The FC guarantees that the paths that satisfy the condition are loop-free; however, not all loop-free paths satisfy the FC. Notice that Ames This will be the metric that Ames reports to NewYork. The RD is thus 2,, The output of show ip eigrp topology shows only feasible successors. The output of show ip eigrp topology all-links shows all neighbors, whether feasible successors or not.
Passive state indicates that the route is in quiescent mode, implying that the route is known to be good and that no activities are taking place with respect to the route. If DUAL finds a feasible successor in its own topology table after one of these events, the route remains in passive state.
If DUAL cannot find a feasible successor in its topology table, it will send a query to all its neighbors and the route will transition to active state.
The next section contains two examples of DUAL reevaluating its topology table. In the first example, the route remains passive; in the second example, the route becomes active before returning to the passive state. These routes become invalid. DUAL attempts to find new successors for both destinations -- DUAL checks the topology table for Since Serial0 is down, the only feasible successor is The FS check is:.
In plain words, this implies that the path available to NewYork via Ames the FS is independent of the primary path that just failed. DUAL installs Ames as the new successor for In our case study, only one FS was available. Since DUAL is searching for the successor s for this destination, it will choose the minimum from this set of metrics via each FS.
Let the lowest metric be Dmin. If multiple FSs yield metrics equal to Dmin , they all become successors subject to the limitation in the maximum number of parallel paths allowed -- four or six, depending on the IOS version number. Since the new successor s is found locally without querying any other router , the route stays in passive state.
After DUAL has installed the new successor, it sends an update to all its neighbors regarding this change. How long does this computation take?
Note that only one ping packet was lost during this computation, implying that the convergence time including the time to detect the failure of the link was in the range of two to four seconds.
Notice that this is a different case in that when Serial0 is down, NewYork has no feasible successors in its topology table see line DUAL knows of no feasible successors, but NewYork has a neighbor that may know of a feasible successor. DUAL places the route in active state see line 15 and sends a query to all its neighbors:.
NewYork sets the reply flag on line 16 , which indicates that NewYork expects a reply to the query. Ames receives the query and marks its topology table entry for Next, Ames checks its topology table for a feasible successor:. Ames sends a reply packet to NewYork with an RD of 2,, line NewYork marks the route as passive and installs a route for In general, if DUAL does not find a feasible successor, it forwards the query to its neighbors.
Routers that did not find a feasible successor would return an unreachable message. So, if Ames did not have a feasible successor in its topology table, it would mark the route as active and propagate the query to its neighbor, if it had another neighbor. If Ames had no other neighbor and no feasible successor it would return an unreachable message to NewYork and mark the route as unreachable in its own table.
When DUAL marks a route as active and sets the r flag on, it sets a timer for how long it will wait for a reply. The default value of the timer is three minutes. DUAL waits for a reply from all the neighbors it queries. If a neighbor does not respond to a query, the route is marked as stuck-in-active and DUAL deletes all routes in its topology table that point to the unresponsive neighbor as a feasible successor. The successors in the DUAL topology table are eligible for installation in the routing table.
This route is not marked as a summary route in any way; it looks like an internal route. The metric is the best metric from among the summarized routes. Note that the minimum bandwidth on this route is k, although there are links in the The route to The topology table entry for this summary route looks like the following:. To make Router Two advertise the components of the There are some caveats when dealing with the summarization of external routes that are covered later in the "Auto-Summarization of External Routes" section.
EIGRP allows you to summarize internal and external routes on virtually any bit boundary using manual summarization. For example, in Figure 14, Router Two is summarizing the Note the ip summary-address eigrp command under interface Serial0, and the summary route via Null0. On Router One, we see this as an internal route:.
EIGRP will not auto-summarize external routes unless there is a component of the same major network that is an internal route. To illustrate, let us look at Figure Router Three is injecting external routes to Although auto-summary normally causes Router Three to summarize the However, if you reconfigure the link between Routers Two and Three to The actions in the table above impact the range of the query in the network by determining how many routers receive and reply to the query before the network converges on the new topology.
To see how these rules affect the way queries are handled, let us look at the network in Figure 16, which is running under normal conditions. Router One chooses the path through Router Three and keeps the path through Router Two as a feasible successor.
Suppose that What activity can we expect to see on this network? Figures 16a through 16h illustrate the process. Router Four, upon receiving a query from its successor, attempts to find a new feasible successor to this network. It does not find one, so it marks Routers Two and Three, in turn, see that they have lost their only feasible route to For simplicity, let us assume that Router One receives the query from Router Three first, and marks the route as unreachable.
Router One then receives the query from Router Two. Although another order is possible, they will all have the same final result. Router One replies to both queries with unreachables; Router One is now passive for Router Five, upon receiving the reply from Router Four, removes network Router Five sends updates back to Router Four so the route is removed from the topology and routing tables of the remaining routers.
