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CE4226 Network Systems Analysis and Design Switching and Routing Protocols Virtual LAN An emulation of a standard LAN that allows data transfer to take place without the traditional physical restraints placed on a network A VLAN is a set of LAN devices that belong to an administrative group Group membership is based on configuration parameters and administrative policies rather than physical location 2 Virtual LAN (Contd.) Members of a VLAN communicate with each other as if they were on the same wire or hub, when in fact they may be located on different physical LAN segments Members of a VLAN communicate with members in a different VLAN as if they were on different LAN segments, even when they are located in the same switch 3 Virtual LAN (Contd.) Switch A Switch B Station A1 Station A2 Station A3 Station B1 Station B2 Station B3 Network A Network B 4 Virtual LAN (Contd.) VLAN A Station A1 Station A2 Station A3 Station B1 Station B2 Station B3 5 VLAN B Virtual LAN (Contd.) VLANs can span multiple switches Trunk links VLANs that span multiple switches together IEEE 802.1Q standard Cisco Inter-Switch Link (ISL) protocol 6 Virtual LAN (Contd.) VLAN A VLAN A Station A1 Station A2 Station A3 Station A4 Station A5 Station A6 Switch A Switch B Station B1 Station B2 Station B3 Station B4 Station B5 Station B6 VLAN B VLAN B 7 Virtual LAN (Contd.) Each router interface defines a broadcast domain boundary Routers prevent broadcast traffic from propagating across interfaces VLANs define broadcast domain boundaries in a layer 2 network If the VLAN switch receives a broadcast on one port, it determines what other ports are allowed to receive 8 Virtual LAN (Contd.) Router Router 9 Virtual LAN (Contd.) 10 Virtual LAN (Contd.) 11 Virtual LAN (Contd.) Broadcast domain in switch network is difficult to see without access to the configuration files Each virtual bridge created within a switch defines a broadcast domain (VLAN) VLAN is a group of devices participating in the same broadcast domain They can communicate with each other without needing to communicate through a router 12 Virtual LAN (Contd.) Traffic from one VLAN cannot pass directly to another VLAN (between broadcast domains) within the same switch Use layer 3 devices to interconnect the VLANs 13 VLAN Memberships Port-based VLANs A switch port is manually configured to be a member of a VLAN All machines on the port belong to the same VLAN Cisco’s Catalyst MAC-based VLANs VLAN membership is based on the MAC address of the workstation The switch has a table listing of the MAC address of each machine, along with the VLAN to which it belongs 14 VLAN Memberships Protocol-based VLANs Layer 3 data within the frame is used to determine VLAN membership For example, IP machines can be classified as the first VLAN, and AppleTalk machines as the second The major disadvantage of this method is that it violates the independence of the layers, so an upgrade from IPv4 to IPv6, for example, will cause the switch to fail 15 Spanning Tree Protocol (STP) Host A LAN X Switch 1 Switch 2 LAN Y Host B 16 Spanning Tree Protocol (STP) Host A LAN X Switch 1 Switch 2 LAN Y Host B 17 Spanning Tree Protocol (STP) (Contd.) Spanning Tree Protocol (STP) IEEE802.1d Rapid Spanning Tree Protocol (RSTP) IEEE802.1w Provide rapid convergence of the spanning tree after a topology change Supersede the STP in IEEE802.1d 2004 edition Common Spanning Tree Protocol (CSTP) IEEE802.1q trunking protocol One instance of STP for all VLANs 18 Spanning Tree Protocol (STP) (Contd.) Per VLAN Spanning Tree (PVST) Cisco’s proprietary protocol Build a separate logical tree topology for each VLAN Allow load sharing by having different forwarding paths per VLAN Use ISL trunking Per VLAN Spanning Tree+ (PVST+) provide the same functionality as PVST Use 802.1Q trunking rather than ISL 19 Spanning Tree Protocol (STP) (Contd.) Multiple Spanning Tree Protocol (MSTP) IEEE 802.1s Use RSTP for rapid convergence Allow several VLANs to be mapped to a reduced number of spanning tree instances Reduce the number of spanning trees required to support a large number of VLANs Provide multiple forwarding paths for load sharing 20 Spanning Tree Protocol (STP) (Contd.) Multi-Instance Spanning Tree Protocol (MISTP) Cisco’s proprietary protocol Allow a set of VLANs to be grouped into a single spanning tree 21 Redundancy and Load Sharing in VLANs It is common practice to design redundant links between LAN switches STP avoids loops but allows only one active path STP provides redundancy, but not load sharing MSTP, PVST+, and MISTP offer both load sharing and redundancy 22 Virtual LAN (Contd.) Advantages: Group