Which parameter does the router use to choose the path to the destination when there are multiple routes available?

IGPs select the path with the lowest metric. For example: • RIP selects the path with the lowest hop count. • OSPF selects the path with the lowest cost. • EIGRP selects the path with the highest bandwidth and lowest delay (unless you change the K values). BGP however, selects the best path based on a list of attributes. On the Internet, it’s more important that you have granular control over how you forward your traffic and to which autonomous systems instead of just going for the shortest path based on a metric. Let’s look at a quick example. Below I have the output of the BGP table of a oute-views.optus.net.au> show ip bgp BGP table version is 781755060, local router ID is 203.202.125.6 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale, m multipath, b backup-path, x best-external Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path * 1.0.0.0/24 203.202.143.34 0 7474 4826 13335 i * 192.65.89.161 1 0 7474 4826 13335 i * 202.139.124.130 1 0 7474 4826 13335 i * 203.13.132.7 10 0 7474 4826 13335 i *> 203.202.143.33 0 7474 4826 13335 i This BGP router has 5 paths for network 1.0.0.0/24. Look at the > symbol at the bottom left. The > symbol means that BGP has selected this path as the best path. This path will be installed in the routing table. Out of all those 5 paths, why did BGP select this path as the best path? Attributes This path was selected based on the following attributes: Prio...

Introduction This document describes how routers work, are configured, and how to select a route for them. Prerequisites Requirements There are no specific prerequisites for this document. Components Used This document is not restricted to specific software and hardware versions. The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command. Conventions For more information on document conventions, see the Background Information One aspect of Cisco routers is how the router chooses the best route among those presented by protocols, manual configuration, and various other means. Route selection requires some knowledge about the way Cisco routers work. Processes Involved There are three processes involved to build and maintain the routing table in a Cisco router: • Various routing processes, which actually run a network (or routing) protocol, such as Enhanced Interior Gateway Routing Protocol (EIGRP), Border Gateway Protocol (BGP), Intermediate System-to-Intermediate System (IS-IS), and Open Shortest Path First (OSPF). • The routing table itself, which accepts information from the routing processes and also replies to requests for information from the forwarding process. • The forwarding process, which requests information from the routing table to make a packet forwardi...

Introduction This document describes the function of the Border Gateway Protocol (BGP) best path algorithm. BGP routers typically receive multiple paths to the same destination. The BGP best path algorithm decides which is the best path to install in the IP routing table and to use for traffic forwarding. Why Routers Ignore Paths Assume that all paths that a router receives for a particular prefix are arranged in a list. The list is similar to the output of the show ip bgp longer-prefixes command. Routers ignore paths in these circumstances: • Paths that are marked as not synchronized in the show ip bgp longer-prefixes output. If BGP synchronization is enabled, there must be a match for the prefix in the IP routing table in order for an internal BGP (iBGP) path in order to be considered a valid path. BGP synchronization is enabled by default in Cisco IOS ® Software. If the route that matches is learned from an Open Shortest Path First (OSPF) neighbor, its OSPF router ID must match the BGP router ID of the iBGP neighbor. Most users prefer to disable synchronization with use of the Note:Synchronization is disabled by default in Cisco IOS Software Release 12.2(8)T and later. • Paths for which the NEXT_HOP is inaccessible. Be sure that there is an Interior Gateway Protocol (IGP) route to the NEXT_HOP that is associated with the path. • Paths from an external BGP (eBGP) neighbor if the local autonomous system (AS) appears in the AS_PATH. Such paths are denied upon ingress into ...

Last Updated on November 2, 2020 by Which parameter does the router use to choose the path to the destination when there are multiple routes available? • the lower metric value that is associated with the destination network • the lower gateway IP address to get to the destination network • the higher metric value that is associated with the destination network • the higher gateway IP address to get to the destination network Answers Explanation & Hints: When a packet arrives at the router interface, the router examines its header to determine the destination network. If there is a route for the destination network in the routing table, the router forwards the packet using that information. If there are two or more possible routes to the same destination, the metric is used to decide which route appears on the routing table. The lower the metric, the better the route. For more question and answers: Click Here Modules 8 – 10: Communicating Between Networks Exam Answers Full 100%

Whenever possible, Routers should have multiple paths (or routes) to get to a target network — this is a key component for network resiliency. Which means Routers routinely compare routes to choose one preferred path among multiple that may exist. The Route Precedence process is accomplished by comparing three different attributes. This video steps through the three step process sequentially, and also shows you what a Router does when all the attributes are identical. The three attributes for Route Preference are the Route Specificity, the Administrative Distance, and the Metric. If multiple routes exist and all three of these attributes are identical, the Router will load balance across the available paths. The topology involves R1 with four possible paths (through R2, R3, R4, and R5) to get to the 9.9.9.128/25 network behind R6. Each path is sequentially enabled to show you exactly what R1 is comparing to choose one best path among the multiple that will exist. Load Balancing is the first scenario illustrated in the video. Initially, the OSPF paths from R1 to 9.9.9.128/25 through R5 and R4 are identical, and R1 will load balance its packets across both links. Then, we’ll tweak the Metric on the link between R5 and R6 and show you that R1 still knows of both the paths through R4 and R5, but chooses the path with the lowest Metric. Following that, we’ll enable the advertisement from R3 via EIGRP and show you how R1 compares the Administrative Distance to choose between an ...