Cisco CCNA Study Guide - Router Alley

More documents

Recommendations

Info

CCNA Study Guide v2.62 – Aaron Balchunas 204 The EIGRP Topology Table Once EIGRP neighbors form adjacencies, they will begin to share routing information. Each router’s update contains a list of all routes known by that router, and the respective metrics for those routes. All such routes are added to an EIGRP router’s topology table. The route with the lowest metric to each network will become the Feasible Distance (FD). The Feasible Distance for each network will be installed into the routing table. The Feasible Distance is derived from the Advertised Distance of the router sending the update, and the local router’s metric to the advertising router. Confused? Consider the following example: Router A has three separate paths to the Destination Network, either through Router B, C, or D. If we add up the metrics to form a “distance” (the metrics are greatly simplified in this example), we can determine the following: • Router B’s Feasible Distance to the Destination Network is 8. • Router C’s Feasible Distance to the Destination Network is 23. • Router D’s Feasible Distance to the Destination Network is 9. Router B sends an update to Router A, it will provide an Advertised Distance of 8 to the Destination Network. Router C will provide an AD of 23, and D will provide an AD of 9. Router A calculates the total distance to the Destination network by adding the AD of the advertising router, with its own distance to reach that advertising router. For example, Router A’s metric to Router B is 8; thus, the total distance will be 16 to reach the Destination Network through Router B. * * * 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 205 The EIGRP Topology Table (continued) Remember, however, that Router A’s Feasible Distance must be the route with the lowest metric. If we add the Advertised Distance with the local metric between each router, we would see that: • The route through Router B has a distance of 16 to the destination • The route through Router C has a distance of 27 to the destination • The route through Router D has a distance of 11 to the destination Thus, the route through Router D (metric of 11) would become the Feasible Distance for Router A, and is added to the routing table as the best route. This route is identified as the Successor. To allow convergence to occur quickly if a link fails, EIGRP includes backup routes in the topology table called Feasible Successors (FS). A route will only become a Successor if its Advertised Distance is less than the current Feasible Distance. This is known as a Feasible Condition (FC). For example, we determined that Router A’s Feasible Distance to the destination is 11, through Router D. Router C’s Advertised Distance is 23, and thus would not become a Feasible Successor, as it has a higher metric than Router A’s current Feasible Distance. Routes that are not Feasible Successors become route Possibilities. Router B’s Advertised Distance is 8, which is less than Router A’s current Feasible Distance. Thus, the route through Router B to the Destination Network would become a Feasible Successor. Feasible Successors provide EIGRP with redundancy, without forcing routers to re-converge (thus stopping the flow of traffic) when a topology change occurs. If no Feasible Successor exists and a link fails, a route will enter an Active (converging) state until an alternate route is found. * * * 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.
Page 1 and 2:
CCNA Study Guide v2.62 - Aaron Balc
Page 3 and 4:
Page 5 and 6:
Page 7 and 8:
Page 9 and 10:
Page 11 and 12:
Page 13 and 14:
Page 15 and 16:
Page 17 and 18:
Page 19 and 20:
Page 21 and 22:
Page 23 and 24:
Page 25 and 26:
Page 27 and 28:
Page 29 and 30:
Page 31 and 32:
Page 33 and 34:
Page 35 and 36:
Page 37 and 38:
Page 39 and 40:
Page 41 and 42:
Page 43 and 44:
Page 45 and 46:
Page 47 and 48:
Page 49 and 50:
Page 51 and 52:
Page 53 and 54:
Page 55 and 56:
Page 57 and 58:
Page 59 and 60:
Page 61 and 62:
Page 63 and 64:
Page 65 and 66:
Page 67 and 68:
Page 69 and 70:
Page 71 and 72:
Page 73 and 74:
Page 75:
Page 78 and 79:
Page 80 and 81:
Page 82 and 83:
Page 84 and 85:
Page 86 and 87:
Page 88 and 89:
Page 90 and 91:
Page 92 and 93:
Page 94 and 95:
Page 96 and 97:
Page 98 and 99:
Page 100 and 101:
Page 102 and 103:
Page 104 and 105:
Page 106 and 107:
Page 108 and 109:
Page 110 and 111:
Page 112 and 113:
Page 114 and 115:
Page 116 and 117:
Page 118 and 119:
Page 120 and 121:
Page 122 and 123:
Page 124 and 125:
Page 126 and 127:
Page 128 and 129:
Page 130 and 131:
Page 132 and 133:
Page 134 and 135:
Page 136 and 137:
Page 138 and 139:
Page 140 and 141:
Page 142 and 143:
Page 144 and 145:
Page 146 and 147:
Page 148 and 149:
Page 150 and 151:
Page 152 and 153:
Page 154 and 155: CCNA Study Guide v2.62 - Aaron Balc
Page 254 and 255:
Page 256 and 257:
Page 258 and 259:
Page 260 and 261:
Page 262 and 263:
Page 264 and 265:
Page 266 and 267:
Page 268 and 269:
Page 270 and 271:
Page 272 and 273:
Page 274 and 275:
Page 276 and 277:
Page 278 and 279:
Page 280 and 281:
Page 282 and 283:
Page 284 and 285:
Page 286 and 287:
Page 288 and 289:
Page 290 and 291:
Page 292 and 293:
Page 294 and 295:
Page 296 and 297:
Page 298 and 299:
Page 300 and 301:
Page 302 and 303:
Page 304:
show all

Cisco CCNA Study Guide - Router Alley

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?