Delay Metric is the measure of the time it takes a bit to be transmitted to an adjacent neighbor. The delay metric (DLY) is a constant value based on the type of link to which the interface is connected. The value is expressed in microseconds, the one-millionth of a second. In the EIGRP metric calculation, the delay value is used in 10’s of microseconds
The delay value is just like the bandwidth value. We can change the default value. Router cannot measure delay dynamically. We can calculate the Delay value, by dividing the DLY in the show interface command by 10. Important the when determining the EIGRP metric, the delay is the cumulative (sum) of all interface delays along the path from the source to destination. The default delay values for different interfaces are following:
Ethernet – 1000
Fast Ethernet – 100
Gigabit Ethernet – 10
16M token Ring – 630
FDDI – 100
T1 Line – 20,000
56 kb/s – 20,000
You can see that the default value is 20,000 microseconds for serial interfaces and 10 microseconds for Gigabit Ethernet interfaces. We can use the show interfaces command to verify the delay value on an interface. Figure 1 illustrates the delay time in R1 interface.
EIGRP and OSPF both are using bandwidth metric which is a static value for calculating routing metrics. The bandwidth is displayed in kilobits per second (kb/s). The default bandwidths for different interfaces are:
Serial Interface – 1544 kb/s
Ethernet is – 10,000 kb/s
FastEthernet – 100,000
GigaEthernet – 1,000,000
TenGig – 10,000,000
Figure 1 illustrates the topology we have used throughout this section. We can verify the bandwidth with the “show interfaces” command. The default value of the bandwidth may or may not reflect the real physical bandwidth of the interface. If real bandwidth of the link is different from the default bandwidth value, the bandwidth value should be modified.
Configuring the Bandwidth Parameter
Most of the links uses the bandwidth metric defaults values as written above. The correct value for bandwidth is very important to the accuracy of routing information. To modify the bandwidth the command syntax is as under:
If the bandwidth is already changed from the default value and you want to restore the default bandwidth you can use the “no bandwidth” command to restore the default value. In topology, the link between R1 and R2 has a bandwidth of 1544 kb/s, and the link between R1 and R3 has a bandwidth of 10,000 kb/s.
Verifying the Bandwidth Parameter
We can verify the interface bandwidth using the show interfaces command”, as shown in Figure 2. Modifying the bandwidth value on the interface does not change the real bandwidth of the link. The bandwidth only modifies the bandwidth metric used by routing protocols, such as EIGRP and OSPF.
A composite metric is a number calculated based on several different components that determine the preferred route. By default, EIGRP uses bandwidth and delay to calculate the preferred path to a network. The reliability and load can also be used, but are not recommended, because they typically result in a frequent recalculation of the topology table.
Bandwidth: The bandwidth value used in the EIGRP metric is determined by dividing 10,000,000 by the bandwidth (in kbps) of the slowest outgoing interface along the path from the source to the destination network.
Delay: It is the cumulative value of all delays associated with all of the interfaces along the path from the source to the destination (in tens of microseconds). We can display the delay in the output of the show interfaces command.
Reliability: It is the worst reliability between the source and destination based on keep alive.
Load– Represents the worst load on a link between the source and destination, determined based on the packet rate and the configured bandwidth of the interface.
Figure 1 illustrates the EIGRP composite metric formula. The formula consists of K1 to K5 values, known as EIGRP metric weights. K1 bandwidth, K3 represent delay, K2 represents load, and K4 and K5 represent reliability. The bandwidth and delay values are used in the default composite metric. EIGRP for IPv4 and EIGRP for IPv6 both use the same formula for the composite metric. K1 and K3 values are set to 1 in the default formula, K2, K4 and K5 are set to 0.
The metric calculation technique (k values) and the autonomous system number of the EIGRP must match between neighbors; otherwise, routers do not perform an adjacency. The command for changing the default k values is “metric weights” router configuration mode command. The command syntax is as under:
TOS stands for the Type of Service Byte in an IPv4 packet header. It is intended to be used for a quality of service marking. But, the value of TOS must be 0. The five k values can be set into the range between 0 – 255. The metric weights value must be the same for establishing neighbor adjacencies. If metric weights values are different, then adjacency does not form.
