Configuration of EIGRP on Cisco With Lab Example
This post is on Configuration of EIGRP on Cisco. You will find an example of how to configure EIGRP for Cisco in a small routed network is explained . All the basic commands for its configuration will be described in detailed.EIGRP Configuration Example on Cisco
Following is the topology example of EIGRP, you will find complete Cisco commands for configuration of the LAB. The types of serial interfaces and their associated bandwidths may not necessarily reflect the most frequent types of connections found in networks today. The bandwidths of the serial links used in this topology were chosen to help explain the calculation of the routing protocol metrics and the process of selecting the best route.
The routers in the topology have an initial configuration, which includes the addresses of the interfaces. At this time, none of the routers have static routing or dynamic routing configured .
The interface configurations for the three EIGRP routers in the topology are shown below.
R1 # show running-config
<result omitted>
!
interface GigabitEthernet0 / 0
ip address 172.16.1.1 255.255.255.0
!
Serial interface0 / 0/0
ip address 172.16.3.1 255.255.255.252
clock rate 64000
!
Serial interface0 / 0/1
ip address 192.168.10.5 255.255.255.252
R2 # show running-config
<result omitted>
!
interface GigabitEthernet0 / 0
ip address 172.16.2.1 255.255.255.0
!
Serial interface0 / 0/0
ip address 172.16.3.2 255.255.255.252
!
Serial interface0 / 0/1
ip address 192.168.10.9 255.255.255.252
clock rate 64000
!
Serial interface
ip address 209.165.200.225 255.255.255.224
R3 # show running-config
<result omitted>
!
interface GigabitEthernet0 / 0
ip address 192.168.1.1 255.255.255.0
!
Serial interface0 / 0/0
ip address 192.168.10.6 255.255.255.252
clock rate 64000
!
Serial interface0 / 0/1
ip address 192.168.10.10 255.255.255.252
Only routers R1, R2, and R3 are part of the EIGRP routing domain. The ISP router is used as the gateway of the routing domain to the Internet.EIGRP Autonomous System Numbers
EIGRP uses the router eigrp autonomous system command to enable the EIGRP process. The autonomous system number mentioned in the EIGRP configuration does not relate to the autonomous system numbers assigned globally by the Internet Assigned Numbers Authority (IANA), which use external routing protocols.So what is the difference between the autonomous system number assigned globally by IANA and the autonomous system number of EIGRP?
The autonomous system assigned globally by IANA is a set of networks under the administrative control of a single entity that presents a routing policy common to the Internet. In Image 2, companies A, B, C and D are all under the administrative control of ISP1. When announcing routes to ISP2, ISP1 presents a common routing policy for all these companies.
The guidelines for the creation, selection and registration of an autonomous system are described in RFC 1930 . IANA assigns global autonomous system numbers and is the same authority that allocates the IP address space.
The local regional Internet Registry (RIR) is responsible for assigning entities an autonomous system number from their assigned autonomous system number block. Currently, 32-bit autonomous system numbers are assigned, which increases the number of available autonomous system numbers to more than 4000 million.
ISP and Institutions
In general, Internet service providers (ISPs), backbone Internet service providers and large institutions connected to other entities require an autonomous system number. These use the external gateway routing protocol, the border gateway protocol (BGP), to propagate the routing information.BGP is the only routing protocol that uses a real autonomous system number in its configuration.
The vast majority of companies and institutions with IP networks do not need an autonomous system number, because they are under the control of a larger entity, such as an ISP. These companies use internal gateway protocols, such as RIP, EIGRP, OSPF and IS-IS to route packets within their own networks.
The autonomous system number used for the EIGRP configuration is only important for the EIGRP routing domain. It works as a process ID to help routers keep track of several running EIGRP instances.
This is necessary because it is possible to have more than one instance of EIGRP running on a network. Each instance of EIGRP can be configured to support and exchange routing updates from different networks.
The EIGRP Configuration Commands For Cisco Router
The Cisco IOS includes processes to enable and configure several different types of dynamic routing protocols. The global router configuration mode command is used to start the configuration of any dynamic routing protocol.The topology shown in Image 1 (above) is used to illustrate this command.
As shown in following Image 3, when followed by a question mark ( ? ), The router command of the global configuration mode lists all available routing protocols that support the specific version of the IOS running on the router.
The following global configuration mode command is used to enter router configuration mode for EIGRP and begin configuring the EIGRP process:
Router (config) # router eigrp autonomous-system
The autonomous-system argument can be assigned to any 16-bit value between numbers 1 and 65 535. All routers within the EIGRP routing domain must use the same autonomous system number.
