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Do I Know This Already? For instance, an address might be written Another way of expressing the same address is to create a subnet mask where 1 shows the position of the network portion and 0 shows the host portion.
Consider a case where three computers need to communicate, as shown in Figure To determine the topology, an IP device takes a bit-wise binary AND of its own address and subnet mask and compares it to an AND of the destination address. PC A PC B Because PC C is on the PC C The following procedure may be used to determine the range of addresses supported by a network. Step 2 To determine the network address, copy the network bits from the address as shown by the CIDR notation.
Fill in the remaining bits with zeros. Step 3 The last address in the range is the broadcast address. Step 4 The usable set of addresses on this network falls between these two numbers. There should be 2n-2 host addresses, where n is the number of host bits. As an example, consider PC C The usable set of addresses on this network falls between these two numbers from 96 to , so addresses from Understanding Summarization This section describes the process of summarization.
Summarization is the technique of grouping IP networks together to minimize advertisements. For instance, imagine that a division's network consisted of the subnets To advertise each network using a routing protocol, the division will send advertisements to other divisions. To extend the example, consider Figure There are many routers in this company, but the three routers shown are the three that tie the divisions together.
If each router announces every route in its division, there will be advertisements! As an alternative, Router A could advertise One of the keys to scalable routing is to take large complicated sets of advertisements and reduce them as much as possible. Summarization reduces router resource consumption CPU and memory required to store and process routes by reducing the number of routes. Summarization also saves network capacity, because fewer advertisements are required and each advertisement is smaller.
In Figure , imagine that the In an unsummarized network, Router C has to advertise In a summarized network, Router C does not pass on this level of detail. This might strike you as counter-intuitive, but IP devices are capable of recognizing when responses are not received. Convergence is sped up in a summarized network because each router has a smaller set of routes to consider, because each router can receive updates faster, and because each router has fewer routes to process.
However, it is important that you understand how to compose the summary address. The method for determining the summary is Step 1 Write each network in binary. Step 2 Determine the number of bits that match. This gives a single summary that includes all the routes, but may include a range of addresses that is too large also called over-summarization.
Take the remaining addresses and start this process again. Suppose a network is composed of the links Following the procedure: Step 1 Write each network in binary. A summary of On the BSCI exam, you may see cases where a range of addresses can be summarized in a neat and tidy fashion, just like the previous example.
In the real world, there will be times when you will need to go a step further. One more example will help you in those cases. This summarizes So that will be one advertisement. We take the remaining portion of addresses and start again. Step 4 Write each network in binary. Step 5 Determine the number of bits that match. Step 6 Because step 2 did not over-summarize, the process is complete.
Two advertisements This process results in advertising Address Planning Summarization is not possible as an after-thought. When designing a network, it is extremely important that careful attention is paid to the requirements for summarization. Figure shows that an example corporation might have multiple levels of summarization— within plants, within manufacturing groups, at the divisional level, and to the Internet.
In particular, notice that the Internet is being summarized back to the company as a default route. If the mask is dotted decimal, convert it to CIDR. To determine the network address, copy the network bits from the address. The last address is the broadcast. The usable addresses fall between these two numbers. To check, subtract the CIDR length from 32 to determine the number of host bits. There are 2n—2 host addresses. This gives a single summary that includes all the routes.
Convert the following numbers to binary: — — — — 2. Convert the following binary numbers to decimal: — — — — 3. What is the binary representation of Specify the IP address class for each of the following addresses: — For each address in the following list, compute the range of host addresses found on its subnet: — Summarize the following addresses without over-summarizing: — EIGRP is sometimes referred to as a hybrid routing protocol, although advanced distance vector routing protocol is probably a more accurate description.
Its ability to scale is limited only by the design of the network. EIGRP is designed for use in large networks. As a proprietary routing protocol for Cisco, it is therefore an obligatory subject in a Cisco exam on IP routing protocols. Multicast b. Best effort unicast c. A reliable unicast d. Rapid convergence b. Reduced bandwidth consumption c. Link state routing d. Select the two correct components from the following list.
