Configure And Verify EIGRP Stubs, EIGRP Load Balancing And EIGRP Neighbor Relationship And Authentication

The EIGRP is commonly the routing protocol of choice when using the network with no requirement for the cross vendor operation. The EIGRP supports multiple routed protocols, including IPx, AppleTalk and IP. The EIGRP evolved from the predecessor, IGRP which is the classful routing protocol like RIPv1. The EIGRP in and of it themselves is the hybrid routing protocol that has the characteristic of both link state protocols as well as distance vector protocol. This session will get you to understand how to configure and verify the EIGRP stubs, EIGRP load balancing, EIGRP neighbor relationship and authentication.

3.19 Configure And Verify EIGRP Neighbor Relationship And Authentication

Much like the RIP using a triggered feature, the EIGRP update is sent when the change in a network is identified. At first, the EIFGRP routers will form the neighbor relationship as well as exchange the topological information. After that the routing protocol sends the periodic hello to ensure whether the neighbor is still there or not. Although, when the route changes or link goes down, update is then sent to the neighboring routers through multicast by using its own internet protocol number 88. The EIGRP uses the DUAL which ensures the loop free routing domains by maintaining 2 separate routes in an EIGRP topology table called as successor as well as feasible successor route.

The EIGRP maintains the 3 separate tables such as the routing table, topology table and the neighbor table. If the route goes down and also no feasible successor exist for a route, the EIGRP can query neighboring routers to see that there is any alternate route is available to a failed route. In the poorly designed network, the EIGRP queries will be downfall of a network as the EIGRP route will become a Struck In Active - SIA. If the query response is not getting back from the router in the allotted time a neighbor relationship is dropped and also any routes which is associated with the neighbor relationship is purged resulting in the dropped packets when the network is re-converging.

When configuring the EIGRP neighbor, then the neighbor statement is needed on both of the ends of a neighbor relationship in an EIGRP routing process which operate in a same autonomous system. Take the below topology for example.

Step 1:

Make sure that all the interfaces are configured in right method.

Step 2:

In the configuration mode, make an enter as a router eigrp asn# command. This command will make the EIGRP process on a router.

Step 3:

To perform the network statement the below command turns on the EIGRP on any interfaces with the network address which falls into a range configured in a network command. Here narrow the selection down with the help of wild card mask option.

For example: On R1

It will simply turn on the EIGRP for the interface S0/0/0, it is the only interface which falls into the range. For the broader approach, simply turn on all the interfaces.

It will simply turn on the EIGRP on all the R1 interface, it is because the command is valid for the entire addresses on a router.

Step 4;

The verification of the EIGRP includes many commands as follows;

Show ip protocols:

It will display information about every routing protocol which is running on a router. You can get a passive interface with this command.

Show ip route:

It lets you view the routing table used by a router. The routes which came from the EIGRP can be marked with the Dual as "D". The internal routes from the EIGRP have the AD of 90.

Show ip eigrp topology:

It lets you view the EIGRP process in the topology table.

Show ip eigrp neighbors

It allows you to see the neighbor router. It will not display any of the routers which are supposed to be the neighbors but not due to error.

show ip eigrp interfaces:

It will display what interface is participating in the EIGRP, but it will not display the passive interfaces.

The EIGRP authentication causes the routers to authenticate each EIGRP message. To perform so, the router must use the same preshared key, generating the MD5 digest for the every EIGRP message depends on the shared PSK. If the router configured for the EIGRP authentication gets the EIGRP message and MD5 digest message does not pass an authentication checking depends on a local copy of a key, then the router will silently discard the message. Finally, when authentication failed, 2 routers will not EIGRP neighbor, because it ignores the EIGRP hello message. The EIGRP authentication also helps to prevent the denial of service attacks. The authentication prevents the attackers from forming the neighborships with a legitimate router, preventing an advertisement of the incorrect routing information.

3.20 Configure And Verify EIGRP Stubs

The EIGRP stub feature improves the network stability, simplifies the stub router configuration and reduces the resource utilization. The stub routing is used commonly in a spoke and hub network topology. In the spoke and hub network, 1 or more ends networks are connected to the remote router which is connected to 1 or more distribution router. The typical connection in between the spoke and hub router has less bandwidth than the connection in a network core. Hence, attempting to use that connection as transit route will typically result in the excessive congestion. The EIGRP stub routing will prevent this issue by simply restricting a spoke router from advertising a route from the hub router to the other hub router. Then, as a result, hub router might notice that there an alternative route via the spoke router. Although, a stub routing is not able to stop a Hub router from advertising routes to the spoke router. The manual summarization is needed on a hub to advertise the default route to a spoke. Any neighbor that gets the packet informing it of a stub route cannot query a stub router for the any routes and the router which has the stub peer cannot query the peer. A stub router can depend on the distribution router to send a proper update to all the peers.