The configuration of the EIGRP process on routers R1, R2 and R3 is shown below. Note that the input request changes from the global configuration mode request to the router configuration mode request. (the same EIGRP is enabled on all three routers by using the same autonomous system number 1)
R1 (config) # router eigrp 1
R1 (config-router) #
R2 (config) # router eigrp 1
R2 (config-router) #
R3 (config) # router eigrp 1
R3 (config-router) #
In this example, 1 identifies this particular EIGRP process , which is running on the router. To establish neighbor adjacencies, EIGRP requires that all routers in the same routing domain be configured with the same autonomous system number.
The router eigrp autonomous-system command does not start the EIGRP process itself; The router does not start sending updates. Instead, this command only provides access to configure the EIGRP parameters.
To completely remove the EIGRP routing process from a device, use the no router eigrp autonomous-system command in global configuration mode, which stops the EIGRP process and removes all EIGRP router configurations.
EIGRP Router ID
The EIGRP router ID is used to uniquely identify each router in the EIGRP routing domain. The router ID is used in the EIGRP and OSPF routing protocols, although the function of this router ID is more important in OSPF.In IPv4 EIGRP implementations, the use of the router ID is not so obvious. EIGRP for IPv4 uses the 32-bit router ID to identify the source router for redistribution of external routes.
The need for a router ID is most evident in the EIGRP analysis for IPv6.
Cisco routers derive the router ID based on three criteria, in the following order of priority:
- The IPv4 address configured with the eigrp router-id command of the router configuration mode is used.
- If the router ID is not configured, the router chooses the highest IPv4 address of any of its loopback interfaces.
- If no loopback interface is configured, the router chooses the highest active IPv4 address of any of its physical interfaces.
If the network administrator does not explicitly configure a router ID by using the eigrp router id command , EIGRP generates its own router ID from a loopback address or a physical IPv4 address.
A loopback address is a virtual interface and is automatically up when it is configured. The interface does not need to be enabled for EIGRP, which means that it does not need to be included in one of the EIGRP network commands. However, the interface must be in the up / up state.
According to the criteria described above, the illustration shows the default EIGRP router IDs, which are determined based on the highest active IPv4 address of the routers.
Configuring the EIGRP Router ID
The eigrp router-id command is used to configure the EIGRP router ID and takes precedence over any loopback address or IPv4 physical interface address. The syntax of the command is:Router (config) # router eigrp autonomous-system
Router (config-router) # eigrp router-id ipv4-address
Note : The IPv4 address used to indicate the router ID is actually any 32-bit number that is shown in decimal notation with dots.
The router ID can be configured with any IPv4 address, with two exceptions: 0.0.0.0 and 255.255.255.255 . The router ID must be a unique 32-bit number in the EIGRP routing domain; otherwise, routing inconsistencies may occur.
The EIGRP router ID settings for R1 and R2 routers are shown below:
R1 (config) # router eigrp 1
R1 (config-router) # eigrp router-id 1.1.1.1
R1 (config-router) #
R2 (config) # router eigrp 1
R2 (config-router) # eigrp router-id 2.2.2.2
R2 (config-router) #
Use of loopback address as Router ID
Another option to specify the EIGRP router ID is to use an IPv4 loopback address.The advantage of using a loopback interface instead of the IPv4 address of a physical interface is that, unlike physical interfaces, it cannot fail.
There are no real adjacent cables or devices on which the loopback interface depends to be in the up state. Therefore, using a loopback address as a router ID can provide a more consistent router ID than using an interface address .
If the eigrp router-id command is not used and there are loopback interfaces configured, EIGRP chooses the highest IPv4 address of any of the loopback interfaces. The following commands are used to enable and configure a loopback interface:
Router (config) # interface loopback number
Router (config-if) # ip address ipv4-address subnet-mask
Note : The EIGRP router ID does not change, unless the EIGRP process is removed with the no router eigrp command or the router ID is manually configured with the eigrp router-id command .
EIGRP Process Verification
In Image 5, the result of show ip protocols for R1, including the router ID , is shown .The show ip protocols command shows the parameters and the current status of any active routing protocol process, including EIGRP and OSPF.
The network command
The figure shows that R1, R2, and R3 have networks that should be included within a single EIGRP routing domain.
To enable EIGRP routing on an interface, use the router network configuration mode command and enter the classy network address for each directly connected network.
With the network command in EIGRP:
- Any interface is enabled on the router match the network address in the command network mode router configuration to send and receive EIGRP updates.
- The interface network is included in EIGRP routing updates.
Router (config-router) # network ipv4-network-address
The ip-v4-network-address argument is the class IPv4 network address for this interface.
In Image, the network commands configured for R1 are shown. We can see that a single classed network instruction (network 172.16.0.0) is used in R1 to include both interfaces in subnets 172.16.1.0/24 and 172.16.3.0/30. Note that only the classy network address is used .
The following shows the use of the network command to enable EIGRP on the R2 interfaces for subnets 172.16.1.0/24 and 172.16.2.0/24.