Verifying thekValues
We can verify the k values using the “show ip protocols” command. Figure 2 illustrates the output of this command for R1. The k values on R1 are set to the default.
Examining Interface Values
We can examine the values used in the metric calculation using the “show interfaces” command. The command also displays complete interface information. Figure 3 illustrates the show interfaces command output for the Serial 0/0/0 interface on R1. The parameters used in the metric calculation formula are marked with a red rectangle.
BW– This is the interface bandwidth. The bandwidth is in kilobits per second.
DLY– This is the delay of the interface. The delay is in microseconds.
Reliability– The reliability of the interface is a fraction of 255. If the value is 255/255, it means that the reliability is 100%. The reliability is calculated as an exponential average over five minutes.
Txload, Rxload– This is transmitted and receive load on the interface as a fraction of 255 but 255/255 is completely saturated. The value is calculated as an exponential average over five minutes.
MTU – The MTU (Maximum Transmission Unit) is not used in the metric calculation formula. The default value of the Maximum Transmission Unit is 1500 Bytes. The Maximum Transmission Unit is carried in the EIGRP Update message to be used when tie occurred.
EIGRP uses a neighbor table, topology table, and IP routing table. The neighbor table maintains a state of neighbors. The topology table is used to store information about all known routes received from all neighbors.
EIGRP Update messages send the routers’ EIGRP topology tables. The EIGRP topology table is a database of possible routes. Each router chooses its best routes and installs these routes in its respective IP routing table uses the information in the topology table.
Each EIGRP router maintains an EIGRP topology table for each IPv4 and IPv6. It also includes route entries for every destination that the router learns from its directly connected EIGRP neighbors. The figure below illustrates the continuance of the earlier route discovery process from the previous article “EIGRP Neighbor Adjacency”.
When a router R1 receives an update from neighbor router R2, it adds the routing information into its EIGRP topology table and replies with an EIGRP acknowledgment. The figure now illustrates the update of the topology table.
Router R1 replies with an EIGRP acknowledgment packet informing Router R2 about the confirmation of receiving the update of the routing information.
Router R1 sends an EIGRP update to Router R2 advertising the routes in its topology table, except route learned from Router R2.
Router R2 receives the EIGRP update from Router R1 and adds this information to its topology table.
Router R2 replies to Router R1’s EIGRP update packet with an EIGRP acknowledgment packet.
Convergence
When Router R1 receives the EIGRP update packets from Router R2, R1 updates its IP routing table using the information in the topology table with the best path to each destination, including the metric and the next-hop router. Similarly to Router R1, Router R2 updates its IP routing table with the best path routes to each network. This is the EIGRP converged state for both routers.
Before exchanging any EIGRP update packets between routers, EIGRP must first discover its neighbor. EIGRP neighbor is an adjacent router running EIGRP on directly connected networks. EIGRP Hello packets are used to establish and maintain neighbor adjacencies.
Several parameters between the two routers must match to become neighbors; for example, the same autonomous system number and EIGRP metric must be required for establishing neighbor adjacencies.
Each EIGRP-enabled router maintains a list of routers that have EIGRP neighbor adjacencies with this router. This list is known as the neighbor table.
The router uses the neighbor table to track the status of EIGRP neighbors. The figure below illustrates exchanging two EIGRP routers’ initial EIGRP Hello packets and discovering the neighbor process.
The EIGRP-enabled router sends an EIGRP hello packet. When another EIGRP-enabled router receives that Hello packet, it adds that router to its neighbor table. For example
Router R1 has powered up or enabled EIGRP, sending an EIGRP Hello packet through its EIGRP-configured interfaces.
Router R2 receives the Hello packet from router R1 on an EIGRP-enabled interface and replies with an EIGRP update packet. The update packet contains all the routes in the R2 routing table, excluding the routes learned through that interface.
The neighbour adjacency is still not established until R2 sends an EIGRP Hello packet to R1. Now R2 sends hello packet to R1, the neighbour adjacency is now established. R1 and R2 update their neighbour tables adding the adjacent router as a neighbour.
We can verify the EIGRP in the routing table using the “show ip route” command. It is important to verify the information in the routing table to ensure that it is populated as estimated, based on configurations entered. The automatic summarization is enabled by default before in the ISO 15.
