Routing tables can either be maintained manually by static routing or dynamically by using routing protocols, like OSPF, IS-IS, EIGRP, etc. Route type Administrative distance Connected Interface 0 Static Route 1 EIGRP Summary Route 5 External Border Gateway Protocol (eBGP) 20 Internal EIGRP 90 IGRP 100 OSPF 110 IS-IS 115 RIP 120 External Gateway Protocol EGP 140 On-Demand Routing 160 External EIGRP 170 Internal BGP (iBGP) 200 Unknown 255 How to maintain a Routing Table? For example, routers can simultaneously describe the same path with a metric of 5 in RIP, 100 in OSPF, and 206175 in EIGRP.īelow is the table of routing protocols and their Administrative Distance (AD). As a result, a single network link can have vastly different metrics depending on the protocol it is connected to. The routing protocol used to learn the route determines the metric value. Routing protocols calculate the best route based on their own metrics, which can vary widely. It’s up to each protocol to decide which routes to keep in its database. The BGP best-path selection process will be used if there is a tie within BGP routes.Įach routing protocol maintains a unique database of routes.ĮIGRP has the topology tables, whereas OSPF has the OSPF database.If a tie exists within OSPF routes, a router will select the best route in the following order, O> O*IA> E1> E2.A router cannot install the two best routes from different protocols in the routing information base (RIB).If two protocols have the same AD (and one has been modified) and the router must determine which one to install in the RIB, it will use the default AD as the tie-breaker.There are some exceptions to the above rules:. Then the router will prefer the device with the lowest metric. For example, if a router has two routes with the same prefix (192.168.100/024), both are learned from OSPF. Then OSPF will be preferred due to lower AD. For example, If a router has two routes with the same prefix (192.168.100.0/24), one from RIP and one from OSPF. The second is Administrative distance (AD), and the lowest AD is always preferred. You can view RIB information in a formatted form by typing the show ip route command.īelow is the Route Selection process to install a route in the RIB:-įirst, it checks the longest match/prefix available in the RIB. The routing table on a Cisco router, or any other vendor’s router, is referred to as the Route Information Base (RIB). And if there is no default gateway configured, then the IP packet will be dropped. If the routing table doesn’t find a match, the packet is sent to the default gateway if one is configured. If a matching route is found then the packet is modified and sent through the correct interface. Then, the router checks the Routing Information Base (RIB) to see if it has a route to the destination network. When an IP packet reaches a router, the router opens it and checks the destination IP address to see if it needs to be sent to another network. Let us look at the fields of a layer 3 IPv4 packet. The specific entry that provides the best match for the destination is used to determine where to send the packet next. When a packet is received, the network device looks inside the IP packet and compares it to the entries in the routing table. The source and destination IP addresses are included in every packet. The data in a routing table is used to find the most direct route for a packet to take to its final destination. Routers and switches use route tables to determine how to forward data packets through a network. In computer networking, a route is a set of instructions that specify how routers should forward IP data packets from one network device to another. The routes are then used to send packets across network links to the correct destination. What is a routing table, and why is it used?Ī routing table is a data table inside routers that stores and retrieves information about routes to specific IP destinations. You can also read about the OSI model and TCP/IP model to get a good grasp of the overall networking concepts. I would suggest reading my article on routing first. In this article, we will explain what is a routing table, the types of routes it contains, the ip routing table example, and the show ip route command in detail. We, as network engineers, frequently use the show ip route command to examine and manage the routing table on a router.
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