Chapter Introduction to Routing and Packet Forwarding Routing Protocols and Concepts quangkien@gmail.com Topics l l Inside the Router Ÿ Routers are computers Ÿ Router CPU and Memory Ÿ Internetwork Operating System Ÿ Router Bootup Process Ÿ Router Ports and Interfaces Ÿ Routers and the Network Layer Path Determination and Switching Function Ÿ Packet Fields and Frame Formats Ÿ Best Path and Metrics Ÿ Equal Cost Load Balancing Ÿ Path Determination Ÿ Switching Function l l CLI Configuration and Addressing Ÿ Implementing Basic Addressing Schemes Ÿ Basic Router Configuration Building the Routing Table Ÿ Introducing the Routing Table Ÿ Directly Connected Networks Ÿ Static Routing Ÿ Dynamic Routing Ÿ Routing Table Principles Inside the Router l Routers are computers l Router CPU and Memory l Internetwork Operating System l Router Bootup Process l Router Ports and Interfaces l Routers and the Network Layer Routers are Computers Leonard Kleinrock and the first IMP l A router is a computer: Ÿ CPU, RAM, ROM, Operating System l The first router: used for the Advanced Research Projects Agency Network (ARPANET): Ÿ IMP (Interface Message Processor) Ÿ Honeywell 516 minicomputer that brought the ARPANET to life on August 30, 1969 l Routers forwarding packets (packet switching): Ÿ From the original source to the final destination Ÿ Selects best path based on destination IP address l A router connects multiple networks: Ÿ Interfaces on different IP networks l Router interfaces: Ÿ LAN Ÿ WAN Routers Determine the Best Path l The router’s primary responsibility: Ÿ Determining the best path Ÿ Forwarding packets toward their destination Routers Determine the Best Path IP Packet enters router’s Ethernet interface Router examines the packet’s destination IP address Router searches for a best match between packet’s destination IP address and network address in routing table Using the exit-interface in the route, the packet is forwarded to the next router or the final destination l Routing table Ÿ Determines best path Ÿ Best match between destination IP address and network address in routing table Router CPU and Memory l CPU - Executes operating system instructions l Random access memory (RAM) Ÿ running copy of configuration file Ÿ routing table Ÿ ARP cache l Read-only memory (ROM) Ÿ Diagnostic software used when router is powered up Ÿ Router’s bootstrap program Ÿ Scaled down version of operating system IOS l Non-volatile RAM (NVRAM) Ÿ Stores startup configuration (including IP addresses, Routing protocol) l Flash memory - Contains the operating system (Cisco IOS) l Interfaces - There exist multiple physical interfaces that are used to connect network Examples of interface types: Ÿ Ethernet / fast Ethernet interfaces Ÿ Serial interfaces Ÿ Management interfaces Router physical characteristics 10 Show Routing Table R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set C C 192.168.1.0/24 is directly connected, FastEthernet0/0 192.168.2.0/24 is directly connected, Serial0/0/0 65 Verifying Interfaces R1# show interfaces FastEthernet0/0 is up, line protocol is up (connected) Hardware is Lance, address is 0007.eca7.1511 (bia 00e0.f7e4.e47e) Description: R1 LAN Internet address is 192.168.1.1/24 MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, rely 255/255, load 1/255 Encapsulation ARPA, loopback not set ARP type: ARPA, ARP Timeout 04:00:00, Last input 00:00:08, output 00:00:05, output hang never Last clearing of “show interface” counters never Queueing strategy: fifo Output queue :0/40 (size/max) minute input rate bits/sec, packets/sec minute output rate bits/sec, packets/sec packets input, bytes, no buffer Received broadcasts, runts, giants, throttles Serial0/0 is up, line protocol is up (connected) Hardware is HD64570 Description: Link to R2 Internet address is 192.168.2.1/24 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255 Encapsulation HDLC, loopback not set, keepalive set (10 sec) Last input never, output never, output hang never 66 Building the Routing Table l Introducing the Routing Table l Directly Connected Networks l Static Routing l Dynamic Routing l Routing Table Principles Introducing the Routing Table R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set C C 192.168.1.0/24 is directly connected, FastEthernet0/0 192.168.2.0/24 is directly connected, Serial0/0/0 l Routing table is a data file in RAM that is used to store route information about: Ÿ Directly connected Ÿ Remote networks 68 Introducing the Routing Table R1# show ip route C C 192.168.1.0/24 is directly connected, FastEthernet0/0 192.168.2.0/24 is directly connected, Serial0/0/0 Exit Interfaces l The routing table contains network/next-hop associations l The “next hop” is the IP address of a next-hop router (coming) l May also include an outgoing or exit interface (more later) 69 Introducing the Routing Table R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP C C 192.168.1.0/24 is directly connected, FastEthernet0/0 192.168.2.0/24 is directly connected, Serial0/0/0 Directly Connected Networks l directly connected network is a network that is directly attached to one of the router interfaces l When a router’s interface is configured with an IP address and subnet mask, the interface becomes a host on that attached network l Active directly connected networks are added to the routing table 70 Introducing the Routing Table R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP