router A would increase, and the load inbound on router B would decrease. The problem is that if the inbound load for router A spikes to more than 100 percent and causes the link to flap, all the sessions crossing that link could be lost. If these sessions were purchases being made on AS 65001 web servers, revenue would be lost, which is a result that administrators want to avoid. If the load averages below 50 percent for the outbound or inbound case, path manipulation might not be needed; however, when a link starts to reach the capacity of the link for an extended period of time, more bandwidth is needed or path manipulation should be considered.
Content 6.6 Manipulating BGP Path Selection with Route Maps 6.6.3 Changing the BGP Local Preference for All Routes Local preference is used only within an autonomous system between IBGP speakers to determine the best path to leave the autonomous system to reach an outside network. The local preference is set to 100 by default; higher values are preferred. The bgp default local-preference command changes the default local preference value. Figure displays the command parameter. With this command, all IBGP routes that are advertised have the local preference set to the value specified. If an EBGP neighbor receives a local preference value, the EBGP neighbor ignores it.
Content 6.6 Manipulating BGP Path Selection with Route Maps 6.6.4 BGP Local Preference Example Figure illustrates an example network running BGP to demonstrate the manipulation of local preference using route maps in AS 65001. The best path to network 172.16.0.0 in AS 65003 from router C in AS 65001 is determined in the following way: The best path from router C to networks in AS 65005 is also selected by Step 4. The shortest path from router C to AS 65005 is through router B, because it consists of one AS (65005) compared to four autonomous systems (65002, 65003, 65004, 65005) through router A. The best path from router C to networks in AS 65004 is also selected by Step 4. The shortest path from router C to AS 65004 is through router B, because it consists of two autonomous systems (65005, 65004) compared to three autonomous systems (65002, 65003, 65004) through router A.
Content 6.6 Manipulating BGP Path Selection with Route Maps 6.6.5 BGP Local Preference Example (continued) Figure demonstrates the BGP forwarding table on router C in AS 65001 with only default settings for BGP path selection. The diagram shows only the networks of interest to this example: The best path is indicated with “>” in the second column of the output. Each network has two paths that are loop-free, synchronization-disabled, and that have a valid next-hop address. All routes have a weight of 0 and a default local preference of 100; thus Steps 1 and 2 in the BGP path selection process are equal. This router did not originate any of the routes (Step 3), so the process moves to Step 4, and BGP chooses the shortest autonomous system path as follows: Neither router A nor router B is using the next-hop-self option in this example. A traffic analysis reveals the following: The network administrator has decided to divert traffic to network 172.30.0.0 and send it out router A to the next hop of 192.168.28.1, so that the loading between routers A and B is more balanced.
Content 6.6 Manipulating BGP Path Selection with Route Maps 6.6.6 BGP Local Preference Example (continued) Figure demonstrates using a route map on router A to alter the network 172.30.0.0 BGP update from router X (192.168.28.1) to have a high local preference value of 400 so that it is more preferred. The first line of the route map is a permit statement with a sequence number of 10 for a route map called “local_pref”; this defines the first route-map statement. The match condition for that statement is checking all networks that are permitted by access list 65. Access list 65 permits all networks that start with the first two octets of 172.30.0.0. The route map sets those networks to a local preference of 400. The second statement of the route map is a permit statement with a sequence number of 20 for the route map local_pref, but it does not have any match or set statements. This statement is a permit–all statement for all route maps. Because there are no match conditions for the remaining networks, they are all permitted with their current settings. In this case, the local preference for network 172.16.0.0 and 172.24.0.0 stays set at the default of 100. The sequence number of 20 is chosen for the second statement in case other policies at a later date have to be implemented before this permit-all statement. This route map is linked to neighbor 192.168.28.1 as an inbound route map; therefore, as router A receives updates from 192.168.28.1, it processes them through the local_pref route map and sets the local preference accordingly as the networks are placed into the router A BGP forwarding table. Figure shows the BGP forwarding table on router C in AS 65001 after the BGP session has been reset. It illustrates that router C has learned about the new local preference value (400) coming from router A for network 172.30.0.0. The only difference in this table compared to the previous example, which did not have local preference manipulation, is that the best route to network 172.30.0.0 is now through 192.168.28.1 because its local preference of 400 is higher than the local preference of 100 for the next hop of 172.20.50.1. The autonomous system path through 172.20.50.1 is still shorter than the path through 192.168.28.1, but path length is not evaluated until Step 4, while local preference is examined in Step 2. The higher local preference path was chosen as the best path.
Content 6.6 Manipulating BGP Path Selection with Route Maps 6.6.7 Setting the MED with Route Maps Recall that the MED is used to decide how to enter an autonomous system. It is used when multiple paths exist between two autonomous systems and one autonomous system is trying to influence the incoming path from the other. Because the MED is evaluated late in the BGP path selection