Bridge/Router Internetworking Protocols

There are a number of additional protocols which are generally used for bridge/router internetworking. These protocols are located in the Data Link Layer, and may carry encapsulated protocols in higher layers, e.g., IP, IPX, Ethernet, Token Ring.

The following bridge/router protocols in this section:
BPDU	Bridge Protocol Data Unit
Cisco HDLC (cHDLC)
Cisco SRB	Cisco Source Routing Bridging
Cisco ISL	Inter-Switch Link
DRIP	Cisco Duplicate Ring Protocol
MAPOS	Multiple Access Protocol over SONET/SDH
RND
SSP	Switch-Switch Protocol
Wellfleet SRB	Wellfleet Source Routing Bridging Protocol
Wellfleet BOFL	Wellfleet Breath of Life
The following bridge/router protocols may be found in other locations in the Protocol Directory:
PPP	including PPP Multilink, LCP, LQR, PAP, CHAP, IPCP, IPXCP, ATCP, BAP, BACP, BCP, PPP-BPDU, CCP, IPv6CP, SNACP, BVCP, NBFCP, DNCP, L2F, L2TP, ECP, OSINLCP, PPTP, and SDCP.
Frame Relay
Cascade
Timeplex (BRE2)
SMDS/DXI

The Bridge router protocols is illustrated here in relation to the OSI model:
Click the protocols on the map to see more details.

 

BPDU

Bridge Protocol Data Unit (BPDU) is the IEEE 802.1d MAC Bridge Management protocol which is the standard implementation of STP (Spanning Tree Protocol). It uses the STP algorithm to insure that physical loops in the network topology do not result in logical looping of network traffic. Using one bridge configured as root for reference, the BPDU switches one of two bridges forming a network loop into standby mode, so that only one side of a potential loop passes traffic. By examining frequent 802.1d configuration updates, a bridge in the standby mode can switch automatically into the forward mode if the other bridge forming the loop fails.

The structure of the Configuration BPDU is shown in the following illustration:

	Octets
Protocol identifier	1-2
Protocol version identifier	3
BPDU type	4
Flags	5
Root identifier	6-13
Root path cost	14-17
Bridge identifier	18-25
Port identifier	26-27
Message age	28-29
Max age	30-31
Hello time	32-33
Forward delay	34-35
Configuration BPDU structure

Protocol identifier
Identifies the spanning tree algorithm and protocol.

Protocol version identifier
Identifies the protocol version.

BPDU type
Identifies the BPDU type: 00000000=Configuration, 10000000=Topology change notification. For the later type, no further fields are present.

Flags
Bit 8 is the Topology Change Acknowledgement flag.
Bit 1 is the Topology Change flag.

Root path cost
Unsigned binary number which is a multiple of arbitrary cost units.

Bridge identifier
Unsigned binary number used for priority designation (lesser number denotes the bridge of the higher priority).

Port identifier
Unsigned binary number used as port priority (lesser number denotes higher priority).

Message age, Max age, Hello time, Forward delay
These are 4 timer values encoded in 2 octets. Each represents an unsigned binary number multiplied by a unit of time of 1/256 of a second. Thus times range from 0 to 256 seconds.

BPDU decode

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Cisco HDLC (cHDLC)

The Cisco company produces communications equipment such as routers and bridges which use a proprietary protocol header (known as Cisco HDLC (cHDLC)) to transfer LAN protocols via WAN.

Cisco Router’s default encapsulation on synchronous serial lines uses HDLC framing with packet contents as defined in the following illustration:

Address	Control	Protocol code	Information
1 byte	1 byte	2 bytes	variable
Cisco HDLC (cHDLC) header structure

Address
Specifies the type of packet:

0x0F	Unicast packets.
0x8F	Broadcast packets.

Control
Always set to zero.