The EIGRP stub router will inform the stub status to upstream the routers through a hello packet. Any neighboring upstream routers, which the hello packets can not query a stub router for the lost routes, as a stub router has no downstream enhanced IGRP neighbors and so may not have any alternative routes for the lost path. Then the upstream router that is connected to a stub router will answer for any query for the stub router, that results in the increased convergence time.

An eigrp stub [receive-only | connected, static, summary, redistributed] router command configures the EIGRP router as the EIGRP stub router. The EIGRP stub router will advertise it connected and also the summary routes to the other neighboring routers as default.

Configuring the EIGRP stub routing may tear down the existing Enhanced IGRP neighbor relationship. Verify a stub configuration on the R5 by using the show ip eigrp neighbor detail EXEC command on the R5.

3.21 Configure And Verify EIGRP Load Balancing

When the router learns multiple routes of the certain network through multiple routing processes, it installs a route without the lowest AD in a routing table. The EIGRP puts up to 4 routes of equal cost in a routing table, which a router then load- balances. The load balancing type is mainly depends on the switching type done with a router. Although, the EIGRP also load balance over the unequal cost links. The coming section will give you idea about both the equal and unequal cost.

3.21.A Equal Cost

The equal cost load balancing is the router's ability to distribute the traffic over entire routes which have a same metric for a destination address. The entire IP routing protocol can perform the equal cost load balancing. The load balancing increases a utilization of the network segment, since increasing the network bandwidth. As default, the IOS balance between the maximum of 4 equal cost path for the Internet protocol. With the help of maximum paths router configuration command, you can also request that up to 16 equally good paths be kept in a routing table. So set the maximum path parameter to one to disable the load balancing.

3.21.B Unequal Cost

The EIGRP balance traffic across the multiple routes which have different metrics which is called as unequal cost load balancing. Then the degree to that EIGRP performs the load balancing is controlled by a variance multiplier router configuration command. Here, the multiplier is the variance value in between 1 and 128 which is used for the load balancing. When the default is 1, that means the equal cost load balancing. Multiplier defines the metric value range which is accepted for the load balancing. Setting the variance value greater than one allows EIGRP to install the multiple loop free routes with the unequal cost in a routing table. The EIGRP can always install the successors in a routing table. Then the variance will allow the feasible successors to be installed in a routing table. There are 2 feasible conditions that determine if the route is the valid, feasible successor and if the route can used for the unequal cost load balancing. The path must be the loop free meaning a good metric learned from a next router has to less than a local best metric. The metric of all the path has to be lower than the variance multiplied by a local best metric.

Take the above figure as an example. In that, find out the feasible successor and a successor. In the routing table, you can find the successor only. Now, change things, feasible successor in a routing table and so it will load balance.

You can also do it using the variance command. This variance command will work as the multiplier. If you set the variance at 2, then you will get:

Successor feasible distance = 20 (10x2)

109 values are higher than the 20, here you could not load balancing

If you set a variance at 5, then you will get:

Successor feasible distance = 50 (10x5)

109 is still higher than the 50, but still no load balancing will occur.

Now, set the variance at 11, you will get:

Successor feasible distance = 110 (10x11)

Here, 109 is lower than the 110, now you can put the feasible successor in a routing table and also start load balancing.

The EIGRP has the ability to use the unequal cost load balancing in the same as that of the IGRP. A router uses the variance as the multiplier in selecting the upper boundary of the route with a greater metric. The EIGRP has the consistent perspective of the network metrics. It also aids in making the route show up in an IP forwarding table.

To configure any unequal cost load balancing follow the below:

  • Configure a bandwidth on the both sides of all interfaces involved in a load sharing group. Then use the command bandwidth xx_kbps to establish it.
  • Define the highest cost metric and lowest cost metric. From that value, compute a variance multiplier as well as add that in an EIGRP routing process. Then the composite metric which the EIGRP is using will be viewed with a command show ip eigrp topology.
  • Set the traffic share variables or the maximum path. This step is optional.

The EIGRP is the extremely common routing protocol due its simplicity and its ease of configuration. The EIGRP neighbor relationship and authentication are discussed with configuration and verification. The EIGRP stub routing is an effective method for limiting or restricting the query range, hence conserves the bandwidth due to the unnecessary queries, improves network stability and prevents the SIA events. The EIGRP has the ability to distribute the traffic over entire routers which have the same metrics for a target destination which is the equal cost load balancing. The EIGRP also distribute the traffic over the router which have the different metrics, this is the unequal cost load balancing.


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