Notes

Outline

ICS 156: Networking Lab Magda El Zarki Professor, ICS UC, Irvine

Course Outline Ch 1: Introduction Ch 2:Bridges Ch 3:Routers Ch 4: Transport Protocols

Ch. 2 Bridges The role of Bridges Transparent Bridges The Spanning Tree Alg.

2.1 What are Bridges Bridges are devices that operate at level 2 of the OSI model (as opposed to repeaters that are purely physical layer devices or routers that operate at layer 3).

Bridge in Operation

2.1 What are Bridges contd. They are used to interconnect/segment LANs to: increase connectivity, coverage and reliability provide secure environments isolate LAN traffic from different departments or buildings increase capacity of the system by reducing the broadcasting area translate from one LAN environment to another, e.g., CSMA/CD to token ring.

2.1.1 Functions of Bridges Bridges are intelligent devices that route packets along an interconnected network to the destination. Bridges only forward packets to destinations that are not connected to the local LAN. Bridges operate in a store and forward fashion. Buffers at the import port read in the whole frame before it is processed, thereby introducing a delay. Bridges perform MAC operations -> any frames with detectable errors are discarded -> simple error control.

2.1.1 Functions of Bridges contd. They do not implement any flow control so under heavy loads may become overwhelmed and drop frames from buffers. They operate in connectionless mode (best effort delivery). Bridges do not segment packets. Frames to the same destination will follow the same route or path so long as the network configuration has not changed due to failures or overloading between two frame transmissions.  There are two kinds of bridges: Transparent and Source Routing. The former are the most popular.

2.1.2 IEEE Addressing 6 bytes are used for addressing A manufacturer developing MAC products buys 224 addresses from IEEE (IEEE sets the first 24 bits and the manufactures sets the last 24 bits) -> each MAC address is unique! Addresses are generally written in HEX format. Hex means each character goes from 0 to 15 (donated by ‘F’) as opposed to decimal where a character goes from 0 to 9 or binary from 0 to 1. Every 4 binary bits translate into 1 hex symbol. E.g., as we have 48 bits -> we have 48/4 hex symbols and an address will be given as: F2-05-AB-39-42-DD.

2.1.2 IEEE Addressing contd. Every workstation that is connected to the network has to have a network address, this is set in software by the network administrator. The MAC (or Physical) address is dependent upon the MAC card being used, i.e., it is hardwired by the manufacturer. Stations desiring to communicate have to translate the network address into a MAC address to forward the frame over a LAN network. Sometimes the MAC address is known sometimes it is not. If not, the station has to broadcast its query for the destination address over the LAN network and wait for the destination to respond (ARP).

Ch 2. Bridges The role of Bridges Transparent Bridges Spanning Tree Algorithm

2.2 Transparent Bridging Transparent bridges  operate in ‘‘promiscuous’’ mode. I.e., they read in every frame that is sent on each LAN attached to its ports (note they do not forward every frame!). Bridges ‘‘learn’’ where destinations are and build up a routing table -> backwards learning algorithm. The bridge maintains the routing table that maps MAC addresses onto outgoing ports by adding, updating and deleting addresses as time goes by.

2.2 Transparent Bridging contd. How does a bridge forward a frame?  Two situations arise: 1) the destination address is in the routing table or 2) the destination address is not in the routing table. For case one: If the destination is on the same LAN as the source it  discards the frame. If the destination is not on the same LAN  it forwards the frame to the appropriate outgoing port.

2.2 Transparent Bridging contd. For case two: It broadcasts  (flooding) the frame on all its outgoing ports (except the one it came in on!).

2.2.1 Backwards Learning When a bridge is first plugged in all its tables are empty. Every frame it receives it has to broadcast. As every frame has both a source and a destination address it reads the source address of every frame (recall it operates in promiscuous mode!) and notes the LAN/port it came in on. This forms an entry in its routing table.  E.g., A2-45-63-FE-90-AA -> LAN 1(port 8). Each entry also has a time associated with it (i.e., time it was entered)

2.2.1 Backwards Learning contd. As the network changes over time, bridges update adddresses continuously, always check to see if the entry they have for an address has not changed (busy bees!). They also purge addresses that have not been checked/ updated in a recent time interval (aging time). Note that each incoming frame involves 2 routing table lookups: 1) for the destination address and 2) for the source address. The latter lookup is either used for adding a new address or updates the time associated with the entry for that source, in other words the time field is constantly being refreshed. Note that a destination lookup does not refresh the time field!

2.2.1 Backwards Learning contd. As broadcasts are used frequently, how do bridges prevent cycles from forming? They use a spanning tree algorithm!

Ch 2. Bridges The role of Bridges Transparent Bridges The Spanning Tree Algorithm

2.3 Spanning Tree Algorithm An interconnected network generally consists of many bridges and LAN segments (that could or could not contain repeaters). The resulting network is generally a mesh with dual paths to most locations. Bridges overlay a tree topology (loop/cycle free) on the mesh for routing purposes. This is done to avoid cycles from forming when flooding is used.

2.3 Spanning Tree Alg. Contd. A minimum spanning tree is one for which the distance between the root node and all other nodes is the shortest (least hops if all LAN segments are of equal bit rate, if unequal bit rates exist, then the higher the bit rate the lower the cost of the link and the shortest path refers to high bit rate path).

2.3 Spanning Tree Alg. Contd. To construct the minimum spanning tree, the bridges have to decide on which one is going to be the root node. To do this they broadcast their MAC address to each other using a MAC multicast address that is used specifically for configuration changes. The bridge with the lowest MAC address becomes the root. This can be overwritten by using a bridge priority number. This is set by the network manager and allows the better management of the network (i.e, choose the root node dependent upon traffic loading, location, etc.)

2.3 Spanning Tree Alg. contd. The root then proceeds to  construct the tree. If two paths are of equal length, then the one pointing to the bridge with the lower  MAC address is chosen (again here bridge priority maybe used instead). Not all bridge ports will be activated at all times, i.e., some LAN segments may not be used by some bridges as they do not form part of the spanning tree (note: that does not mean the LAN segment is isolated, it basically means that it is reachable via another route/bridge).

2.3 Spanning Tree Alg. Contd. When failures occur (bridges continuously send outconfiguration messages to determine their well being), new shortest paths have to be calculated. When new LAN segments or bridges are added to the topology, new shortest paths have to be calculated. When a topology spans across a WAN, remote bridges are used at each end of the long distance WAN link. The spanning tree should make sure that the WAN link is the least loaded (another reason for using bridge priorities to manipulate the algorithm’s operation and outcome).