R2 (config) # router eigrp 1
R2 (config-router) # network 172.16.0.0
R2 (config-router) #
* Feb 28 17: 51: 42,543:% DUAL-5-NBRCHANGE: EIGRP-IPv4 1:
Neighbor 172.16.3.1 (Serial0 / 0/0) is up: new adjacency
R2 (config-router) #
When EIGRP is configured on the S0 / 0/0 interface of R2, DUAL sends a notification message to the console indicating that a neighbor adjacency with another EIGRP router was established on that interface.
This new adjacency occurs automatically, because R1 and R2 use the same autonomous system number eigrp 1 and both routers now send updates on their interfaces on the 172.16.0.0 network.
The eigrp log-neighbor-changes command in the router configuration mode is enabled by default. This command is used for the following:
- Show any changes in the adjacencies of EIGRP neighbors.
- Help verify neighbor adjacencies during EIGRP configuration.
- Notify the network administrator when any EIGRP adjacency is removed.
The network command and the wildcard mask
By default, when using the network command and an IPv4 network address, such as 172.16.0.0, all interfaces on the router that belong to that classy network address are enabled for EIGRP.However, there may be times when the network administrator does not want to include all interfaces within a network when enabling EIGRP. For example, in Image 7, assume that an administrator wishes to enable EIGRP on R2, but only for subnet 192.168.10.8 255.255.255.252, on interface S0 / 0/1.
To configure EIGRP to advertise only specific subnets, use the wildcard-mask option with the network command:
Router (config-router) # network network-address [wildcard-mask]
Think of the wildcard mask as the reverse of a subnet mask. The inverse of a 255.255.255.252 subnet mask is 0.0.0.3. To calculate the inverse value of the subnet mask, subtract the subnet mask from 255.255.255.255 as follows:
255.255.255.255
- 255.255.255.252
---------------
0. 0. 0. 3: wildcard mask
Now, continue the EIGRP network configuration of R2. The network 192.168.10.8 0.0.0.3 command specifically enables EIGRP on the S0 / 0/1 interface, a member of the 192.168.10.8 255.255.255.252 subnet.
R2 (config) # router eigrp 1
R2 (config-router) # network 192.168.10.8 0.0.0.3
R2 (config-router)
Some versions of IOS also allow you to enter the subnet mask instead of a wildcard mask.
An example of the configuration of the same S0 / 0/1 interface on R2 is shown below, only in this case a subnet mask is used in the network command . However, if the subnet mask is used, the IOS converts the command to the wildcard-mask format within the configuration. This is verified in the result of show running-config .
R2 (config) # router eigrp 1
R2 (config-router) # network 192.168.10.8 255.255.255.252
R2 (config-router) # end
R2 # show running-config | eigrp section 1
eigrp router 1
network 172.16.0.0
network 192.168.10.8 0.0.0.3
eigrp router-id 2.2.2.2
R2 #
Passive interface
As soon as a new interface is enabled within the EIGRP network, EIGRP attempts to form a neighbor adjacency with any neighboring router to send and receive EIGRP updates.From time to time it may be necessary, or advantageous, to include a network directly connected in the EIGRP routing update, but not allow any neighbor adjacency to form outside that interface.
The passive-interface command can be used to prevent neighbor adjacencies from forming. There are two main reasons to enable this command:
- To suppress unnecessary update traffic, for example, when an interface is a LAN interface, without other routers connected
- To increase security controls, for example, to prevent unknown unauthorized routing devices from receiving EIGRP updates
Passive interface Example
In Image, it is shown that R1, R2 and R3 have no neighbors on their GigabitEthernet 0/0 interfaces.
The passive-interface command of the router configuration mode disables the transmission and reception of EIGRP hello packets on these interfaces.
Router (config) # router eigrp as-number
Router (config-router) # passive-interface interface-type interface-number
Below is the passive-interface command configured to suppress hello packets on the LANs for R1 and R3.
R1 (config) # router eigrp 1
R1 (config-router) # passive-interface gigabitethernet 0/0
R3 (config) # router eigrp 1
R3 (config-router) # passive-interface gigabitethernet 0/0
Without a neighbor adjacency, EIGRP cannot exchange routes with a neighbor. Therefore, the passive-interface command prevents the exchange of routes in the interface.
While EIGRP does not send or receive routing updates through an interface configured with the passive-interface command , it does include the interface address in routing updates sent by other non-passive interfaces.
Note : To configure all interfaces as passive, use the passive-interface default command . To disable an interface as passive, use the no passive-interface interface-type interface-number command .
Passive Interface Verification
Finally, to verify if any interface on a router is configured as passive, use the show ip protocols command in the privileged EXEC mode, as shown in Image 9.
Note that while the R3 GigabitEthernet 0/0 interface is a passive interface, EIGRP still includes the network address of the 192.168.1.0 network interface in its routing updates.