It is important to understand that although there may be other query paths or processing orders, all routers in the network process a query for network Some routers may end up processing more than one query Router One in this example. In fact, if the queries were to reach the routers in a different order, some would end up processing three or four queries. Router Two has a topology table entry for the Router Three has a topology table entry for the Router Four has a topology table entry for the If Router Two, on receiving the query from Router One, marks the route as unreachable because the query is from its successor and then queries Routers Four and Three:.
Router Three, when it receives the query from Router One, marks the destination as unreachable and queries Routers Two and Four:. Router Four, when it receives the queries from Routers Two and Three, replies that The query, in this case, is bounded by the autosummarization at Routers Two and Three. Router Five does not participate in the query process, and is not involved in the re-convergence of the network.
Queries can also be bound by manual summarization, autonomous system borders, and distribution lists. If a router is redistributing routes between two EIGRP autonomous systems, it replies to the query within the normal processing rules and launches a new query into the other autonomous system.
For example, if the link to the network attached to Router Three goes down, Router Three marks the route unreachable and queries Router Two for a new path:. Router Two replies that this network is unreachable and launches a query into autonomous system toward Router One. Once Router Three receives the reply to its original query, it removes the route from its table. Router Three is now passive for this network:. While the original query did not propagate throughout the network it was bound by the autonomous system border , the original query leaks into the second autonomous system in the form of a new query.
This technique may help to prevent stuck in active SIA problems in a network by limiting the number of routers a query must pass through before being answered, but it does not solve the overall problem that each router must process the query. In fact, this method of bounding a query may worsen the problem by preventing the auto-summarization of routes that would otherwise be summarized external routes are not summarized unless there is an external component in that major network.
Rather than block the propagation of a query, distribution lists in EIGRP mark any query reply as unreachable. Let us use Figure 19 as an example. Router Three has a distribute-list applied against its serial interfaces that only permits it to advertise Network B. When Router One loses its connection to Network A, it marks the route as unreachable and sends a query to Router Three. Router Three does not advertise a path to Network A because of the distribution list on its serial ports.
Router Two examines its topology table and finds that it has a valid connection to Network A. Note the query was not affected by the distribution list in Router Three:. Router Three builds the reply to the query from Router One, but the distribution list causes Router Three to send a reply that Network A is unreachable, even though Router Three has a valid route to Network A:. Some routing protocols consume all of the available bandwidth on a low bandwidth link while they are converging adapting to a change in the network.
EIGRP avoids this congestion by pacing the speed at which packets are transmitted on a network, thereby using only a portion of the available bandwidth. The default configuration for EIGRP is to use up to 50 percent of the available bandwidth, but this can be changed with the following command:. Essentially, each time EIGRP queues a packet to be transmitted on an interface, it uses the following formula to determine how long to wait before sending the packet:.
This allows a packet or groups of packets of at least bytes to be transmitted on this link before EIGRP sends its packet. The pacing timer determines when the packet is sent, and is typically expressed in milliseconds. The pacing time for the packet in the above example is 0. There is a field in show ip eigrp interface that displays the pacing timer, as shown below:. The time displayed is the pacing interval for the maximum transmission unit MTU , the largest packet that can be sent over the interface.
There are two ways to inject a default route into EIGRP: redistribute a static route or summarize to 0. Use the first method when you want to draw all traffic to unknown destinations to a default route at the core of the network. This method is effective for advertising connections to the Internet. For example:. If you use another network, you must use the ip default-network command to mark the network as a default network.
Summarizing to a default route is effective only when you want to provide remote sites with a default route. Since summaries are configured per interface, you do not need to worry about using distribute-lists or other mechanisms to prevent the default route from being propagated toward the core of your network.
Note that a summary to 0. The only way to configure a default route on a router using this method is to configure a static route to 0. EIGRP puts up to four routes of equal cost in the routing table, which the router then load-balances. The type of load balancing per packet or per destination depends on the type of switching being done in the router. EIGRP, however, can also load-balance over unequal cost links. The router, by default, places traffic on both path 1 and 2.
Using EIGRP, you can use the variance command to instruct the router to also place traffic on paths 3 and 4. The variance is a multiplier: traffic will be placed on any link that has a metric less than the best path multiplied by the variance. Similarly, to also add path 4, issue variance 4 under the router eigrp command. How does the router divide the traffic between these paths? It divides the metric through each path into the largest metric, rounds down to the nearest integer, and uses this number as the traffic share count.
The router sends the first three packets over path 1, the next three packets over path 2, the next two packets over path 3, and the next packet over path 4. The router then restarts by sending the next three packets over path 1, and so on. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.
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It doesn't matter which Autonomous System AS number you use—as long as it's the same on all routers that will be talking to each other. Valid options for the AS number are 1 to While you can configure more than one AS on a single router, Cisco doesn't recommend this approach.
You can accomplish this using the network command. The first parameter is the network IP address; the second parameter is the inverse mask. The inverse mask or wildcard mask is the inverse of the subnet mask. This command is similar to the OSPF network command. So, you can have one network statement that covers multiple interfaces. What is the topology table? It shows the metric for these routes as well as the feasible distance to these networks. The topology table contains a lot of information about successors, feasible successors, and feasible distance.
What is a successor?
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