C C 192.168.1.0/24 is directly connected, FastEthernet0/0 192.168.2.0/24 is directly connected, Serial0/0/0 Directly Connected Networks l directly connected network is a network that is directly attached to one of the router interfaces l When a router’s interface is configured with an IP address and subnet mask, the interface becomes a host on that attached network l Active directly connected networks are added to the routing table 71 Introducing the Routing Table R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP C C 192.168.1.0/24 is directly connected, FastEthernet0/0 192.168.2.0/24 is directly connected, Serial0/0/0 Remote Network l A remote network is a network that is not directly connected to the router l A remote network is a network that can only be reached by sending the packet to another router l Remote networks are added to the routing table using Ÿ a dynamic routing protocol or Ÿ by configuring static routes l Dynamic routes are routes to remote networks that were learned automatically by the router, using a dynamic routing protocol l Static routes are routes to networks that a network administrator manually configured 72 Directly Connected Networks R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP C C 192.168.1.0/24 is directly connected, FastEthernet0/0 192.168.2.0/24 is directly connected, Serial0/0/0 Directly Connected Networks l C: Source of the route information, directly connected network, static route, or a dynamic routing protocol Ÿ The C represents a directly connected route l 192.168.1.0/24: The network address and subnet mask of the directly connected or remote network Ÿ In this example, 192.168.1.0/24 is the directly connected network l FastEthernet 0/0: The exit interface and/or the IP address of the next-hop router Ÿ In this example, both FastEthernet 0/0 is the exit interfaces used to reach these networks 73 Directly Connected Networks R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP C C 192.168.1.0/24 is directly connected, FastEthernet0/0 192.168.2.0/24 is directly connected, Serial0/0/0 Directly Connected Networks l Before any static or dynamic routing is configured on a router, the router only knows about its own directly connected networks l These are the only networks that are displayed in the routing table until static or dynamic routing is configured l Static and dynamic routes cannot exist in the routing table without a router’s own directly connected networks l The router cannot send packets out an interface if that interface is not enabled with an IP address and subnet mask, just as a PC cannot send IP packets out its Ethernet interface if that interface is not configured with an IP address and subnet mask 74 Static Routes R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP Gateway of last resort is not set C C S 192.168.1.0/24 is directly connected, FastEthernet0/0 192.168.2.0/24 is directly connected, Serial0/0/0 192.168.3.0/24 [1/0] via 192.168.2.2c Static Route l Static route includes the network address and subnet mask of the remote network, along with the IP address of the next-hop router or exit interface l Note: Configuration of the static route is not shown l Static routes are denoted with the code S in the routing table, l Static routes are examined in detail in the next chapter 75 Dynamic Routes R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set C C S R 192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 192.168.4.0/24 is directly connected, FastEthernet0/0 is directly connected, Serial0/0/0 [1/0] via 192.168.2.2 [120/1] via 192.168.2.2, 00:00:20, Serial0/0/0 l R1 has automatically learned about the 192.168.4.0/24 network from R2 through the dynamic routing protocol RIP (Routing Information Protocol) l RIP was one of the first IP routing protocols and will be fully discussed in later chapters l Note: Configuration of RIP not shown 76 Routing Table Principles l These principles, listed as follows, are from Alex Zinin’s book, Cisco IP Routing: Ÿ Every router makes its decision alone, based on the information it has in its own routing table Ÿ The fact that one router has certain information in its routing table does not mean that other routers have the same information Ÿ Routing information about a path from one network to another does not provide routing information about the reverse, or return, path 77 Asymmetric Routing l Asymmetric routing - Because routers not necessarily have the same information in their routing tables, packets can traverse the network in one direction, using one path, and return through another path l Asymmetric routing is more common in the Internet, which uses the BGP routing protocol, than it is in most internal networks 78 Chapter Introduction to Routing and Packet Forwarding ...Topics l l Inside the Router Ÿ Routers are computers Ÿ Router CPU and Memory Ÿ Internetwork Operating System Ÿ Router Bootup Process Ÿ Router Ports and Interfaces Ÿ Routers and the Network... the Router l Routers are computers l Router CPU and Memory l Internetwork Operating System l Router Bootup Process l Router Ports and Interfaces l Routers and the Network Layer Routers are Computers. .. including T1, DSL, and ISDN 21 Router Interfaces FastEthernet 0/0 MAC: 0c00-41cc-ae12 10 .1. 0 .1/ 16 FastEthernet 0/0 MAC: 0c00-3a44 -19 0a 19 2 .16 8 .1. 1/24 Serial 0/0 17 2 .16 .1. 1/24 Serial 0 /1 172 .16 .1. 2/24