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Route Selection in Cisco Routers Route Selection • One of the intriguing aspects of Cisco routers, especially for those new to routing, is how the router chooses which route is the best. Route Selection • To understand it completely requires some knowledge about the way Cisco routers work. Processes Involved • There are three processes involved in building and maintaining the routing table in a Cisco router: Processes Involved • There are three processes involved in building and maintaining the routing table in a Cisco router: • Various routing processes (Protocols) RIP EIGRP etc. Processes Involved • There are three processes involved in building and maintaining the routing table in a Cisco router: • Various routing processes (Protocols) RIP EIGRP etc. • The routing table itself Processes Involved • There are three processes involved in building and maintaining the routing table in a Cisco router: • Various routing processes (Protocols) RIP EIGRP etc. • The routing table itself • The forwarding process Routing Processes • • • • • Dynamic Routing Protocols IGRP – EIGRP – OSPF – RIP – • These processes populate the routing table. The Routing Table • Accepts information from the routing processes and also replies to requests for information from the forwarding process. Forwarding Process • Requests information from the routing table to make a packet forwarding decisions as to which router interface to forward the packet to. Building the routing table • Let's examine the interaction between the routing protocols and the routing table to understand how the routing table is built. Building the routing table • As each routing process ( EIGRP, RIP etc..) receives updates and other information, it chooses the best path to any given destination and attempts to install this path into the routing table. Building the routing table • For instance, if EIGRP learns of a path towards 10.1.1.0/24, and decides this particular path is the best EIGRP path to this destination, it tries to install the path it has learned into the routing table. Building the routing table • The router decides whether or not to install the routes presented by the routing processes based on the …. • administrative distance of the route in question. Administrative distance • This is the believability of the route trying to be inserted into the routing table. Administrative distance • This is the believability of the route trying to be inserted into the routing table. • Some Routing Processes are more trustworthy than others. Administrative distance • This is the believability of the route trying to be inserted into the routing table. • Some Routing Processes are more trustworthy than others. • Each Routing Process has a given Administrative Distance as a number. Administrative distance • The smaller the number the more believable the route. Administrative distance • The smaller the number the more believable the route. • If two routing processes try to install the same route into the table one will be rejected. Example • To understand this better, let's look at an example. Example • To understand this better, let's look at an example. Example • Assume a router has four routing processes running: EIGRP, OSPF, RIP, and IGRP. • Now, all four of these processes have learned of various routes to the 10.168.24.0/24 network, and each has chosen its best path to that network through its internal metrics and processes . Example • Each of these four processes attempts to install their route toward 10.168.24.0/24 into the routing table Example • The routing processes are each assigned an administrative distance, which is used to decide which route to install. Example • Since the internal EIGRP route has the best administrative distance, it's installed in the routing table Backup Routes • What do the other protocols? RIP, IGRP, and OSPF? do with the routes that weren't installed? Backup Routes • What do the other protocols? RIP, IGRP, and OSPF? do with the routes that weren't installed? • What if the most preferred route, learned from EIGRP, fails? Backup Routes • Cisco IOS® Software uses two approaches to solve this problem: Backup Routes • Cisco IOS® Software uses two approaches to solve this problem: 1. The first is to have each routing process attempt to install its best routes periodically Backup Routes • Cisco IOS® Software uses two approaches to solve this problem: 1. The first is to have each routing process attempt to install its best routes periodically ( RIP, IGRP ) 2. The second method is to store the route in a database and attempt to install it if the primary route fails. ( EIGRP, OSPF ) Prefix Lengths • Let's look at another scenario to see how the router handles another common situation? varying prefix lengths. Prefix Lengths Assume, again, that a router has four routing processes running on it, and