Protocol code
Specifies the encapsulated protocol. The Protocol Code is usually Ethernet type codes; however, Cisco has added some codes to support packet types that do not appear in Ethernet.
Standard Ethernet values include:

0x0200	PUP.
0x0600	XNS.
0x0800	IP.
0x0804	Chaos.
0x0806	ARP.
0x0BAD	Vines IP.
0x0BAF	Vines Echo.
0x6003	DECnet phase IV.
0x8019	Apollo domain.
0x8035	Cisco SLARP.
0x8038	DEC bridge spanning tree protocol.
0x809B	Apple EtherTalk.
0x80F3	AppleTalk ARP.
0x8137	Novell IPX.
Cisco-specific values include:
0x0808	Frame Relay ARP.
0x4242	IEEE bridge spanning protocol.
0x6558	Bridged Ethernet/802.3 packet.
0xFEFE	ISO CLNP/ISO ES-IS DSAP/SSAP.

Information
Higher-level protocol data.

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Cisco SRB

Cisco uses a proprietary header in order to pass Token Ring information over WAN lines. This is known as Source Routing Bridging (SRB).

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Cisco ISL

The Inter-Switch Link or ISL is used to inter-connect two VLAN capable Ethernet switches using the Ethernet MAC and Ethernet media. The packets on the ISL link contain a standard Ethernet, FDDI, or Token Ring frame and the VLAN information associated with that frame. Some additional information is also present in the frame.

The format of the header is shown in the following illustration:

		Octets
Destination Address		1-5
Frame Type		6
User Type		7
Source Address		8-14
Length		15-17
SNAP LLC		18-21
HSA		22-25
Virtual LAN ID	BPDU (1)	26-41
Index		42-44
Reserved		45-47
Cisco HDLC (cHDLC) header structure

Destination address
The Destination Address field contains a 5 byte destination address.

Frame type
The Frame Type indicates the type of frame that is encapsulated. In the future this could be used to indicate alternative encapsulations. The following Type codes are defined:

0000	Ethernet
0001	Token Ring
0010	FDDI
0011	ATM

User type

0	Normal priority.
1	Highest priority.

Source address
This field contains the source address of the ISL packet. It should be set to the 802.3 MAC address of the switch port transmitting the frame. It is a 48 bit value.

Length
A 16 bit field containing the length of the packet in bytes, not including the DA, T, U, SA, LEN and CRC fields. The total length of the fields excluded is 18 bytes so the length field is the total length minus 18 bytes.

HSA
The HSA (High bits of source address) field contains the upper 3 bytes of the SA field.

Virtual LAN ID
Virtual LAN ID. This is the virtual LAN ID of the packet. It is a 15 bit value that is used to distinguish frames on different VLANs. This field is often referred to as the color of the packet.

BPDU and CDP indicator

0	Not forwarded to the CPU for processing.
1	Forwarded to the CPU for processing.

Index
The Index field indicates the port index of the source of the packet as it exits the switch. It is used for diagnostic purposes only and may be set to any value by other devices. It is a 16-bit value and ignored in received packets.

Reserved
A reserved field.

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DRiP

Cisco Ios Release 11.3(4)T

The Cisco Duplicate Ring Protocol (DRiP) runs on Cisco routers and switches that support VLAN networking and is used to identify active Token Ring VLANs. A VLAN is a logical group of LAN segments with a common set of requirements. DRiP information is used for all-routes explorer filtering and detecting the configuration of duplicate TrCRFs across routers and switches, which would cause a TrCRF (Token ring Concentrator Relay Function; a logical grouping of ports) to be distributed across ISL trunks. DRiP sends advertisements to a multicast address so the advertisements are received by all neighboring devices. The advertisement includes VLAN information for the source device only. The DRiP database in the router is initialized when TRISL (Cisco’s Token Ring Inter-Switch Link) encapsulation is configured, at least one TrBRF (Token Ring bridge relay function, a logical grouping of TrCRFs) is d efined, and the interface is configured for SRB (source route bridging) or for routing with RIF.