users together, even though they weren't physically located together Simplify moves, adds, and changes Divide physical LANs into many logical LANs or broadcast domains a VLAN usually has its own IP subnet A router (or a routing module within a switch) provides inter-VLAN communication 23 Routing Choices Routing Static or dynamic Distance-vector and link-state protocols Interior and exterior Etc. 24 Selecting a Routing Protocol Convergence time Resource consumption Bandwidth consumption Support VLSM Multipath load sharing Scalability Open standard Authentication 25 Next Hop Routing Each router makes routing decisions about how to reach a destination based on its routing table Router just selects the next hop leading to the destination This concept relies on the next hop router to select a further hop closer to a destination 26 Next Hop Routing This independent hop-by-hop routing requires all routers have a consistent view of network To achieve consistency: Manually configure Use routing protocol 27 Static Routing Network administrator compute the routing table for all routers in advance Easy to configure because we just simply tell each router how to reach every indirect network segment It is predictable and controllable No overhead on routers and links 28 Static Routing Example 172.16.5.0/24 172.16.3.0/24 172.16.2.0/24 Router 2 3 Com 172.16.3.1 172.16.2.1 172.16.5.1 172.16.3.2 3 Com 172.16.1.2 3 Com Router 1 Router 3 172.16.1.1 172.16.4.1 172.16.1.0/24 172.16.4.0/24 29 Static Routing Example Router 1 ip route 172.16.3.0 255.255.255.0 172.16.1.2 ip route 172.16.4.0 255.255.255.0 172.16.1.2 ip route 172.16.5.0 255.255.255.0 172.16.1.2 Router 2 ip route 172.16.2.0 255.255.255.0 172.16.1.1 ip route 172.16.4.0 255.255.255.0 172.16.3.2 30 Static Routing Example Router 3 ip route 172.16.1.0 255.255.255.0 172.16.3.1 ip route 172.16.2.0 255.255.255.0 172.16.3.1 ip route 172.16.5.0 255.255.255.0 172.16.3.1 Another way ip route 0.0.0.0 0.0.0.0 172.16.3.1 31 Static Routing The price of its simplicity is a lack of scalability Difficult to update the configuration Error prone It cannot use redundant links to adapt to network failure (unless you configure 2 or more static routes to the same destination) 32 Dynamic Routing Main advantages are scalability and adaptability Not require reconfiguration when network changes Adapt to partial failure 33 Dynamic Routing Routers learn about network topology by communicating with other routers by using routing protocol Each router announces its presence, and available routes on the network Other routers hear and adjust routing table accordingly 34 Dynamic Routing Introduce complexity Prepare information to send to others Select the best route among many candidates Remove the old or unusable information Bandwidth overhead to exchange information 35 Hybrid Routing Some parts of the network use static routing, and some other parts use dynamic routing Static routing in the access network Dynamic routing in the core and distribution networks Use static routing for stub networks Use dynamic routing when there are many router connections 36 Dynamic Routing Protocols Exterior, interior protocols Exterior Gateway Protocol (EGP) Interior Gateway Protocol (IGP) Distance vector, link state protocols 37 Exterior VS Interior Protocols Exterior gateway protocol Carry routing information between two independent administrative entities, such as two corporations, two universities Border Gateway Protocol (BGP) is the most widely used 38 Exterior VS Interior Protocols Interior gateway protocol Used within a single administrative domain or among closely cooperative groups Routing Information Protocol (RIP), Open Shortest Path First (OSPF), Interior Gateway Routing Protocol (IGRP), Enhanced Interior Gateway Routing Protocol (EIGRP) are common 39 Exterior VS Interior Protocols It is possible to use IGP as EGP and vice versa, but not a good idea EGPs are designed to scale to the largest of networks Therefore, EGPs has complexity and overhead IGPs are fairly simple and have little overhead, but they don’t scale well 40 Distance Vector VS Link State Protocols Another way to classify is by what information the routers tell each other, and how they use the information to form their routing tables Distance vector protocol A router periodically sends all of its neighbors two pieces of information about the destinations it knows how to reach Distance to the destination Vector (direction) to the destination indicating the next hop 41 Distance Vector VS Link State Protocols Simple to configure and understand Updates are periodically broadcasted Slow convergence Less processor intensive No complete network topology Loop Count to infinity