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IP Routing Principles. Application. Application. Presentation. Presentation. Session. Session. Transport. Transport. Network. Network. Network. Network. Network. Data Link. Data Link. Data Link. Data Link. Data Link. Physical. Physical. Physical. Physical. Physical.
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Application Application Presentation Presentation Session Session Transport Transport Network Network Network Network Network Data Link Data Link Data Link Data Link Data Link Physical Physical Physical Physical Physical Network-Layer Protocol Operations X Y C A B Y X • Each router provides network layer (routing) services B A C
Static versus Dynamic Routes • Static route • Uses a protocol route that a network administrator enters into the router • Dynamic route • Uses a route that a network routing protocol adjusts automatically for topology or traffic changes
Static Route Example Point-to-point or circuit-switched connection A Only a single network connection with no need for routing updates • Fixed route to address reflects administrator’s knowledge B “Stub” Network
Company X Internet 172.34.56.0 A B C 10.0.0.0 Routing Table No entry for destination net Try router B default route Default (static) Route Example • Use if next hop is not explicitly listed in the routing table
Adapting to Topology Change B A D C
Adapting to Topology Change B A X • Can an alternate route substitute for a failed route? D C
Adapting to Topology Change B A X • Can an alternate route substitute for a failed route? D C
Network Dynamic Routing Operations Routing Protocol A router passes routing information to its neighbors Routing Table • Routing protocol maintains and distributes routing information
Network Dynamic Routing Operations Routing Protocol Routing Protocol A router passes routing information to its neighbors Routing Table Routing Table • Routing protocol maintains and distributes routing information
Representing Distance with Metrics A 56 Hop count Ticks Cost T1 56 • Information used to select the best path for routing T1 B
Representing Distance with Metrics A Bandwidth Delay Load Reliability MTU 56 Hop count Ticks Cost T1 56 • Information used to select the best path for routing T1 B
B A C D Classes of Routing Protocols Distance Vector Hybrid Routing B Link State A C D
One Issue: Time to Convergence • Convergence occurs when all routers use a consistent perspective of network topology • After a topology changes, routers must recompute routes, which disrupts routing • The process and time required for router reconvergence varies in routing protocols
B A C D Distance Vector Concept A • Pass periodic copies of routing table to neighbor routers and accumulate distance vectors D C B Routing Table Routing Table Routing Table Routing Table
B C A D Link-State Concept Link-State Packets Topological Database Routing Table • After initial flood, pass small event-triggered link-state updates to all other routers SPF Algorithm Shortest Path First Tree
Comparing Distance Vector Routing to Link-State Routing Distance Vector Link-State Views net topology from neighbor’s perspective Adds hops from router to router Frequent, periodic updates: slow convergence Passes copies of routing table to neighbor routers Gets common view of entire network topology Calculates the shortest path to other routers Event-triggered updates: faster convergence Passes link-state routing updates to other routers
Choose a routing path based on distance vectors Converge rapidly using change-based updates Balanced Hybrid Routing Hybrid Routing • Share attributes of both distance-vector and link-state routing
Static Routing • Advantages: • predictable • no additional CPU overhead • no additional network overhead • easy to configure • Disadvantages: • does not scale • hard to maintain • does not adapt to network changes
Dynamic Routing • Advantages: • scalability • adaptability • Disadvantages: • increased complexity • increased bandwidth overhead • increased resource usage
Dynamic Routing • Moral of the story: • Use static routing where you can,use dynamic routing only where you must
Autonomous Systems • Internetwork • The big “I” Internet • AS = a group of routers and their networks (administered by the same owner) • AS peering
Routing Protocols • Where is it used? • Interior Gateway Protocols (IGP) • RIP • OSPF • EIGRP • (IS-IS) • Exterior Gateway Protocols (EGP) • BGP
Routing Protocols • How does it work? • Distance-Vector Protocols • RIP • EIGRP • BGP • Link State • OSPF
Protocol Features • RIPv2 • hop count metric • unreliable transport • passive RIP • simple
Protocol Features • OSPF • full CIDR support • trust • route injection • hierarchical routing • a smarter protocol
Protocol Features • EIGRP • more information in advertisements • improved convergence properties • partial and incremental updates • no support for areas
Protocol Features • BGP • BGP-4: RFC 1771 • created to support CIDR • even more information • Policy routing • Reliable transport • can also be used as an IGP (IBGP) • high overhead
Real World Routing • multiple routing protocols are generally necessary • interior routing is a LOT different than exterior routing • multi-protocol issues
ASBR ASBR Implementation Considerations EIGRP RIP 172.16 172.16 RIP 172.16.0.0 AS 300EIGRP RIP EIGRP 172.16 172.16 • Routing feedback • Suboptimal path selection • Routing loops • Incompatible routing information • Inconsistent convergence time
172.16.12.1 172.16.3.2 Trans 172.16.2.2 172.16.2.1 T-1 172.16.3.1 How can we prevent routing update traffic from crossing some of these links? 172.16.1.1 172.16.1.2 Cen R200 172.16.4.1 172.16.5.1 172.16.7.2 172.16.4.2 T-1Frame Relay 64Kb Rem 172.16.11.1 172.16.7.1 172.16.5.2 64Kb R100 R300 172.16.6.1 172.16.6.2 172.16.9.1 172.16.10.1 Controlling Routing Update Traffic
Defining Distance • Different protocols use different metrics • Metrics are difficult to compare algorithmically
Defining Distance • Different protocols use different metrics • Metrics are difficult to compare algorithmically • Therefore, need a selection process: • 1—Which protocol do you believe the most? • 2—Then decide which metric is the best
Route Source Default Distance Connected Interface 0 Static Route 1 Enhanced IGRP Summary Route 5 External BGP 20 Internal Enhanced IGRP 90 IGRP 100 OSPF 110 IS-IS 115 RIP 120 EGP 140 External Enhanced IGRP 170 Internal BGP 200 Unknown 255 What Protocol to Believe?
Routers! • Cisco CRS-1 IP/MPLS Router • 92Tbps total switching capacity • 1152 x OC-768c/STM-256c (40Gbps) Line cards • 707 Kg, 15 KW per chassis • IPv4, IPv6, MPLS
Routers! • Juniper T640 Internet Routing Node • 640 Gbps total switching capacity • 770 Million packet per second forwarding • 40 Gbps per slot (4 * OC-192c, 1 * OC-768c) • 8 slots per rack
Routers! • Foundry NetIron 1500 Internet Router • 480 Gbps total switching capacity • 178 Million packet per second forwarding • 10 Gbps per slot (1 * 10G Ethernet) • 15 slots per rack