Neighbor discovery b. SPF algorithm c. Areas d. Bandwidth b. Reliability d. Authentication must be enabled. The K-values of the metric must be the same on both routers. The autonomous system number must be the same on both routers. The holddown timer must be the same. What do the letters SIA stand for? Stuck in Active b. Shortest IP Address c. Stuck in Area d. The administrative distance b.
The metric of neighbors c. The feasible distance between neighbors d. Shortest Remote-Trip Time b. Smooth Round-Trip Time c. Shortest Reliable-Trip Time d.
Which of the following would trigger the topology table to recalculate? LSP received b. SRT packet received c. A new router coming online d. Link loss detected Feasible successor information is stored in which table? Topology b. Routing c. Neighbor d. When the router is actively forwarding b. When the router is actively recalculating paths c. When the router is searching for replacement paths d. When the router is discovering neighbors The factors that can affect the scaling of EIGRP include which of the following?
The amount of information sent between neighbors b. The number of routers that receive updates c. The distance between neighboring routers d. EIGRP is scalable in terms of hardware resources and network capacity. EIGRP is also lightning fast. The two that are of interest today are IP and IPv6. Reliable, in a networking context, means that the receiver acknowledges that the transmission was received and understood. These packets are directly encapsulated by IP.
Hellos are sent as periodic multicasts and are not acknowledged directly. Updates are sent as multicasts only when there is a change. If an update indicates that a path is down, multicast queries are used to ask other neighbors if they still have a path. If the querier does not receive a reply from each of its neighbors, it repeats the query as a unicast to each unresponsive neighbor until it either gets a reply or gives up after sixteen tries.
Each neighbor responds to the query with a unicast reply indicating an alternative path or the fact that it does not have a path. EIGRP produces hellos periodically. If hellos are missed over a long period of time—the hold time—then the neighbor is removed from the EIGRP table and routing reconverges. EIGRP starts by discovering its neighbors. Advertisements are multicast, and individual unicast acknowledgements come back. The neighbor table is used to make sure that each neighbor responds.
Unresponsive neighbors receive a follow-up unicast copy, repeatedly, until they acknowledge. If a neighbor is still unresponsive after 16 attempts, the neighbor is removed from the neighbor table and EIGRP continues with its next task. Presumably, the neighbor will at some point be able to communicate. When it is able to do so, it will send a hello and the process of routing with that neighbor will begin again.
That metric is metric K1 bandwidth K2 bandwidth load K3 delay K5 reliability K4 EIGRP Features and Advantages 65 Although this equation looks intimidating, a little work will help you understand the math and the impact the metric has on route selection. To do that it uses K-values to balance bandwidth and delay. The K-values are constants that are used to adjust the relative contribution of the various parameters to the total metric. In other words, if you wanted delay to be much more relatively important than bandwidth, you might set K3 to a much larger number.
Because routing protocols select the lowest metric, inverting the bandwidth using it as the divisor makes faster paths have lower costs. Both are multiplied by a zero K-value, so neither is used. For example, serial links have a delay of 20, microseconds and Ethernet lines have a delay of microseconds. EIGRP uses the sum of all delays along the path, in tens of microseconds. Therefore, given the default K-values the equation becomes metric 1 bandwidth bandwidth 0 bandwidth load 1 delay 0 reliability 0 delay Substituting the earlier description of variables, the equation becomes 10,, divided by the chokepoint bandwidth plus the sum of the delays: metric min bandwidth delays 10 An example of the metric in context will make its application clear.
There really is not a compelling reason to change the default K-values and Cisco does not recommend it. This concept is easier to grasp if you imagine it geographically. Consider the map of North Carolina shown in Figure The numbers show approximate travel time by car, in minutes. Pretend that you live in Hickory. From Hickory, you need to determine the best path to Raleigh. Each neighbor advertises its cost travel time to get to Raleigh and the cost Hickory would use.
