transitions too soon, it is possible that not all network links will be ready to change their state and loops can result.
To support Token Ring, another ISL frame format was developed. In addition to the fields found in the original ISL frame format, the ISL Token Ring Frame format includes:
Figure A-15 shows the format of an ISL Token Ring frame.
The Destination Address field is a 40-bit multicast address and is set to 0x01000C0000
The Type field indicates the type of frame that is encapsulated. For Token Ring frames, this field is set to 0001.
The User field extends the meaning of the Type field. For example, Token Ring frames may have more than one type. The default User field value is 0000.
The Source Address field indicates the 802.3 MAC address of the MAC transmitting the frame.
The Length field indicates the length, in bytes, of the frame excluding the Destination Address, Type, User, Source Address, Length, and CRC fields.
The SNAP LLC of the frame. For ISL frames this field is set to AAAA03.
The HSA (high bits of source address) field indicates the upper 3 bytes, which identifies the manufacturer, of the Source Address field.
The Destination VLAN field indicates the ID of VLAN for which the packet is destined. This value is used to distinguish frames on different VLANs. This field is often referred to as the color of the packet.
The BPDU field indicates whether the encapsulated frame is a BPDU. This field is also used to indicate whether the encapsulated frame is a CDP or VTP frame. All frames received with this field set are forwarded to the CPU for processing.
The Index field indicates the port index of the source of the frame as it comes out from the Catalyst switch. It is used for diagnostic purposes only and may be set to any value by other devices.
The Source VLAN field indicates the ID of VLAN from which the packet was sent.
The Explorer field indicates whether the encapsulated frame is a data frame or and explorer (ARE or STE) frame.
The Destination Route Descriptor field indicates the route descriptor to be used for forwarding. If there is no route descriptor following the routing information field match in the routing information field or if there is no routing information field present in the frame, this field is set to 0 and the destination address is used for forwarding.
The Source Route Descriptor field indicates the route descriptor to be used for source learning. If there is no route descriptor prior to the ring-in in the routing information field or if there is no routing information field present in the frame, this field is set to 0 and the source address is used for source learning.
The FCS Not Included field indicates whether the Frame Check Sequence field is included in the Encapsulated Frame field.
The E Size field indicates the frame size for frames less than 64 bytes. This field is use to account for the case where a frame crosses a router and is padded to 64 bytes (minimum Ethernet frame).
The actual Token Ring frame. For more information on the format of the Token Ring frame, see the "Token Ring Frame Format" section.
The CRC field is the frame checksum. This CRC is in addition to the one at the end of the Encapsulate Frame field. It contains a standard 32-bit CRC value calculated on the entire encapsulated frame from the Destination Address field to the Encapsulated Frame field. The receiving device checks this CRC and may discard packets that do not have a valid CRC on them.
The ATM cell is a fixed-length, standard unit of data transmission for all cell relay services in an ATM network. The first five bytes of the ATM cell serve as the cell header. The cell header contains information essential to routing the cell through the network and ensuring that the cell reaches its destination. Figure A-16 shows the format of an ATM cell.
The Generic Flow Control field is used when passing ATM traffic through a user-to-network (UNI) interface to alleviate short-term overload conditions. A network-to-network (NNI) interface does not use this field for generic flow control purposes; rather, an NNI uses this field to define a larger VPI value for trunking purposes.
The VPI field identifies the virtual paths. In an idle or null cell, the VPI field is set to all zeros. (A cell containing no information in the payload field is either idle or null). VPIs provide a way to bundle ATM traffic being sent to the same destination.
In an ATM UNI header, part of the VPI field (bits 5 through 8 of byte 1) is reserved as a generic flow control field. However, the ATM NNI header provides a larger range of VPI values (using bits 5 through 8 of byte 2, in addition to bits 1 through 4 of byte 1). This larger range of VPI values that can be defined in an ATM NNI cell header reflects the greater use of virtual paths in the network for trunking purposes between ATM inter-switch and ATM inter-network interfaces.
The VCI field identifies a particular VCC. In an idle or null cell (one containing no payload information), the VCI field is set to all zeros. Other non-zero values in this field are reserved for special purposes. For example, the combined values of VPI = 0 and VCI = 5 are used exclusively for ATM signaling purposes when requesting an ATM connection.
The Payload Type Identifier indicates the type of payload the cell contains: either user data or special network management data used to perform certain network operation, administration, and maintenance functions in the network.
The Cell Loss Priority field is set by the AAL to indicate the relative importance of a cell. This field is set to 1 to indicate that a cell can be discarded, if necessary, such as when an ATM switch is experiencing traffic congestion. This field is set to 1 to indicate that the cell should not be discarded, such as when supporting a specified or guaranteed quality of service. This field may also be set by the ATM layer if an ATM connection exceeds the QOS parameters established during connection setup.
The Header Error Check field is an 8-bit CRC computed on all fields in an ATM UNI/NNI cell header. The header error check is capable of detecting all single-bit errors and certain multiple-bit errors. This field provides protection against incorrect message delivery caused by addressing errors. However, it provides no error protection for the ATM cell payload proper. The physical layer uses this field for cell delineation functions during data transport.
Posted: Wed Oct 2 03:34:38 PDT 2002
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