each process has received these routes: • EIGRP (internal): 10.168.32.0/26 • RIP: 10.168.32.0/24 • OSPF: 10.168.32.0/19 Prefix Lengths • Which of these routes will be installed in the routing table? • EIGRP (internal): 10.168.32.0/26 • RIP: 10.168.32.0/24 • OSPF: 10.168.32.0/19 Prefix Lengths • Since EIGRP internal routes have the best administrative distance, it's tempting to assume the first one will be installed. Prefix Lengths • Since EIGRP internal routes have the best administrative distance, it's tempting to assume the first one will be installed. • However, since each of these routes has a different prefix length (subnet mask), they're considered different destinations, and they will all be installed in the routing table. Making Forwarding Decisions • Which routing table entry show I use to get to the required destination ? Making Forwarding Decisions Let's look at the three routes we just installed in the routing table, and see how they look on the router. .... Making Forwarding Decisions Pod1_1603#show ip route D 10.168.32.0/26 [90/25789217] via 10.1.1.1 R 10.168.32.0/24 [120/4] via 10.1.1.2 O 10.168.32.0/19 [110/229840] via 10.1.1.3 Identifies which Process installed the route. D = EIGRP, R = RIP, O = OSPF Making Forwarding Decisions Pod1_1603#show ip route D 10.168.32.0/26 [90/25789217] via 10.1.1.1 R 10.168.32.0/24 [120/4] via 10.1.1.2 O 10.168.32.0/19 [110/229840] via 10.1.1.3 Identifies the Admin Distance. Making Forwarding Decisions Pod1_1603#show ip route D 10.168.32.0/26 [90/25789217] via 10.1.1.1 R 10.168.32.0/24 [120/4] via 10.1.1.2 O 10.168.32.0/19 [110/229840] via 10.1.1.3 Identifies the next HOP IP address or Interface to Forward the packet to. Making Forwarding Decisions Pod1_1603#show ip route D 10.168.32.0/26 [90/25789217] via 10.1.1.1 R 10.168.32.0/24 [120/4] via 10.1.1.2 O 10.168.32.0/19 [110/229840] via 10.1.1.3 If a packet arrives on a router interface destined for 10.168.32.1, which route would the router choose? Making Forwarding Decisions Pod1_1603#show ip route D 10.168.32.0/26 [90/25789217] via 10.1.1.1 R 10.168.32.0/24 [120/4] via 10.1.1.2 O 10.168.32.0/19 [110/229840] via 10.1.1.3 It depends on the prefix length, or the number of bits set in the subnet mask. Longer prefixes are always preferred over shorter ones when forwarding a packet. Making Forwarding Decisions Pod1_1603#show ip route D 10.168.32.0/26 [90/25789217] via 10.1.1.1 R 10.168.32.0/24 [120/4] via 10.1.1.2 O 10.168.32.0/19 [110/229840] via 10.1.1.3 It depends on the prefix length, or the number of bits set in the subnet mask. Longer prefixes are always preferred over shorter ones when forwarding a packet. Making Forwarding Decisions Pod1_1603#show ip route D 10.168.32.0/26 [90/25789217] via 10.1.1.1 R 10.168.32.0/24 [120/4] via 10.1.1.2 O 10.168.32.0/19 [110/229840] via 10.1.1.3 Before I tell you which route is preferred lets break down each route to see what it is describing ! Routing table entries • • • • • • • D 10.168.32.0/26 /26 = 255.255.255.192 256 – 192 = 64 So networks are 0, 64, 128, 192 This route is describing Subnetwork 0 10.168.32.0/26 IP host addresses covered = ( 1 thru 63 ) Routing table entries • • • • • • R 10.168.32.0/24 /24 = 255.255.255.0 256 – 255 = 1 So networks are 0,1, 2, 3, 4 in 3rd Octet This route is describing Network 10.168.32.0 IP host addresses covered = ( 1 thru 255 ) Routing table entries • • • • • • O 10.168.32.0/19 /19 = 255.255.224.0 256 – 224 = 32 So networks are 0,32,64,96,etc.. in 3rd Octet This route is describing Network 10.168.32.0 IP host addresses covered = ( 1 thru 255 ) Back to our Example • If a packet arrives on a router interface destined for 10.168.32.1, which route would the router choose? • /26 IP host addresses covered = ( 1 thru 63 ) • /24 IP host addresses covered = ( 1 thru 255 ) • /19 IP host addresses covered = ( 1 thru 255 ) Back to our Example • If a packet arrives on a router interface destined for 10.168.32.1, which route would the router choose? • /26 IP host addresses covered = ( 1 thru 63 ) • /24 IP host addresses covered = ( 1 thru 255 ) • /19 IP host addresses covered = ( 1 thru 255 ) • This is for Host 1 on network 10.168.32.0 Back to our Example • If a packet arrives on a router interface destined for 10.168.32.1, which route would the router choose? • /26 IP host addresses covered = ( 1 thru 63 ) • /24 IP host addresses covered = ( 1 thru 255 ) • /19 IP host addresses covered = ( 1 thru 255 ) • This is for Host 1 on network 10.168.32.0 • All entries cover this host, so the longest Prefix is used. Back to our Example • If a packet arrives on a router interface destined for 10.168.32.100, which route would the router choose? • /26 IP host addresses covered = ( 1 thru 63 ) • /24 IP host addresses covered = ( 1 thru 255 ) • /19 IP host addresses covered = ( 1 thru 255 ) • This is for Host 100 on network 10.168.32.0 Back to our Example • If a