The cost from the neighbor to the destination is called the advertised distance. The cost from Hickory is called the feasible distance. Table Feasible and Advertised Distance City Feasible Distance Advertised Distance Asheville Charlotte Greensboro 60 Hickory will select the route with the lowest feasible distance, which is the path through Greensboro.
If the Hickory-Greensboro link goes down, Hickory knows it may fail-over to Charlotte without creating a loop. Notice that the distance from Charlotte to Raleigh minutes is less than the distance from Hickory to Raleigh minutes.
Because Charlotte is closer to Raleigh, routing through Charlotte does not involve driving to Charlotte and then driving back to Hickory before going to Raleigh.
Charlotte is a guaranteed loop-free path. Neighbors that meet the feasibility requirement are called feasible successors. In emergencies, EIGRP understands that using feasible successors will not cause a routing loop and instantly switches to the backup paths. Notice that Asheville is not a feasible successor. For all we know, driving to Raleigh through Asheville involves driving from Hickory to Asheville, then turning around and driving back to Hickory before continuing on to Raleigh in fact, it does.
Asheville will still be queried if the best path is lost and no feasible successors are available because potentially there could be a path that way; however, paths that do not meet the feasibility requirement will not be inserted into the routing table without careful consideration.
Now consider how DUAL works in terms of routers and networks. A feasible successor is a backup path, and it can be substituted for a lost path at any point. When a path is lost and no feasible successor exists, the router will send queries to its remaining neighbors.
If a neighbor does not know of an alternative path, it will recursively ask its neighbors. Recursive queries can loop without being resolved, forcing the router to time-out the query.
This situation is known as stuck in active SIA. Fortunately, it is uncommon; understanding its causes can prevent it entirely. EIGRP uses split-horizon, which says that a router should not advertise a network on the link from which it learned about the network. As shown in Figure , because Router A learned about If the link between B and C goes down, B loses its only path to The query process allows B to actively search remaining neighbors for a replacement route.
Router E has a route and so replies to A, which passes the news on to B. Queries continue propagating until an answer is found or until no one is left to ask. When queries are produced, the router changes to an active state and sets a timer typically three minutes. If the timer expires before an answer comes back then the router is considered stuck in active. SIA typically occurs because queries loop or are not properly limited to an area.
The primary way to limit how far queries travel called query scoping is to summarize. Queries will not cross summarization because the summary answers the query—the route is either behind the summary or not. When new networks are added or advertisements are withdrawn, routers may ask each other for additional information, allowing EIGRP to converge quickly even when there is not a feasible successor.
When a route is added or withdrawn, an incremental update is sent that includes only those changes. This is an important feature because it prevents EIGRP from monopolizing link access, which was occasionally a problem with older protocols. These packets are sent with sequence numbers to make the transmission of data reliable.
Hellos and ACKs do not require acknowledgement. Incremental updates cannot be anticipated; therefore, update, query, and reply packets must be acknowledged by the receiving neighbor.
Updates are sent using a reliable multicast. The address is the reserved class D address, When the neighbor receives a multicast, it acknowledges receipt of the packet with an unreliable unicast. The use of multicast by EIGRP to send updates is also important because it represents an improvement over other protocols.
Older protocols used broadcast, which created issues. Although they might not be directly connected to the same physical cable, if they are in a switched environment, from a logical Layer 2 or Layer 3 perspective, they are on the same link.
If a broadcast is sent out, all the devices within the broadcast domain will hear the message and will expend resources determining whether it is addressed to them. EIGRP is unique in its support for unequal-cost load sharing. Unequal-cost load balancing takes the best FD and multiplies it by variance. Any other path with an FD less than this product is used for load sharing. That is exciting because now a kbps link and a kbps link can work together—but EIGRP actually goes one better than that.
EIGRP does proportional unequal-cost load sharing. This allows all links to a destination to be used to carry data without saturating the slower links or limiting the faster links.