When a switch receives a DRiP advertisement from a router, it compares the information in the advertisement with its local configuration to determine which TrCRFs have active ports and then denies any configuration that would allow a TrCRF that is already active on another box to be configured on the local switch. If there is a conflict between 2 identical TrCRFs, all ports attached to the conflicting TrCRFs are shut down in the switches and the router’s ports remain active. A DRiP advertisement is sent every 30 seconds by the router.

DRiP is assigned the Cisco HDLC protocol type value 0x0102. A Cisco proprietary SNAP value is used. The following fields appear in DRiP frames:

Version
The version number.

Code
The code number.

VLAN info count
The number of VLAN information elements.

VLAN 1... VLAN2...
Various VLAN information elements.

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MAPOS

RFC 2171-6

The MAPOS (Multiple Access Protocol over SONET/SDH) protocol provides multiple access capability over SONET/SDH. It has the scalability of SONET/SDH and also provides a seamless network environment. MAPOS is connectionless, thus well suited for IP traffic. In addition it supports both broadcasts and multicasts. An efficient and simple forwarding mechanism makes it an excellent solution for high-speed networking at all levels: SONET LAN, SONET campus backbone, SONET Internet backbone, and Internet exchange using SONET WAN. MAPOS uses an HDLC-like framing. MAPOS supports a wide range of line rates from 155 Mbps to 10 Gbps, with potential for higher rates in the future.

The fields are transmitted from left to right.

MAPOS frame format

Flag 01111110	Address (16 bits)	Protocol (16 bits)
Information	FCS (16/32 bits)	Flag 01111110
MAPOS frame structure

Flag sequence
Flag sequence is used for frame synchronization. Each frame begins and ends with a flag sequence.

Address
This field contains the destination HDLC address.

Protocol
The protocol field indicates the protocol to which the datagram encapsulated in the information field belongs; for example, 0xFE01 is ARP and 0x0021 is IP.

Information
The information field contains the datagram for the protocol specified in the protocol field.

FCS (frame check sequence)
This is 16 bits long (but may be 32). It is calculated over all bits of the address protocol and information fields.

Note on Interframe fill:
A sending station continuously transmits the flag sequence as Inter-frame fills after the FCS field. The inter-frame flag sequences is silently discarded by the receiving station. When an under-run occurs during DMA in the sending station, it aborts the frame transfer and continuously sends the flag sequence to indicate the error.

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RND

The RND company produces communications equipment such as routers and bridges. The company uses a proprietary protocol header (known as RND) to transfer LAN protocols via WAN.

The structure of the RND header is shown in the following illustration:

1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16
MPCC
Destination bridge ID								Destination bridge entity
Source bridge ID								Source bridge entity
Message broadcast ID								Message broadcast bridge
Cost
Routing flag								Link count
Data length
RND header structure

MPCC
Specifies normal case or swap bytes case.

Destination bridge ID
Specifies the type of message:

0xF4	IPX router message.
0xF4	DECnet router message.
0xF7	IP router message.
0xF8	TRE management message.
0xF9	ETE management message.
0xFB	Routing message bridge ID.
0xFC	This bridge entity.
0xFD	Channel status message.
0xFE	Common LAN bridge ID.
0xFF	Broadcast bridge ID.

Destination bridge entity
Value of the entity:

0x0F	LAN broadcast entity.
0x64	Smap entity.
0x65	Reml entity.
0x6F	C5 reml entity.
0x79	RS232 rem entity.

Source bridge ID
Refer to destination bridge ID above.

Source bridge entity
Refer to destination bridge entity above.

Message broadcast ID

0	Point-to-point.

Message broadcast bridge

0	Point-to-point.

Cost
Accumulated cost.

Routing flag
Routing attributes.

Link count
Count of router hops.

Data length
Length of the data in bytes (swapped).

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Wellfleet SRB

Wellfleet, which is known today as Bay Networks, is a manufacturer of routers and bridges. They use a proprietary header in order to pass Token Ring information over WAN lines. This is known as Source Routing Bridging (SRB).