Part 3

Bridges and Routers

Bridges and routers are used to expand beyond the confines of a single LAN.

Bridges are used for interconnection of LANs that use identical protocols at the MAC layer. Bridges are used for several reasons:

• Reliability: one network is a single point of failure. By using bridges, the network can be partitioned into self-containing LANs
• Performance: performance declines with an increase in the number of devices or the length of the wire.
• Security: keep different types of traffic on physically separate media.
• Geography

See figure 4.8, page 136. The bridge is attached to both LANs. The functions of the bridge are:

• Read all frames transmitted on A and accept those addressed to any station on B
• Using the MAC protocol for B, retransmit each frame on B
• Do the same for B-to-A traffic

Design aspects:

• The bridge makes no modification to the content or format of the frames it receives. It does not encapsulate or add additional headers. It simply copies frames from one LAN to the other.
• The bridge must contain addressing and routing intelligence. It must know which physical addresses reside on which LAN. There may be more than two LANs interconnected by a number of bridges. In that case, a frame may have to be routed through several bridges.
• A bridge may connect more than two LANs

See figure 4.9, page 138.

Routers

Routers connect dissimilar networks and operate on layer 3 of the OSI model, the network layer. Routers must cope with several issues:

• Addressing schemes. The networks may use different schemes for assigning addresses to devices. For example an IEEE 802 LAN uses either 16- or 48-bit binary addresses for each attached device; an X.25 uses 12-digit decimal addresses.
• Maximum frame sizes. Frames from one network may have to be broken into smaller pieces, fragments. For example, Ethernet imposes a maximum frame size of 1500 bytes, X.25 imposes a max frame size of 1000 bytes.
• Interfaces. The hardware and software interfaces to various networks differ. The concept of a router must be independent of these differences.
• Reliability: The operations of the router should not depend on an assumption of network reliability.

Go through an IP routing example.

IEEE 802

• 802.1 Higher Level Interface (HILI)
• 802.2 Logical Link Control (LLC)
• 802.3 CSMA/CD
• 802.4 Token Bus
• 802.5 Token Ring
• 802.6 MAN
• 802.7 Broadband Technical Advisory Group (BBTAG)
• 802.8 Fiber Optic Technical Advisory Group (FOTAG)
• 802.9 Integrated Services LAN (ISLAN) Interface
• 802.10 Standard for Interoperable LAN Security (SILS)
• 802.11 Wireless LAN (WLAN)
• 802.12 Demand Priority
• 802.14 Cable-TV Based Broadband Communication Network

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