Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 212 EIGRP Auto-Summarization EIGRP is a classless routing protocol that supports Variable Length Subnet Masks (VLSMs). The above example would pose no problem for EIGRP. However, EIGRP will still automatically summarize when crossing major network boundaries. For example, when Router A sends an EIGRP update to Router B via Serial0, by default it will still summarize the 10.1.0.0/16 network to 10.0.0.0/8. This is because the 10.1.0.0/16 and 192.168.123.0/24 networks do not belong to the same major network. Likewise, the 66.115.33.0/24 network will be summarized to 66.0.0.0/8. An auto-summary route will be advertised as a normal internal EIGRP route. The best metric from among the summarized routes will be applied to this summary route. The router that performed the auto-summarization will also add the summary route to its routing table, with a next hop of the Null0 interface. This is to prevent routing loops. This auto-summarization can be disabled: RouterA(config)# router eigrp 10 RouterA(config-router)# no auto-summary The no auto-summary command will prevent Router A from summarizing the 10.1.0.0/16 and 66.115.33.0/24 networks. (Reference: http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#summarization) * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 213 EIGRP Load-Balancing By default, EIGRP will automatically load-balance across equal-metric routes (four by default, six maximum). EIGRP also supports load-balancing across routes with an unequal metric. Consider the following example: Earlier in this section, we established that Router A would choose the route through Router D as its Feasible Distance to the destination network. The route through Router B became a Feasible Successor. By default, EIGRP will not load-balance between these two routes, as their metrics are different (11 through Router D, 16 through Router B). We must use the variance command to tell EIGRP to load-balance across these unequal-metric links: RouterA(config)# router eigrp 10 RouterA(config-router)# variance 2 RouterA(config-router)# maximum-paths 6 The variance command assigns a “multiplier,” in this instance of 2. We multiply this variance value by the metric of our Feasible Distance (2 x 11 = 22). Thus, any Feasible Successors with a metric within twice that of our Feasible Distance (i.e. 12 through 22) will now be used for load balancing by EIGRP. Remember, only Feasible Successors can be used for load balancing, not Possibilities (such as the route through Router C). The maximum-paths command adjusts the number of links EIGRP can loadbalance across. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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Cisco CCNA Study Guide - Router Alley

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