How Routers Make Routing Decisions PDF

ADMINISTRATIVE DISTANCE = TRUST WORTHINESS CWE091 - 5 Industrial Networks II Oct. 2021 How Routers make their routing decisions Defaul...

ADMINISTRATIVE DISTANCE = TRUST WORTHINESS CWE091 - 5 Industrial Networks II Oct. 2021 How Routers make their routing decisions Default Administrative Distance (AD) ROUTES Don't STATIC Connected 0 SCALE WELL Static 1 RIP 120 · OSPF B 110 7 YNAMIC ROUTING Eigrp D 90 Know these. I cisco pri How Metrics Determine the Route Selection Process Routes are chosen and built in the routing table based on the routing protocol's LOWEST administrative distance. The routes learned from the routing protocol with the lowest AB administrative distance are installed in the routing table. If there are multiple paths to the same destination from a single routing protocol, then the multiple paths would ↓ have the same administrative distance and the best path is selected based on the metrics. Metrics are values associated with specific routes, ranking them from most LOWEST preferred to least preferred. The parameters used to determine the metrics differ for METRI different routing protocols. The path with the lowest metric is selected as the optimal path and installed in the routing table. If there are multiple paths to the same ↓ destination with equal metrics, load balancing is done on these equal cost paths. IF SAME Prefix Lengths A ne Let's look at another scenario to see how the router handles another common situation: varying prefix lengths. Assume, again, that a router has three routing processes running on it, and each process has received these routes:  EIGRP: 192.168.32.0/26  RIP: 192.168.32.0/24  OSPF: 192.168.32.0/19 Which of these routes will be installed in the routing table? Since EIGRP 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. Let's see how the forwarding engine uses the information from the routing table to make forwarding decisions. Keith Smyth Page 1 of 2 CWE091 - 5 Industrial Networks II Oct. 2021 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. · LONGEST PREFIY VINs DESTINATION · Ensure Is In ADDRESS THE SUBNET If a packet arrives on a router interface destined for 192.168.32.1, which route would the router choose? 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. (Longest prefix match (also called Maximum prefix length match)) In this case, a packet destined to 192.168.32.1 is directed toward 10.1.1.1, because 192.168.32.1 falls within the 192.168.32.0/26 network (192.168.32.0 to 192.168.32.63). It also falls within the other two routes available, but the 192.168.32.0/26 has the longest prefix within the routing table (26 bits verses 24 or 19 bits). Likewise, if a packet destined for 192.168.32.100 arrives on one of the router's interfaces, it's forwarded to 10.1.1.2, because 192.168.32.100 doesn't fall within 192.168.32.0/26 (192.168.32.0 through 192.168.32.63), but it does fall within the 192.168.32.0/24 destination (192.168.32.0 through 192.168.32.255). Again, it also falls into the range covered by 192.168.32.0/19, but 192.168.32.0/24 has a longer prefix length. Summary In summary, making a forwarding decision actually consists of three sets of processes: the routing protocols, the routing table, and the actual process which makes a forwarding decision and switches packets. These three sets of processes are illustrated, along with their relationship, below. The longest prefix match always wins among the routes actually installed in the routing table, while the routing protocol with the lowest administrative distance always wins when installing routes into the routing Forwarding tables often contain a default route, which has the shortest possible prefix match, to fall back on in case matches with all other entries fail. Keith Smyth Page 2 of 2