CSE/EE 461 Lecture 7 Bridging LANs. Last Two Times. This Time -- Switching (a.k.a. Bridging)

Transcription

1 S/ 461 Lecture 7 ridging LNs Last Two Times Medium ccess ontrol (M) protocols Part of the Link Layer t the heart of Local rea Networks (LNs) ow do multiple parties share a wire or the air? Random access protocols (SM/) ontention-free protocols (turn-taking, reservations) Wireless protocols (SM/ and RTS/TS) This Time -- Switching (a.k.a. ridging) ocus: What to do when one shared LN isn t big enough? nterconnecting LNs ridges and LN switches preview of the Network layer pplication Presentation Session Transport Network ata Link Physical 1

2 Limits of a LN One shared LN can limit us in terms of: istance Number of nodes Performance nodes (wire) ub (repeater) ow do we scale to a larger, faster network? We must be able to interconnect LNs Switching (a.k.a. ridging) Transferring a packet from one LN to another LN uild an extended LN ifferent varieties of switching Packet switched vs. circuit switched onnection vs. onnectionless We ll focus on connectionless, packet switched thernet ridges and xtended LNs Transparently interconnect LNs with bridge Receive frames from each LN and forward to the other ach LN is its own collision domain; bridge isn t a repeater ould have many ports bridge 2

3 Learning ridges To optimize overall performance: Shouldn t forward or, should forward and bridge ow does the bridge know? Learn who is where by observing source addresses and prune orward using destination address; age for robustness Why stop at one bridge? llows you to incrementally build out network, across organizations 3 5 ut must avoid loops -- bridge must forward only on some bridge ports! The Spanning Tree algorithm does this t is separate from previous idea of learning K Spanning Tree xample Spanning tree uses select bridge ports so there are no cycles Prune some ports Only one tree K Q: ow do we find a spanning tree? utomatically with a distributed algorithm

4 Spanning Tree ompute ST with a bridge as root such that Root forwards onto all of its outgoing ports Other bridges forward TO the root if a packet is coming from a bridge further from the root, else they forward away from the root Packet traversal: forwards (UP)* then (OWN*) Spanning tree vs. learning Once the spanning tree is in place the bridge uses the regular learning algorithm to figure out which ports to forward / flood packet on ob of spanning tree algorithm is to disable some ports to eliminate cycles Spanning Tree lgorithm istributed algorithm to compute spanning tree Robust against failures, needs no organization eveloped by Radia Perlman at spec Outline: oal is to turn some bridge ports off 1. lect a root node of the tree (lowest address) 2. row tree as shortest distances from the root (using lowest address to break distance ties) ll done by bridges sending periodic configuration messages over ports for which they are the best path Then turn off ports that aren t on best paths 4

5 lgorithm Overview ach bridge has a unique id e.g., 1, 2, 3 Select the bridge with the smallest id as root Select bridge on each LN that is closest to the root as that LN s designated bridge use ids to break ties K ach bridge forwards frames over each LN on which it is the designated bridge lgorithm continued ridges exchange configuration messages, containing: id for bridge sending the message id for what the sending bridge believes to be the root bridge distance (hops) from sending bridge to root bridge ach bridge records current best configuration message for each port nitially, each bridge believes it is the root when learn not root, stop generating configuration messages instead, forward root s configuration message incrementing distance field by 1 in steady state, only root generates configuration messages lgorithm More When learn not designated bridge on LN, stop forwarding configuration messages in steady state, only designated bridges forward configuration messages Root bridge continues to send configuration messages periodically f a bridge does not receive config. message after a period of time: assumes topology has changed starts generating configuration messages claiming to be root 5

6 lgorithm xample Message format: (root, dist-to-root, sending bridge) Sample messages sequences to and from 3: 1. 3 sends (3, 0, 3) to 2 and receives (2, 0, 2) and (5, 0, 5) and accepts 2 as root 3. 3 sends (2, 1, 3) to receives (1, 1, 2) and (1, 1, 5) and accepts 1 as root K 5. 3 could send (1, 2, 3) but doesn t as its nowhere best 2 and 5 are better choices. so 3 is NOT a designated bridge 6. 3 receives (1, 1, 2) and (1, 1, 5) again stable 3 turns off data forwarding to LNs and 6 4 Some other tricky details onfiguration information is aged f the root fails a new one will be elected Reconfiguration is damped dopt new spanning trees slowly to avoid temporary loops LN Switches LN switches are multi-port bridges Modern, high performance form of bridged LNs Looks like a hub, but frames are switched, not shared very host on a separate port, or can combine switches thernet Switch thernet Switch 6

7 Limitations of ridges/switches LN switches form an effective small-scale network Plug and play for real! Why can t we build a large network using bridges? Little control over forwarding paths Size of bridge forwarding tables grows with number of hosts roadcast traffic flows freely over whole extended LN Spanning tree algorithm limits reconfiguration speed Poor solution for connecting LNs of different kinds Key oncepts We can overcome LN limits by interconnection ridges and LN switches ut there are limits to this strategy Next Topic: Routing and the Network layer ow to grow large and really large networks 7