|A and B
|A and D
|A and D
|B and C
|C and D
|A, B, C, D
|If the router does not have a feasible successor in its topology table, it sends a query packet to its neighbors asking whether they have a feasible successor.
|The timers that the neighbor table keeps track of are the holdtime, the SRTT, and the RTO.
|An update is the routing information packet that a router will send out to inform its neighbors of a change in the network. In a query, the router has no feasible successor in its topology table for a network that is down. At this point, it queries its neighbors to ascertain whether they have a feasible successor. If they do, this route becomes the feasible successor for the original router.
|EIGRP recalculates the topology table whenever it receives a change input to the topology table. This could be a change of metric for a physically connected link; a change of status of a physically connected link; or an EIGRP routing packet—either an update, a query, or a reply packet.
|The rules for scaling an EIGRP network include the following:
|The packets that EIGRP sends reliably are updates, queries, and replies. EIGRP uses RTP. This is necessary because EIGRP does not send out periodic updates, and the RTP mechanism ensures a loop-free synchronized network.
|The holdtime is three times the Hello timer. The holdtime is 15 seconds on a LAN or 180 seconds on a low-speed WAN.
|The topology table contains all links received with a metric other than infinity, in other words, every valid path. The topology table does not include all links because of split horizon. The topology table holds the metric for every path along with the metric from the next logical hop or neighbor. The table contains the outgoing interface on the router through which to reach the remote network and the IP address of the next-hop address. The status of the route (passive or active) is also recorded. The topology table also keeps track of the routing protocol packets that have been sent to the neighbors.
|Advertised distance is the metric that is reported by the neighbor routers. Feasible distance is the metric that is reported by neighbor routers, plus the cost associated with the forwarding link from the local interface to the neighbor routers.
|The Diffusing Update Algorithm (DUAL) is run on the topology table. It is used to determine the best path and to build the routing table.
|EIGRP places a network into active mode when there is no feasible successor in its topology table.
|By default, EIGRP summarizes at the classful network boundary.
|Stuck in Active (SIA) is a state in which a router will place a route after it has failed to hear a reply to a query that was sent to a neighbor.
EIGRP sends a query when a route is lost and another feasible route does not exist in the topology table. The SIA is caused by two sequential events: First, a route has gone away. Second, an EIGRP neighbor (or neighbors) has not replied to the query for that route. When the SIA occurs, the router removes the neighbor that has not replied to the query from the neighbor table.
|A hierarchical tiered design and summarization are both critical to being able to scale an EIGRP network. If these are in place, it is possible to summarize the network, which reduces the network resources needed for large tables and limits the query range of the router.
It is also important to ensure that the router has sufficient memory, the network has sufficient bandwidth on its WAN links, and, where appropriate, the bandwidth command has been configured.
|The reply packet is used to update the topology table. It is a response to a query sent out by a neighbor asking about suspect routes.
|To become a neighbor, the following conditions must be met: