“Internetworking”

• Bridges– Transparent bridges– Source Routing - Transparent Bridges

• Routers (Network Layer)

• Brouters 1 1

2

3

2 2

1 1

Why Bridges

• Isolation of Physical Layer Effects

• Bandwidth Multiplication

• Security or Traffic Isolation

Segmenting Traffic

Workstations

File server

Terminals

Terminal server

LAN traffic

LAN traffic

Host

Bridge

Transparent Bridges• Interconnect multiple cable segments to allow for extension of a network.

• Can be used to interconnect different access methods (Ethernet to Token Ring) and different physical layers.

• Operate at the data link layer.• They are protocol transparent.

–They are designed to operate regardless of the upper-layer protocol.

–They operate on the source and destination address in the MAC header.

T-L-F Bridges

• Bridges only forward traffic destined for other cable segments.

• They operate transparently to any stations that are active on the network.

• Packet formats and software drivers on the workstations remain the same.

• Bridges do not have to be programmed with the addresses of all the devices on the network.

Learning, Filtering, and Forwarding

Terminals

Node A Node B

Node C

Node D

Cable segment 2

Cable segment 1

Node E

Forwarding table

Node C, D and F are on this cable segment through port 2.

Nodes A, B, and E are on thiscable segment though port 1.

Port 2

Port 1

Node F

Bridge

Filtering - An Example

A 1B 1

C 2 D 2

Node A Node B

Node C Node D

Fowarding Table

Filtered

Cable segment 1

Cable segment 2

Port ID 1

Port ID 2

Packet transmitted

Forwarding - An Example

Node A Node B

Node C Node D

Forwarded

Cable segment 1

Cable segment 2

Forwarding tableA 1B 1

C 2 D 2

Port ID 1

Port ID 2

Forwarding Beyond One Bridge

Node A

Node D

Node B

Node C

Cable segment Z

Cable segment Y

Cable segment X

Cable segment V

A B

A B

A B

C D

C D

C D

Bridge table

Bridge 3

Bridge 2

Bridge 1

Loops• Complexity of bridging arises when two or more bridges interconnect the same two cable segments.

• This is called providing redundancy or providing a loop.

• There are problems with this type of design including:

–duplicate packets,

–broadcast packets, and

–unknown destination packets.

Duplicate PacketsNode A Node B

Node C Node D

Single packet transmitted

Two packets received

Cable segment 1

Cable segment 2

Bridge 2Bridge 1

Broadcasts

Node A Node B

Node C Node D

Broadcast packet transmitted

Cable segment 1

Cable segment 2

Packet received andtransmitted back by

second bridge

Loop

Bridge 2Bridge 1

Unknown Destination AddressNode A Node B

Node C Node D

Destination Z packet transmitted

Cable segment 1

Cable segment 2

Packet received andtransmitted back by

second bridge

Loop

Bridge 2Bridge 1

Spanning Tree Algorithm• Bridged networks must allow for redundancy. Only one path should be enabled to any destination on the network.

• STA is a protocol unto itself. Don’t confuse it with the transparent bridge protocol. IEEE 802.1d

• In an active STA topology certain bridges are allowed to forward packets.

–Other bridges will participate in the STA but do not forward packets.

–These are backup bridges that dynamically become available.

• Bridges that do not forward packets are placed in blocking mode.

–These bridges still participate in the spanning tree protocol.

Source Routing Bridges• Developed as a bridge protocol for Token Ring LANs.

• Source routing gained popularity due to IBM’s support of it.

–It is easy to install a source route network.–It is not easy to grow a source route network into a large network.

• Invented due to technical limitations of the source route chip set.. Early source route chip sets could not be set for promiscuous mode.

• Source routing was also invented to allow two non-routing protocols to be placed on a LAN: NetBIOS and SNA.

• Source Routing does not build forwarding tables based on MAC addresses.

• Most of the intelligence for this algorithm is found in the network stations.

• Each frame carries complete route information with it.

Source Routing Features

• Source routing requires split intelligence to be carried in the node and the bridge.

• All frames contain routing information, which does produce more overhead.

• Uses STA to configure which bridges will forward single route broadcast frames.

• All paths are active which legally allows loops to be designed.

• Provided a routing solution for those protocols that could not be routed (NetBIOS).

• Easy to follow ring/MAC address for troubleshooting.

Source Routing Features (cont.)

• Source Routing originated as an alternative to transparent bridging

• Originally, Token Ring could not be placed in promiscuous mode ( requirement for transparent bridging) and therefore an alternative model was created

• Allowed for SNA and NetBIOS traffic an attempt to enjoy the benefits of routing

–As a data link layer implementation.

Source Routing Overview• Each separate ring is assigned a unique ring number, assigned on the source

route bridge port and not on the ring station.

• Each bridge is assigned a bridge number. There is a single number for the whole bridge, no matter how many ports it has.

• End stations try to find destination ring stations by broadcasting special discovery frames.

• A frame will contain source route information based on one bit in the source address.

• A source route frame may not cross more than seven bridges.

–At the eighth bridge, the frame is discarded.

Source Routing Example

Ring 4 Ring 3

Node 1 Node 2

MAU

Find a station on

the local ring

Find a station off

ring

1

2

MAU

Bridge 5

Bridge 6

Bridge 7

Routing Information Field

Starting Delimiter

AccessControl

Frame Control

Destination Address

Source Address

OptionalRouting

Information Field

Destination ServiceAccess Protocol

(DSAP)

Source ServiceAccess Protocol

(SSAP)

Rest of TokenRing frame

RoutingControl

RouteDesignator

RouteDesignator . . . . . .

B B B L L L L L D F F F r r r r Ring number Bridgenumber

12 bits 4 bits1 - F

bridge IDs

2 bytes Up to 8 RD fields

Routing Information Indicator (RII)

1 - 4095

rings

The Route Designator

Ring B Ring A

Routing Control

TokenFrame Header

Token FrameTrailer

RoutingInformation

Field

Routing Control

TokenFrame Header

Token FrameTrailer

RoutingInformation

Field

00B1 00A0

RD1 RD2RCRC

Discoveryframe

Bridge 1

Source Route Frame Types• Four types of Source Route frames:

–Single Route Explorer (SRE)•Also known as Spanning Tree Explorers (STE)

–So named by the IEEE 802.5 working group

–All Routes Explorer (ARE)

–Specifically Routed Frame (SRF)

–Single Route Explorer with a specific route return.

Token Ring to Ethernet Conversion

SD AC FC DA SA RIF DSAP SSAP CTRL OUI Type Info FCS ED FS

DA SA Type Info FCSPreamble

Copy andbit reverse

CopyDiscard

Token Ring frame

Ethernet frame

SNAP header

Ethernet to Token Ring Conversion

DA SA Type Info FCSPreamble

SD AC FC DA SA RIF DSAP SSAP CTRL OUI Type Info FCS ED FS

Copy andbit reverse

Copy

Insert

Ethernet frame

Token Ring frame SNAP header

Token Ring to IEEE 802.3 Conversion

Copy andbit reverse Token Ring frame

CutInsert Copy

IEEE 802.3 frame

DA SA Info FCSPreamble SFD Length DSAP SSAPCTRL PAD

SD AC FC DA SA RIF DSAP SSAP CTRL Info FCSED FS

IEEE 802.3 to Token Ring Conversion

Copy andbit reverse

Token Ring frame

CutInsert Copy

IEEE 802.3 frame

SD AC FC DA SA RIF DSAP SSAP CTRL Info FCSED FS

DA SA Info FCSPreamble SFD Length DSAP SSAPCTRL PAD