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Joining LANs - Bridges: 

Joining LANs - Bridges

Connecting LANs: 

Connecting LANs Repeater Operates at the Physical layer no decision making, processing signal boosting only Bridges operates at the Data Link layer forwarding decisions based on addresses no fragmentation/reassembly Routers operates at the Network Layer fragmentation/reassembly routing to distant networks

Two types of Bridges: 

Two types of Bridges Transparent or Spanning Tree Processing overhead is in the bridges Host interface is simple Source Routing Processing overhead is in the host interface

Sample topology: 

Sample topology

Spanning Tree Bridge Behavior: 

Spanning Tree Bridge Behavior If the destination address is found to be in the direction of the port on which it arrived, do nothing ex. source on LAN E and destination on LAN A not forwarded by B1

Learning the topology: 

Learning the topology If the destination is not known forward the frame Problem if the topology contains a loop


A B B1 B2 A B B1 routing table is empty B2 routing table is empty Node A port 0 Node A port 0 0 0 1 1 B1 B B2 B


A B B1 B2 B1 routing table is empty B2 routing table is empty Node A port 0 Node A port 0 0 0 1 1 B2 B B1 B


A B B1 B2 B1 routing table is empty B2 routing table is empty Node A port 0 0 0 1 1 B2 B B1 B Node A port 0

Creating a Spanning Tree: 

Creating a Spanning Tree Set of all nodes in a graph plus a subset of the links so that all nodes are connected but there is only one path between any pair of nodes Redundant bridges provide backup, but do not participate in routing

Bridge Protocol: 

Bridge Protocol Create the Spanning Tree Bridge with lowest ID becomes the root All others determine which port provides best access to the root (root port) For each LAN, determine the one bridge to be used to access the root of the spanning tree. Designated bridge for that LAN For each bridge, root port and designated bridge ports are placed in forwarding state Other ports are in blocking state

Sample topology with root ports and designated bridge ports identified: 

Sample topology with root ports and designated bridge ports identified * = root port === = designated bridge port LAN A


Examples LAN B ==> LAN F LAN C ==> LAN D path taken is not always optimal, because not all bridge ports are available

One virtual LAN : 

One virtual LAN All the LANs connected by the bridges one virtual LAN one address space Each bridge learns how to forward to all nodes on all LANs

Forwarding behavior: 

Forwarding behavior Destination goes to port of arrival no action by the bridge LAN A ==> LAN A, not forwarded by B4 LAN E ==> LAN A, not forwarded by B1 Destination known to be other than arrival port forward toward destination not always toward the root LAN A ==> LAN D, forwarded by B4 on port 3 LAN B ==> LAN C, forwarded by B3 on port 2 Destination not known forward on all non-blocking ports except arrival port LAN B==> unknown destination forwarded by B3 on ports 2 and 3

An abstraction of the spanning tree: 

An abstraction of the spanning tree Unique path from each source to each destination clearly visible Two links with the same (E) label not desirable = destination host

Better diagram: 

Better diagram No longer have two links with the same label. Now have a node that represents two bridges, though. Call this a multibridge node.

Use of the diagram: 

Use of the diagram Calculate the length of the journey for a frame traveling between any two LANs number of LANs traversed number of bridges that forward the frame Transfer between children of a multibridge node count one LAN traversed, two bridges used If there are more than two bridges represented by the multibridge node, there still are two bridges and one LAN used in the transfer.

Larger Example: 

Larger Example

Spanning tree for larger example: 

Spanning tree for larger example


Examples Source: G Destination: I Number of LANs: 1 (A) Number of Bridges (ids): 2 (B6, B8)

Another example: 

Another example Source: H Destination: E Number of LANs: 1 (A) Number of Bridges: 2 (B7, B4)

One more example: 

One more example Source: I Destination: E Number of LANs: 2 (A, E) Number of Bridges: 3 (B7, B4, B5)

Maximum path length between nodes: 

Maximum path length between nodes Distance from the lowest leaf to the root plus distance from the root to the lowest node that is not a descendent of the same multinode as the first leaf.

Example: : 

Example: Maximum path is from any of {G, H, I, J} to the root plus the distance from the root to either of {B, C}

Source Routing Bridges: 

Source Routing Bridges Source specifies which bridges are to forward the frame Route is inserted in the frame header following the source address Special use of a bit in the source address that would indicate a group address (group address not possible for source) to indicate that routing information follows.

Bridges summary: 

Bridges summary Two basic types: spanning tree and source routing Spanning tree is easier to manage, but does not always give the optimal route Source Routing always gives the optimal route, but involves more overhead. Spanning tree is the standard, with source routing as an allowed option.

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