It is important to know that auto summarization can make a difference in the information displayed in the IPv4 routing table. If auto summarization is enabled by default then we can disable it using a “no auto-summary” command in router configuration mode.
Figure 1, illustrates the routing table of R1 for the topology we have used in the previous lesson. EIGRP routes are represented in the routing table with the letter D because the protocol is based upon the DUAL algorithm.
Thecommand verifies EIGRP in the routing table. It displays the entire routing table; including remote networks learned dynamically, directly connected, and static routes. It is the first command used to check for convergence.
If routing is correctly configured on all routers, the show ip route command displays a full routing table. Notice that R1 has installed routes to three IPv4 remote networks in its IPv4 routing table:
168.1.0/24 network, received from router R2 via 10.10.10.6, on the Gigabit Ethernet 0/0 interface
168.2.0/24 network, received from router R3 via 10.10.10.2 on the Gigabit Ethernet 0/1 interface
10.10.8/30 network, received from both R2 on the Gigabit Ethernet 0/0 interface, and from R3 on the Gigabit Ethernet 0/1 interface
R1 has two different paths to the 10.10.10.8/30 network because its cost to reach that network is the same or equal using both routers. R1 uses both paths to accomplish this network, which is known as load balancing.
EIGRP-enabled routers to establish neighbor adjacencies with other EIGRP-enabled routers by exchanging EIGRP Hello packets. Without establishing neighbor adjacencies routers cannot send or receive any updates. Using the “show ip eigrp neighbors” command, we can examine the neighbor’s table and verify EIGRP adjacencies. Figure 1 illustrates the output of the “show ip eigrp neighbors” command.
We can see the IPv4 address for each adjacent router and the interface that this router uses to reach the EIGRP neighbors. Each router uses this topology to list two adjacent neighbors in the neighbors table. The output also includes the following:
H column– This field lists the neighbors in the order they were learned. The first neighbor will have a value of 0, the second neighbors a value of 1, and so on.
Address– This is the IPv4 address of the adjacent neighbors.
Interface– This is the local interface on which this Hello packet was received.
Hold– This field specify how long EIGRP will wait to hear from the neighbour before declaring it down. When a Hello packet is received, the time value is reset to the maximum hold time for that interface, and then again counts down to zero. If zero is reached, the neighbour is considered down.
Uptime– This is the time since this neighbour was added to the neighbour table. The time in hours:minutes: seconds.
Smooth Round Trip Timer (SRTT) –The time it takes to send an EIGRP packet and receive an acknowledgement from the neighbour.
Retransmission Timeout (RTO) – The time that EIGRP will wait previous to retransmitting a packet from the retransmission queue to a neighbour for reliable EIGRP packets.
Queue Count– This values always be zero, if the value is more than zero, then EIGRP packets wait to be sent.
Sequence Number– The sequence number is used to track updates, queries, and reply packets.
This command is useful for verifying and troubleshooting EIGRP problems. We can check the neighbour IP address in the adjacencies table. We can also use the “show ip interface brief” command to verify the interface state. If the interface is an active state then we can try pinging the IPv4 address of the neighbour. If the ping not received, it means that the neighbour interface is down and must be activated. If the ping is successful and doing well then we should check that the EIGRP autonomous system number must be the same.
A passive interface is used in all routing protocols, where we can stop sending updates from a specific interface. The behavior varies from one protocol to another. In EIGRP, using the passive-interface, we stop sending outgoing hello packets; therefore, the router cannot form any neighbor adjacencies via the passive interface. This behavior stops both outgoing and incoming routing updates.
The passive-interface command can be used to stop the neighbor adjacencies. The command can be used in router configuration mode. We enable a passive interface to suppress unnecessary update traffic, for example, when an interface is a LAN interface with no other routers connected. It also increases security controls, such as stopping an unknown scoundrel routing device from receiving EIGRP updates. Figure 1 illustrates the R1, R2, and R3 where neighbor routers are not attached with interfaces GigabitEthernet 0/2 interfaces of R1, GigabitEthernet 0/2 interfaces of R2, and GigabitEthernet 0/2 interfaces of R3. The command syntax is the following:
The passive-interface command prevents the exchange of routes on these interfaces, but EIGRP still includes these interfaces and their addresses in routing updates. The passive interface configuration for the above topology is as follows:
Using the passive-interface default command, we can configure all interfaces as passive. To disable an interface as passive, we can use the no passive-interface interface-type interface-number command in router configuration mode.
