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46 Cards in this Set
- Front
- Back
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What are the 5 packet types in EIGRP, and which must be acknowledged?
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Hello - identifies neighbors, Update - advertises routes, ACK - Acknowledges receipt of an update, Query - Asks about routes where previous best path has been lost, Reply - Answers a query. Update, query and reply packets must be acknowledged.
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Whats the metric used to work out path cost?
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256 x (10,000,000/min bw + delays/10)
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What is feasible distance?
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Is the metric that is reported by the neighbors routers, plus the cost associated with the forwarding link from the local interface to the neighbors router.
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What is advertised distance?
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Is the metric that is reported by the neighbors routers. The neighbor with the best path is referred to as the successor.
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What is the feasible requirement?
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Is when the AD of the alternative path is less than the FD of the best path. Neighbors that meet the feasible requirement are called feasible successors. In emergencies EIGRP knows that switching to the FS will not cause loops so can do so immediately without the need to send queries.
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What happens if a router doesn't have a feasible successor?
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It sends a query packet to its neighbors asking if they have one. If they do, this route becomes the feasible successor for the original router
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What settings must be the same for routers to become neighbors?
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To become a neighbor the routers must hear hello packets, and the AS number and K-values must be the same. Each L3 protocol supported by EIGRP (ipv4, IPv6, IPX, etc) has its own neighbor table. Information about routes or costs are not shared between protocols.
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What does spilt horizon do?
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It stops a router advertising a network back on the link from which it learned that network
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What does the topology table contain and what does it do?
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Topology table has a record of not only feasible successors and successors, but also of all received routes (with "all-links" keyword). The other routes are referred to as possibilities. It also holds the metric of every path, metric of the next hop, the status of the route (passive or active) and also keeps track of update, query and reply packets sent and received. It manages the selection of routes to be added to the routing table.
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When does EIGRP recalculate the topology table?
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EIGRP recalculates the topology table whenever it receives a change input to the topology table. This could be a change of metric or status of a physically connected link, or an EIGRP routing packet (either update, query or reply)
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What is an update packet?
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Is the routing information packet that a router will send out to inform its neighbors of a change in the network. Eigrp only send updates when a new route is advertised or an existing route is withdrawn (changes state to down).
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When does a EIGRP network send queries?
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When new networks are added or advertisements withdrawn, routers may use queries to ask each other for additional information, allowing EIGRP to converge quickly even when there is not a feasible successor. It uses incremental updates. The query process allows EIGRP to avoid periodically transmitting entire routing tables.
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What is Diffusing Update Algorithm (DUAL)?
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It is a modification of the way distance vector routing typically works that allows the router to identify loop-free failover paths. When the router has an understanding of the topology, it runs DUAL to determine the best path to the remote network. The result is entered into the routing table.
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What does Stuck in Active (SIA) mean?
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Queries continue to propagate until an answer is found or no one is left to ask. When they are produced, the router changes to an active state and sets a timer (typically 3mins). If the timer expires before an answer comes back the router is considered SIA. The neighbors that fail to reply are removed from the neighbor table.
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What is the difference between Passive and Active routes?
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A healthy network is marked as passive, an unhealthy one as active. A lost route is active because the router is actively attempting to find an alternative path to the remote network.
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How is the routing table built?
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The Routing table is built from the topology table using DUAL. The topology table holds all routing information known to the router and from this information successors and feasible successors are selected. Successor paths are then transferred to the routing table and used as a basis for routing decisions.
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How can you prevent SIA?
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Summarizing at logical points in the network is important to prevent SIA. Route convergence cant proceed until all replies have been received, and summarization limits these queries.
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Where does EIGRP summarize by default?
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By default at the classful network boundary, but can be manually applied at any point in the network and configured to summarize on any router interface. Upstream routers should receive a consolidated route, and downstream a default route.
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What are the benefits of using summarization?
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Summarization limits advertisements, minimizes the size of advertisements, and optimizes table lookups and calculations. It also reduces the scope of the queries sent by the router.
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Where is it possible to summarize?
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Summarization is possible over all interfaces in EIGRP as long as the addressing scheme allows for it to be implemented. This is one of the major advantages of eigrp over OSPF which can only summarize at ABRs.
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Whats the default hold timer?
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It is 3 times the hello, so on a normal LAN is 15 secs, or 180 secs on a low speed LAN
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Whats the difference between the "passive interface" and the "network mask" techniques?