Split horizon Poisonous reverse Holddown timer Triggered update 42 Distance Vector VS Link State Protocols Link state protocol A router provides information about the topology of the network in its vicinity to all routers in the network Links it is attached to State of those links 43 Distance Vector VS Link State Protocols Complex Update only when state changes Fast convergence High processing power Have complete network topology Loop-free Scale better than DV 44 RIP Routing Information Protocol RIP was developed originally for the Xerox Network System (XNS) protocols and was adopted by the IP community in the early 1980s RIP version 1 (RIPv1): RFC 1058 RIP version 2 (RIPv2): RFC 2453 45 RIP RIP broadcasts its routing table every 30 seconds RIP sends triggered updates when the metric of a route changes (only the changes) RIP allows 25 routes per update packet Easy to configure and troubleshoot for flat or edge network Uses a single routing metric (hop count) to measure the distance to a destination network The hop count can not go above 15 A hop count of 16 means the distance to the destination is infinity (unreachable) 46 RIP RIP uses split horizon with poison reverse route updates are sent out an interface with an infinite metric for routes learned (received) from the same interface RIP supports equal-cost load balancing to a destination RIP has an administrative distance of 120 RIP summarizes IP addresses at network boundaries RIP uses UDP port 520 RIPv1 sends update to 255.255.255.255 RIPv2 sends update to 188.8.131.52 47 RIP Timer Update Timer Specifies the frequency of the periodic broadcasts By default, the update timer is set to 30 seconds Invalid Timer When the invalid timer expires, the route is marked as invalid (possibly down) The router marks the route invalid by setting the metric to 16 The route is retained in the routing table and packets can be forwarded as normal By default, the invalid timer is 180 seconds, or six updates periods (30 x 6 = 180) 48 RIP Timer Holddown Timer Cisco implements an additional timer for RIP The holddown timer stabilizes routes The holddown timer starts when route is unreachable After invalid timer expires After receiving update that route is unreachable Router accepts no updates for the route until the holddown timer expires By default, the holddown timer is 180 seconds 49 RIP Timer Flush Timer A route entry marked as invalid is retained in the routing table until the flush timer expires By default, the flush timer is 240 seconds, which is 60 seconds longer than the invalid timer Start automatically after last update is received (along with invalid timer) 50 RIP RIPv1 is a classful routing protocol Discontiguous subnets are not visible to each other, and VLSM is not supported RIPv2 is classless RIPv2 supports authentication simple plain-text password MD5 authentication It still sends updates every 30 seconds and retains the 15-hop limit 51 RIP Downed link This failure is detected by the interface hardware and indicated to the router This change is sent out as a triggered update Downed router Downed router couldn’t inform neighboring routers It takes up to 3 minutes for the neighboring routers to mark the routes learned from the downed router as unreachable Then use triggered updates 52 IGRP Interior Gateway Routing Protocol (IGRP) developed by Cisco to overcome RIP IGRP update timer is 90 second IGRP has a maximum hop limit of 100, by default, and can be configured to support a network diameter of 255 IGRP uses a vector metric and a composite metric based on the following factors: Bandwidth Delay Reliability Load 53 IGRP Metric Bandwidth The bandwidth of the lowest-bandwidth link on the path Equal to 107 / Data rate (kbps) Delay The sum of all the delays for outgoing interfaces in the path Delay is not dynamically calculated Each router interface has a default delay (can be changed by administrator) Equal to Delay (microseconds) / 10 54 IGRP Metric Reliability Based on the interface reliability reported by routers in the path 255 is 100 percent reliable and 1 is minimally reliable It is dynamically calculated (5-min interval) Load Based on the interface load reported by routers in the path 255 is 100 percent loaded and 1 is minimally loaded Load is dynamically calculated (5-min interval) 55 IGRP Metric BW k5 metric k1 BW k 2 k3 delay 256 load reliabilit y k 4 By default, K1 = K3 = 1 and K2 = K4 = K5 = 0 metric BW delay K1 to k5 can be changed but must be same for all routers 56 IGRP Metric Media Data rate (kbps) BW Delay (us) Delay ATM 155000 65 100 10 155Mb/s Fast 100000 100 100 10 Ethernet FDDI 100000 100 100 10 Token Ring 16000 625 630 63 Ethernet 10000 1000 1000 100 T1 1544 