packet arrives on a router interface destined for 10.168.32.100, which route would the router choose? • /26 IP host addresses covered = ( 1 thru 63 ) • /24 IP host addresses covered = ( 1 thru 255 ) • /19 IP host addresses covered = ( 1 thru 255 ) • This is for Host 100 on network 10.168.32.0 • /26 Does not cover this host address • ( 10.168.32.64 / 26 now this does !!!!! But that’s not in the table ) Back to our Example • If a packet arrives on a router interface destined for 10.168.32.100, which route would the router choose? • /24 IP host addresses covered = ( 1 thru 255 ) • /19 IP host addresses covered = ( 1 thru 255 ) • This is for Host 100 on network 10.168.32.0 • Both cover host 100 • So longest Prefix is used = /24 Back to our Example • If a packet arrives on a router interface destined for 10.168.32.100, which route would the router choose? • /24 IP host addresses covered = ( 1 thru 255 ) • /19 IP host addresses covered = ( 1 thru 255 ) • This is for Host 100 on network 10.168.32.0 • Both cover host 100 • So longest Prefix is used = /24 Conclusion • These examples are to make you think and to examine just what the routing table entry is describing. Conclusion • Its tempting to just go for the Longest prefix Conclusion • That’s OK if the longest prefix describes your host !!!! IP Classless • Where the ip classless configuration command falls within the routing and forwarding processes is often confusing. IP Classless • In reality, IP classless only affects the operation of the forwarding processes in IOS; it doesn't affect the way the routing table is built. IP Classless • If IP classless isn't configured (using the no ip classless command), the router won't forward packets to subnets it does not specifically know about !!!! • As an example, let's again place three routes in the routing table and route packets through the router IP Classless router#show ip route.... 172.30.0.0/16 is variably subnetted, 2 subnets, 2 masks D 172.30.32.0/20 [90/4879540] via 10.1.1.2 D 172.30.32.0/24 [90/25789217] via 10.1.1.1 S* 0.0.0.0/0 [1/0] via 10.1.1.3 IP Classless • Remembering that the 172.30.32.0/24 network = Addresses 172.30.32.1 through 172.30.32.255 ( /24 = Network bits = 255.255.255.0 ) ( Networks increment in 1’s ) ( As such just one network is described ) IP Classless • Remembering that the 172.30.32.0/24 network = Addresses 172.30.32.1 through 172.30.32.255 ( /24 = Network bits = 255.255.255.0 ) ( Networks increment in 1’s ) ( As such just one network is described ) IP Classless • Remembering that the 172.30.32.0/24 network = Addresses 172.30.32.0 through 172.30.32.255 ( /24 = Network bits = 255.255.255.0 ) ( Networks increment in 1’s ) ( As such just one network is described ) IP Classless • and the 172.30.32.0/20 network includes the addresses • 172.30.32.0 through 172.30.47.255 • • • • ( /20 = 255.255.240.0 ) ( Networks increment in 16’s ) Networks 0,16,32,48,64 etc….. This mask therefore describes networks 32 - 47 NO IP Classless • Finally the routing table entry 0.0.0.0/0 • Is the DEFAULT route to take • However the NO IP CLASSLESS command affects how this works. NO IP Classless • You only take the Default route for a subnet if your table knows of NO other subnet for the CLASSFULL MAJOR NETWORK NO IP Classless • We can try switching three packets through this routing table and see what the results are. • D 172.30.32.0/20 10.1.1.2 • D 172.30.32.0/24 • S* 0.0.0.0/0 [90/4879540] via [90/25789217]via 10.1.1.1 [1/0] via 10.1.1.3 NO IP Classless • A packet destined to 172.30.32.1 is forwarded to 10.1.1.1, since this is the longest prefix match. • D 172.30.32.0/20 10.1.1.2 • D 172.30.32.0/24 • S* 0.0.0.0/0 [90/4879540] via [90/25789217]via 10.1.1.1 [1/0] via 10.1.1.3 NO IP Classless • A packet destined to 172.30.33.1 is forwarded to 10.1.1.2, since this is the longest prefix match. • D 172.30.32.0/20 10.1.1.2 • D 172.30.32.0/24 • S* 0.0.0.0/0 [90/4879540] via [90/25789217]via 10.1.1.1 [1/0] via 10.1.1.3 NO IP Classless • A packet destined to 10.168.10.1 is forwarded to 10.1.1.3; since this network doesn't exist in the routing table, this packet is forwarded to the default route. • D 172.30.32.0/20 10.1.1.2 • D 172.30.32.0/24 • S* 0.0.0.0/0 [90/4879540] via [90/25789217]via 10.1.1.1 [1/0] via 10.1.1.3 NO IP Classless • A packet destined to 172.30.254.1 is dropped. • D 172.30.32.0/20 10.1.1.2 • D 172.30.32.0/24 • S* 0.0.0.0/0 [90/4879540] via [90/25789217]via 10.1.1.1 [1/0] via 10.1.1.3 NO IP Classless • A packet destined to 172.30.254.1 is dropped. • Because No table entry described this route • IP CLASSLESS will not allow the default route to be taken, because we know some routes to the MAJOR network 172.30.0.0 • So its dropped !! No IP Classless No IP Classless Blocks access to any 10.x.x.x subnet in the cloud but allows internet access to any other Major Network / Subnetwork. Lets Break and then attempt LAB 4