A neighbor table is used to make sure all acknowledgements are received. A topology table is used to understand paths through the network. Finally, the best paths from the topology table are fed into the IP routing table.
Creating the Neighbor Table The neighbor table is maintained by means of Hello packets. Hello packets are multicast announcements that the router is alive. Reciprocal hellos build the local neighbor table. Information about neighbors, routes, or costs is not shared between protocols. Holdtime is three times the value of the Hello timer by default. The neighbor table tracks all the packets sent between the neighbors. It tracks both the last sequence number sent to the neighbor and the last sequence number received from the neighbor.
SRTT is the time in milliseconds that it takes a packet to be sent to a neighbor and a reply to be received. On hearing Hellos, the receiving routers add an entry in their neighbor table. The continued receipt of these packets maintains the neighbor table.
If a Hello from a known neighbor is not heard within the holdtime, the neighbor is treated as no longer operational and removed from the table. The holdtime, by default, is three times the Hello timer. Therefore, if the router misses three Hellos, the neighbor is declared dead. The Hello timer on a LAN is set to 5 seconds; the holdtime, therefore, is 15 seconds. On DS1 1. Example demonstrates a neighbor table. The topology table has a record not only of feasible successors and successors but also of all received routes.
The other routes are referred to as possibilities. The topology table is built from the update packets that are exchanged by the neighbors and by replies to queries sent by the router. If a router does not hear an acknowledgment within the allotted time, it retransmits the packet as a unicast. If there is no response after 16 attempts, the router marks the neighbor as dead.
Each time the router sends a packet, RTP increments the sequence number by one. The router must hear an acknowledgment from every router before it can send the next packet.
The capability to send unicast retransmissions decreases the time that it takes to build the tables. 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.
Maintaining the Topology Table The topology table may be recalculated because a new network is added to the network, successors change, or because a network is lost. Just as the neighbor table tracks the receipt of the EIGRP packets, the topology table records the packets that have been sent by the router to the neighbors. Like a Sunday afternoon, passive is good and active is bad. Because the routing table is built from the topology table, the topology table must have all the information required by the routing table.
This includes the next hop or the address of the neighbor that sent the update, and the metric which is taken from the feasible distance.
At the start of this process, the old interface has converged routing. The following list describes how the new network is propagated to all the routers in the EIGRP autonomous system: 1. As soon as Router A becomes aware of the new network, it starts to send Hello packets out of the new interface.
No one answers—no other routers are on the segment. There are no new entries in the neighbor table because no neighbors have responded to the Hello protocol. There is a new entry in the topology table, however, because it is attached to a new network. EIGRP, sensing a change, is obliged to send an update to all its neighbors on the old interface, informing them of the new network. These updates are tracked in the topology table and the neighbor table because the updates are connection-oriented and the acknowledgments from the neighbors must be received within a set timeframe.
Router A has completed its work. However, its neighbors on the old network still have work to do. On hearing the update from Router A, they will update the sequence number in their neighbor table and add the new network to the topology table.
They calculate the FD and the successor to place in the routing table. The next section describes the process for removing a router or path from the topology table. If a network connected to Router A is disconnected, Router A updates its topology and routing table and sends an update to its neighbors.
When a neighbor receives the update, it updates the neighbor table and the topology table. The neighbor searches for an alternative route to the remote network. It examines the topology table for alternatives. Because there is only one path to the remote network, no alternatives are found. The neighbor then sends out a query to its neighbors requesting that they look in their tables for paths to the remote network.
The route is marked active in the topology table at this time. The query number is tracked, and when all the replies are in, the neighbor and topology tables are updated.
DUAL, which starts to compute as soon as a network change is registered, runs to determine the best path, which is placed in the routing table. Before they respond, they query their own neighbors; in this way, the search for an alternative path extends or diffuses throughout the organization.
If no alternative route is available, the neighbors reply to the query stating that they have no path. When no router can supply a path to the network, all the routers remove the network from their routing and topology tables.
