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Name:MooreJoin In:Feb 21, 2019
CCNP: BGP PROTOCOL BEST PATH SELECTION ALGORITHM

Post by Moore » Dec 18, 2018

It is a path vector routing protocol, that is, its job is to exchange routing information between autonomous systems in order to discover the most efficient path to access somewhere on the Internet. Each BGP router is connected to one or more routers in the vicinity by neighbor names. Once the neighbor relationship is established, these BGP routers exchange routing information with each other. According to my latest statistics, there are about 125,000 routing information on the entire Internet, so a powerful router is required to receive all BGP routing information. CCNP question bank
What is the BGP best path selection algorithm?
Since the BGP routing table of the entire Internet has more than 200,000 routes, and a BGP router may receive multiple routing tables from multiple sources, there is definitely a way to compare different BGP routing tables and choose the best one. Routing scheme. This method is the BGP best path selection algorithm.
As you may notice, CiscoBGP routers use the application weight as the first standard for routing tables, unlike other brands of routers. The standards referenced are detailed in Cisco s official BGP Best Path Selection Algorithm document. Next I will list each standard and give explanations and examples.
By default, BGP best paths are based on the principle of the shortest autonomous system (AS). But many times, standards such as weight, localpreference, and MED are set by the network administrator.
Next, we will introduce these standards according to the reference order of BGP to choose the best path:
#1 Weight —The weight is a custom parameter Cisco sets for the local router and does not change with the router update. If there are multiple paths to an IP address (which is common), BGP will look for the path with the highest weight. There are many reference factors for setting weights, including neighbor commands, as-path access lists, or route mirroring.
#2 Local Preference — The local egress priority parameter tells the AS which path has local priority. The higher the value, the higher the priority. The default is 100. For example:
Bgp default local-preference 150 #3 Network or Aggregate—This parameter selects the locally initiated network or aggregate as the path. Adding a specific path to the route will make the route more efficient and save network space. For more information on aggregation, refer to the Cisco article "UnderstandingRouteAggregation in BGP."
#4 Shortest AS_PATH — BGP uses this parameter only when weight, localpreference, and localorioriated are fairly close.
#5 Lowest origin type — This parameter handles the Interior Gateway Protocol (IGP) protocol with a lower priority than the Exterior Gateway Protocol (EGP) protocol.
#6 Lowest multi-exit discriminator (MED) — The lower MED value is better than the higher MED value.
#7 eBGP over iBGP — Similar to #5, BGP AS Path prefers eBGP instead of iBGP.
#8 Lowest IGP metric — This parameter tends to use the lowest IGP as the BGP next hop.
#9 Multiple paths — This parameter determines whether multiple paths are to be loaded in the routing table. See BGPMultipath for more information.
#10 External paths — When all paths are external, select the path first received (the older path).
#11 Lowest router ID — Select the path from the BGP router with the lowest router ID.
#12 Minimum cluster list — If the originator or router ID of multiple paths is the same, select the path with the shortest cluster list length.
#13 Lowest neighbor address — This is the path from the lowest neighbor address. Source: Exam-C Some commands require constant use of BGP parameters, they are show ip bgp and show ipbgpsummary. In fact, I prefer to create aliases for these two commands so that they can be used more quickly (see my article "Importing Commands More Effectively with Cisco Command Alias"). These commands allow you to quickly get BGP and relationships with routers. The following is an example of the situation after using the show ip bgp command:CCNP question bank
RTA# show ip bgp BGP table version is 14, local router ID is 203.250.15.10 Status codes: s suppressed, d damped, h history, * valid,>best, i -internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *>i128.213.0.0 128.213.63.2 0 200 0 200 i > 192.208.10.0 192.208.10.5 0 300 0 300 i *>i200.200.0.0/16 128.213.63.2 200 0 200 400 i >i203.250.13.0 203.250.13.41 0 100 0 i *>i203.250.14.0 203.250.13.41 0 100 0 i *> 203.250.15.0 0.0.0.0 0 32768 i The asterisk is the best path chosen by the BGP best path selection algorithm.
to sum up Whether you are using BGP network management, or just learning BGP students for CCIE or CCNP exams, knowing this knowledge is very beneficial. As you can see above, using parameters and the command line does allow you to quickly implement the required functionality.
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