|Exam 400-101||Question id=729||Layer 3 Technologies|
Which of the following commands can you issue to limit EIGRP queries?
router eigrp as-number|
ip summary-address eigrp as-number address mask|
ip hello-interval eigrp as-number seconds|
ip hold-time eigrp as-number seconds|
You can issue the eigrp stub command or the ip summary-address eigrp as-number address mask command to limit Enhanced Interior Gateway Routing Protocol (EIGRP) queries. Query packets are sent to find feasible successors to a destination network. When a router does not have a feasible successor, it floods query packets to its neighbors. If a neighbor has a route to the destination network, it replies with the route. However, if a neighbor does not have a route to the destination network, it queries its neighbors, those neighbors query their neighbors, and so on. This process continues until either a router replies with the route or there are no routers left to query. The network cannot converge until all the replies have been received, which can cause a router to become stuck in active (SIA). Limiting EIGRP queries prevents queries from consuming bandwidth and processor resources and prevents routers from becoming SIA.
The eigrp stub command limits EIGRP queries by creating a stub router. Stub routers advertise only a specified set of routes and therefore typically need only a default route from the hub router. A hub router detects that a router is a stub router by examining the TypeLengthValue (TLV) field within EIGRP hello packets sent by the router. The hub router will specify in its neighbor table that the router is a stub router and will no longer send query packets to that stub router, thereby limiting how far EIGRP queries spread throughout a network.
The ip summary-address eigrp as-number address mask command limits EIGRP queries by configuring route summarization. If a neighbor router has a summarized route but does not have the specific route to the destination network in the query, the neighbor router will reply that it does not have a route to the destination network and will not query its neighbors. Thus route summarization creates a query boundary that prevents queries from propagating throughout the network.
You cannot limit EIGRP queries by issuing the router eigrp as-number command, which is used to create an EIGRP process for an autonomous system (AS). Queries are sent from neighbor to neighbor throughout a network, even from one AS to another. Therefore, creating a separate AS will not limit EIGRP queries.
You cannot limit EIGRP queries by issuing the ip hello-interval eigrp as-number seconds command, which is used to adjust the hello timer interval. By default, the hello timer is set to five seconds on high-bandwidth links and 60 seconds on low-bandwidth multipoint links slower than 1.544 Mbps.
You cannot limit EIGRP queries by issuing the ip hold-time eigrp as-number seconds command, which is used to adjust the hold timer interval. The hold timer is set to three times the hello timer value by default. Therefore, the hold timer is typically set to 15 seconds on high-bandwidth links and 180 seconds on low-bandwidth multipoint links. If you adjust the hello timer values, you must also adjust the hold timer values because they are not adjusted automatically.