The next section describes what happens if a neighbor does have an alternative route. The router looks in the topology table, which has every advertisement received, to determine whether there is an alternative route. It is looking for a FS. If a successor is found, the router adds the FS route to its routing table. If the router did not have a FS, it would have placed the route into an active state while it actively queried other routers for an alternative path.
After interrogating the topology table, if a feasible route is found, the neighbor replies with the alternative path. This alternative path is then added to the topology table. If no answer is heard, the messages are propagated.
When the router sends a query packet, it is recorded in the topology table. This is to ensure a timely reply. If the router does not hear a reply, the neighbor is removed from the neighbor table and all networks held in the topology table from that neighbor are removed from the topology table.
Occasionally, problems can occur because of slow links and burdened routers in a large network. In particular, a router might not receive a reply from all the queries that it sent out. This leads to the route being declared SIA; the neighbor that failed to reply is removed from the neighbor table. 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 the basis for routing decisions. Up to sixteen paths can be held for one destination. There are three different types of paths. These three path types are described in Table If one is found, the router stays in passive mode passive, in this sense, means that the router is not actively querying for an alternative path. Figure provides an example network. This means that the feasible distance meets the feasibility condition, allowing it to become a feasible distance.
If you follow the diagram, it is very straightforward and less algebraic. Therefore, this is a feasible successor and can be replaced as a route without Router A changing from passive to active mode.
After the link between D and G dies, the routing table would be updated from the topology table while the route remains passive. The following section illustrates what happens when the topology table is interrogated and no feasible route is found.
The feasible distance is 20, and the advertised distance from Router D is These neighbors have advertised distances of 27, 27, 20, and 21, respectively. Because all the neighbors have an advertised distance that is the same or greater than the successor feasible distance, they do not meet the feasibility requirement.
The feasible distance is acceptable, the topology and routing tables will be updated, and there is no need for further convergence. EIGRP refers to this neighboring router as a successor.
The details on how EIGRP computes successors are complex, but the concept is simple, as described in the next section. Scaling a network is a major concern in organizations. New demands are constantly driving the networks to use applications that require more bandwidth and less delay; at the same time networks are becoming larger and more complex.
Mergers and acquisitions, for instance, do nothing for good design. Careful design and placement of network devices can remedy many of the problems seen in a network. These are particularly important over slow WAN links. By sending less information about the network, the capacity available for the data between clients and servers increases. A balance between summarization and full information must be struck, but generally this balance will tilt toward more summarization and not less.
EIGRP automatically summarizes at classful network boundaries because summarization is generally helpful and the EIGRP process is built to recognize opportunities such as this to optimize the network. A hierarchical network design meets this criteria.
If a router does not have a feasible successor, what action will it take? Which timers are tracked in the neighbor table? What is the difference between an update and a query? How long is the holdtime by default? What is Stuck in Active? What conditions must be met for a router to become a neighbor? If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.
The router prompts you for the networks. Updates are sent. The routing table is created. Hellos are sent on appropriate interfaces. Networks are advertised. At a classful network boundary b. At the ASBR c. At the ABR d. At the routing process b. At the interface c. After the network command d. Updates b. Hellos c. Queries and replies d. What is displayed in the command debug ip eigrp summary? A summary of the contents of the neighbor database c.
The process taken when a change is made in a summary route d. Classless routing protocols are also able to arbitrarily summarize. EIGRP summarizes automatically at classful network boundaries, but you will also want to summarize within your network. This bit number is arbitrary. Remember that the AS must be consistent between routers to exchange updates. EIGRP will not produce hellos or advertise networks until it is activated on particular links. There are two ways to utilize the network command.
A router with three interfaces— In some cases, you might want to leave an interface that is within the classful network out of EIGRP. The command to do this is passive-interface. The passive-interface command prevents EIGRP from speaking on an interface; it does not send hellos or advertisements. The passive-interface command can be used on interfaces with no neighbors, or on interfaces that run another routing protocol such as BGP. For example, a router with two interfaces addressed Michael Strebensen wtf this great ebook for free?!
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