For example, if we want to use a key only from January 1st, to December 1st, , the following commands are used:. Equal-cost means that multiple routes must have the same metric to reach the destination, so that router can choose to load balance across equal cost links. Routers R2 and R3 are connected to the subnet Both routers advertise the route to reach that subnet to R1. Router R1 receives the two routing updates for the subnet Router R1 places both routes in the routing table and load balances across three links.
You can set up your router to load balance over links with different metric to reach the destination. To accomplish unequal-cost load balancing, the variance command is used. The command takes one parameter, the multiplier, which tells the router to load balance across each link with the metric for the destination less than the feasible distance multiplied by the multiplier value. Router R1 chooses the route from R2 as the best route.
But what if we want to load balance traffic across the other link? EIGRP performs an auto-summarization each time it crosses a border between two different major networks. For example, in Figure 13, Router Two advertises only the 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. Note: Even with variance configured, EIGRP will not send traffic over an unequal cost path if the reported distance is greater than the feasible distance for that particular route. The bandwidth should always be set to the real bandwidth of the interface; multipoint serial links and other mismatched media speed situations are the exceptions to this rule.
Because EIGRP uses the interface bandwidth to determine the rate at which to send packets, it is important that these be set correctly. At lower bandwidths, the bandwidth has more influence over the total metric; at higher bandwidths, the delay has more influence over the total metric. External administrative tags are useful for breaking redistribution routing loops between EIGRP and other protocols. It is not possible to modify the administrative distance for a default gateway that was learned from an external route because, in EIGRP, the modification of the administrative distance only applies for internal routes.
In order to raise the metric, use a route-map with prefix-list; do not change the administrative distance. A basic example of configuring these tags follows, but this example does not show the entire configuration used for breaking redistribution loops.
The output of this command shows the information that has been exchanged between the neighboring EIGRP router. An explanation of each output field follows the table. This command only displays feasible successors.
To display all entries in the topology table, use the show ip eigrp topology all-links command. FD is shows the feasible distance, which is the best metric to reach this destination or the best metric known when the route went active.
Q means a query is pending. This field can also be: U, for update pending; or R, for reply pending. This field can also be: Multiple origins, meaning that multiple neighbors have sent queries on this destination, but not the successor; or Successor origin, meaning the successor originated the query.
This field can also be: Connected, if the network is directly connected to this router; Redistributed, if this route is being redistributed into EIGRP on this router; or Summary, if this is a summary route generated on this router. Q is the send flag for this route, meaning there is a query pending.
This field can also be: U, meaning there is an update pending; or R, meaning there is a reply pending. Via This command displays all entries in the topology table for this destination, not just feasible successors. State is Passive means the network is in passive state, or, in other words, we are not looking for a path to this network. Routes are almost always in a passive state in stable networks. Query origin flag is 1 If this route is active, this field provides information on who originated the query.
This router has received more than one query for this route from more than one source. Similar to 2, but it also means there is a query origin string which describes the queries outstanding for this path. FD is shows the best current metric to this network. In order to include this path also in load sharing, the variance should be changed to 3. In this case, the traffic share count ratio is:.
Similarly, when you use the traffic-share command with the keyword min , the traffic is sent only across the minimum-cost path, even when there are multiple paths in the routing table. This is identical to the forwarding behavior without use of the variance command. However, if you use the traffic-share min command and the variance command, even though traffic is sent over the minimum-cost path only, all feasible routes get installed into the routing table, which decreases convergence times.
Similarly, you can do the same in IGRP, except for the feasibility condition. This condition is not applicable to IGRP. Cisco Express Forwarding CEF is an advanced Layer 3 switching technology which can be used for load balancing in routers.
By default, CEF uses per-destination load balancing. If it is enabled on an interface, per-destination load balancing forwards packets based on the path to reach the destination.
If two or more parallel paths exist for a destination, CEF takes the same path single path and avoids the parallel paths. This is a result of the default behavior of CEF. CEF takes the single path in cases when load sharing is done simultaneously on interfaces of different physical types, such as serial and tunnel. The hash algorithm determines the path to be chosen. In order to utilize all the parallel paths in CEF and load balance the traffic, you must enable per-packet load balancing when you have different physical interfaces like serial and tunnel.
So, on the basis of the configuration and topology serial or tunnel , load sharing can fail to work correctly with the default CEF load balancing mode.
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