1. “Internetworking” Bridges –Transparent bridges –Source Routing - Transparent Bridges Routers (Network Layer) Brouters 11 2 3 22 11

2. Why Bridges Isolation of Physical Layer Effects Bandwidth Multiplication Security or Traffic Isolation

3. Segmenting Traffic Workstations File server Terminals Terminal server LAN traffic Host Bridge

4. 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.

5. 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.

6. Learning, Filtering, and Forwarding Terminals Node ANode 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 this cable segment though port 1. Port 2 Port 1 Node F Bridge

7. Filtering - An Example A 1 B 1 C 2 D 2 Node A Node B Node CNode D Fowarding Table Filtered Cable segment 1 Cable segment 2 Port ID 1 Port ID 2 Packet transmitted

8. Forwarding - An Example Node A Node B Node C Node D Forwarded Cable segment 1 Cable segment 2 Forwarding table A 1 B 1 C 2 D 2 Port ID 1 Port ID 2

9. 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 C D Bridge table Bridge 3 Bridge 2 Bridge 1

10. 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.

11. Duplicate Packets Node A Node B Node C Node D Single packet transmitted Two packets received Cable segment 1 Cable segment 2 Bridge 2 Bridge 1

12. Broadcasts Node A Node B Node C Node D Broadcast packet transmitted Cable segment 1 Cable segment 2 Packet received and transmitted back by second bridge Loop Bridge 2 Bridge 1

13. Unknown Destination Address Node ANode B Node C Node D Destination Z packet transmitted Cable segment 1 Cable segment 2 Packet received and transmitted back by second bridge Loop Bridge 2Bridge 1

14. 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.

15. 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.

16. 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.

17. 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.

18. 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.

19. Source Routing Example Ring 4Ring 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

20. Routing Information Field Starting Delimiter Access Control Frame Control Destination Address Source Address Optional Routing Information Field Destination Service Access Protocol (DSAP) Source Service Access Protocol (SSAP) Rest of Token Ring frame Routing Control Route Designator Route Designator...... B B B L L L L LD F F F r r r r Ring number Bridge number 12 bits4 bits 1 - F bridge IDs 2 bytes Up to 8 RD fields Routing Information Indicator (RII) 1 - 4095 rings

21. The Route Designator Ring B Ring A Routing Control Token Frame Header Token Frame Trailer Routing Information Field Routing Control Token Frame Header Token Frame Trailer Routing Information Field 00B1 00A0 RD1 RD2RC Discovery frame Bridge 1

22. 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.

23. Token Ring to Ethernet Conversion SD AC FCDASARIFDSAPSSAPCTRL OUI TypeInfoFCSEDFS DASAType Info FCS Preamble Copy and bit reverse Copy Discard Token Ring frame Ethernet frame SNAP header

24. Ethernet to Token Ring Conversion DASA Type Info FCS Preamble SDACFCDASARIFDSAPSSAPCTRL OUI TypeInfoFCSEDFS Copy and bit reverse Copy Insert Ethernet frame Token Ring frame SNAP header

25. Token Ring to IEEE 802.3 Conversion Copy and bit reverse Token Ring frame Cut Insert Copy IEEE 802.3 frame DASA InfoFCS PreambleSFD Length DSAP SSAP CTRL PAD SDACFCDASARIFDSAPSSAPCTRL InfoFCS EDFS

26. IEEE 802.3 to Token Ring Conversion Copy and bit reverse Token Ring frame Cut Insert Copy IEEE 802.3 frame SDACFCDASARIFDSAPSSAPCTRL InfoFCS ED FS DASA Info FCS PreambleSFD Length DSAP SSAP CTRL PAD