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The "passive interface" cmd forces an interface to be quiet, EIGRP doesn't speak on it (no hellos or advertisements), but it still advertises the connected network out to other interfaces. Interfaces left out of "network mask" don`t speak EIGRP and subnets attached to those interfaces are not advertised.
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When is a stub configuration typically used?
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On small capacity routers in a hub-and-spoke WAN environment. The router has a limited set of neighbors and doesn't need to be involved in convergence. It isn't therefore necessary for this remote router to have a complete routing table that might overwhelm it. It needs only a default route to the dist router that can serve all its needs. It also optimizes all queries in the network, since when a query is sent to the router it will immediately respond, thus accelerating the process.
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To enable EIGRP on a router. Routers in the same Administrative domain should be identified and grouped by a common AS number.
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router(config)# router eigrp [AS num]
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To advertise a classful network
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router(config-router)# network [network num]
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To advertise specific interfaces, classless. Routes that match the network and wildcard mask will run EIGRP and those subnets will be advertised.
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router(config-router)# network [network num] [wildcard mask]
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To disable automatic summarization. All known subnets will be advertised out of every interface, limited only by split-horizon
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router(config-router)# no auto-summary
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To announce a specific range on a particular interface (summarize).
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router(config-if)# ip summary-address eigrp [AS num] [address] [mask]
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To configure a stub router, which uses EIGRP to send limited info between the stub and core routers. The stub router has only 1 neighbor, a dist layer router. Receive-only=prevents the router from advertising routes, Connected (default)=Permits advertisement of connected routes, Static= Permits redistribution of static routes, Summary(default)=Advertises summary routes
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router(config-router)# eigrp stub [receive-only | connected | redistributed | static | summary]
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To change the hello timer, default for high b/w links 5 secs, low b/w links 60 secs. Timer values aren't communicated between neighbors and aren't a requirement for neighborship. They are locally specific and changed on a per interface basis.
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router(config-if)# ip hello-interval eigrp [AS num] [seconds]
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To change the hold timer, which is how long the router waits for a hello before pronouncing a neighbor unavailable. Default is 3 times the hello timer.
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router(config-if)# ip hold-time eigrp [AS num] [seconds]
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To adjust the percentage of b/w that EIGRP will use for routing protocol traffic. By default, uses no more than half of the link capacity. This is normally sufficient.
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router(config-if)# bandwidth-percent eigrp [AS] [%]
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To change the b/w setting. It doesn't actually affect the actual speed of the link, the value is only used for routing protocol calculations and load calculations
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router(config-if)# bandwidth [speed-of-the-line]
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EIGRP automatically does proportional unequal-cost load sharing. When the variance is used, the metric of the best path is multiplied by the variance. Default is 1, which means equal-cost load balancing. If it is higher than 1, all FS paths to the same dst with a metric of less than the result of the equation are now included in load-balancing. Any Feasible successor in the topology table can be used in the variance cmd.
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router(config-router)# variance [multiplier]
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To check the links bandwidth.
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router# show interface
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To show detailed information about neighbors. It records the communication between the router and neighbors in addition to the interface and address by which they communicate.
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router# show ip eigrp neighbors
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To show details about the routes held in the topology table, detailed info on the networks that the router is aware of and the preferred paths of those networks, and the next hop in the path. Also allows the router to track EIGRP packets that have been sent to neighbors and the effect of DUAL
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router# show ip eigrp topology [AS | Ip add & mask]
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To see information about the aggregated traffic sent to and from the EIGRP process. Gives details of hellos, updates, queries, replies and Acks sent/received
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router# show ip eigrp traffic
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Shows the EIRGP packets sent and received by the router. The packet types to be monitored can be selected. Upto 11 types are available.
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router# debug ip eigrp packet
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Shows the hello packets sent and received by the router and the neighbors discovered by this process
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router# debug ip eigrp neighbors
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Shows dynamic changes made to the routing table
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router# debug ip eigrp
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Shows the process taken when a summary (manual or auto) is changed on the router.
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router# debug ip eigrp summary
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To specify MD5 authentication
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router(config-if)# ip authentication mode eigrp [AS] md5
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Once MD5 is specified, use to set the key
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router(config-if)# ip authentication key-chain eigrp [AS] [chain-name]
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To configure the key chain (the name)
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router(config)# key-chain [chain-name]
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To configure the key-id (the number)
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router(config-keychain)# key [key-id]
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