6476 20000 2000 E1 2048 5000 20000 2000 DS0 64 156250 20000 2000 56 kbps 56 178571 20000 2000 57 IGRP Metric Link1 is serial 1.544 Mbps Link2 is Ethernet 10 Mbps IGRPmetric = (10,000,000/1544) + (20000 + 1000)/10 IGRPmetric = 6476 + 2100 = 8576 58 IGRP Metric IGRP uses composite metric (CM) to make routing decision But updates vector metric (VM) to other neighbors NewVM.BW = min(UpdtVM.BW, Link.BW) NewVM.DL = UpdtVM.DL + Link.DL NewVM.RL = min(UpdtVM.RL, Link.RL) NewVM.LD = max(UpdtVM.LD, Link.LD) CM = min(NewCM, OldCM) 59 IGRP Timer Update Timer IGRP sends its routing table to its neighbors every 90 seconds Invalid Timer IGRP uses an invalid timer to mark a route as invalid after 270 seconds (three times the update timer) 60 IGRP Timer Holddown Timer The router accepts no new changes for the route until the holddown timer expires This setup prevents routing loops in the network The default holddown timer is 280 seconds Flush Timer IGRP uses a flush timer to remove a route from the routing table The default flush timer is set to 630 seconds 61 IGRP IGRP is classful IGRP implements split horizon with poison reverse, triggered updates, and holddown timers for stability and loop prevention By default, IGRP will load-balance traffic if there are several paths with equal cost (up to 6) IGRP can load-balance over unequal-cost paths 62 IGRP 104 routes per IGRP message Administrative distance is 100 IGRP uses IP directly, with protocol number 9 IGRP sends update to 255.255.255.255 No support for authentication Largely replaced by EIGRP 63 EIGRP Enhanced Interior Gateway Routing Protocol (EIGRP) EIGRP is compatible with IGRP EIGRP can also redistribute routes for RIP, IS-IS, BGP, and OSPF EIGRP has Protocol-Dependent Modules that can deal with AppleTalk and Novell’s IPX 64 EIGRP EIGRP is a classless protocol Hybrid distance-vector protocol EIGRP uses hellos and forms neighbor relationships (as link-state protocol) to detect the new or the loss of a neighbor EIGRP uses the composite metric as IGRP but multiplied with 256 EIGRP does not send periodic updates 65 EIGRP By default, EIGRP will load-balance traffic if there are several paths with equal cost EIGRP can load-balance over unequal- cost paths EIGRP supports authentication simple plain-text password MD5 authentication 66 EIGRP EIGRP packet uses both multicast (184.108.40.206) and unicast Reliable Transport Protocol (RTP) ensures delivery in order EIGRP uses IP protocol number 88 Administrative distance EIGRP internal routes are 90 EIGRP external routes are 170 EIGRP summary routes are 5 67 EIGRP EIGRP uses Diffusing-Update Algorithm (DUAL) for fast convergence and less bandwidth utilization Updates are Nonperiodic: updates are sent only when a metric changes rather than at regular intervals Partial: updates include only routes that have changed, not every entry in the routing table Bounded: updates are sent only to affected routers 68 OSPF Open Shortest Path First (OSPF) RFC 2328 Link state protocol Classless protocol Hierarchical routing protocol Divide into areas Support equal cost multipath load balancing for up to 6 paths 69 OSPF OSPF propagates only changes to minimize bandwidth utilization OSPF networks converge far more quickly than RIP networks Within a network multiple areas can be created to help ease CPU use in SPF calculations, memory use and the number of LSAs being transmitted 60-80 routers are considered to be the maximum to have in one area 70 OSPF The default area is 0.0.0.0 and should exist even if there is only one area in the whole network All other areas must connect to the backbone area (0.0.0.0) via Area Border Router (ABR) ABR sends and receives summary route from backbone area Autonomous System Boundary Router (ASBR) connects an OSPF network to another network that uses a routing protocol other than OSPF 71 OSPF 72 OSPF OSPF is sent over IP, protocol type 89 OSPF uses two multicast addresses for broadcast and point to point networks 220.127.116.11 for all OSPF routers 18.104.22.168 for all designed routers OSPF uses unicast for Non-Broadcast Multi-Access (NBMA) networks such as ISDN, X.25, frame relay, ATM 73 OSPF OSPF calculates cost by Cost = 108 / data rate (bps) Cost = 1 for data rate > 100 Mbps OSPF supports authentication simple plain-text password MD5 authentication 74 Summary of Routing Protocol Features Protocol RIP OSPF IGRP EIGRP Type DV LS DV DV Convergence Time Slow Fast Slow Fast VLSM Yes (v2) Yes No Yes Bandwidth usage High Low High Low Resource usage Low High Low Low Multipath support Yes (Cisco) Yes Yes Yes Scale well No Yes Yes Yes Proprietary No No Yes Yes Non-IP protocols No No No Yes 75
"CE 4226 Network Systems Analysis and Design"