The passive-interface increases security by preventing the hello packet from An example of using the passive-interface to increase security controls is
When a network connects to a third-party organization over which the network administrator has no control, such as an ISP network, the local network must advertise the interface link through the local network. This security risk occurs if the ISP sends or receives a routing update to the local network devices. Anyone can compromise the local network through an ISP. So, in this case, we can set the interface connected to the ISP as a passive interface.
Verifying the Passive Interface
We can verify the interface on a router configured as passive using the “show ip protocols” command in privileged EXEC mode. Figure 2 illustrates the output of this command on router R1. Notice that a GigabitEthernet 0/2 interface of R1 is a passive interface, but the routing update still includes the address for this interface, 192.168.0.0.
FAQs: EIGRP Passive-Interface
1. What is EIGRP?
EIGRP (Enhanced Interior Gateway Routing Protocol) is an advanced distance-vector routing protocol used in computer networks to automate routing decisions and configuration.
2. What is a passive-interface in EIGRP?
A passive interface in EIGRP prevents sending EIGRP hello packets on a particular interface. This effectively stops EIGRP from forming neighbor relationships on that interface.
3. Why would I use a passive-interface in EIGRP?
Using a passive-interface can improve network security and efficiency by preventing unwanted EIGRP traffic and avoiding unnecessary formation of EIGRP neighbor relationships on certain interfaces.
4. How do I configure a passive-interface in EIGRP?
To configure a passive-interface in EIGRP, you use the command “passive-interface <interface>” in the EIGRP configuration mode. This disables the sending of EIGRP hello packets on the specified interface.
5. Can I use the passive-interface command on multiple interfaces?
Yes, you can configure multiple interfaces as passive by specifying each interface in the EIGRP configuration mode.
6. What happens to existing EIGRP neighbors when I configure an interface as passive?
Existing EIGRP neighbors on an interface that is newly configured as passive will be dropped, as the interface will no longer send EIGRP hello packets to maintain the neighbor relationship.
7. Is there a way to make all interfaces passive by default in EIGRP?
Yes, you can use the “passive-interface default” command in EIGRP configuration mode to make all interfaces passive by default. You can then use the “no passive-interface <interface>” command to selectively enable EIGRP on specific interfaces.
8. What are the benefits of using a passive-interface in EIGRP?
The main benefits include enhanced security by restricting EIGRP traffic and more efficient use of network resources by reducing unnecessary EIGRP communications.
9. Can the passive-interface command affect network performance?
Using passive-interface judiciously can improve network performance by limiting unnecessary EIGRP traffic. However, misconfiguring it might prevent essential EIGRP neighbor relationships, potentially causing network disruptions.
10. How can I verify the configuration of passive interfaces in EIGRP?
You can use the “show ip eigrp interfaces” command to verify which interfaces are configured as passive in EIGRP.
The network command has the same function as in all IGP routing protocols. It enables any interface on this router that matches the network address. It enables EIGRP routing on an interface, using the command in router configuration mode and enter the classful network address for each directly connected network. The command syntax is as under
The ipv4-network-address is the classful IPv4 network address for this interface. I am using the same topology as already used in the “EIGRP router command” article. The topology is illustrated in figure 1. The configuration of the network is the following:
Router R1
R1(config)#router eigrp 1
R1(config-router)# eigrp router-id 1.1.1.1
R1(config-router)# network 10.0.0.0
R1(config-router)# 192.168.0.0
Router R2
R2(config)#router eigrp 1
R2(config-router)# eigrp router-id 2.2.2.2
R2(config-router)# network 10.0.0.0
R2(config-router)# 192.168.1.0
Router R3
R3(config)#router eigrp 1
R3(config-router)# eigrp router-id 3.3.3.3
R3(config-router)# network 10.0.0.0
R3(config-router)# 192.168.2.0
Figure 2 illustrates the network command used to enable EIGRP on R1’s interfaces for subnets 10.0.0.0. When EIGRP is configured on R1’s for network 10.0.0.0, DUAL sends a notification message to the console stating that a neighbour adjacency with another EIGRP router on that interfaces GigabitEthernet 0/0 and GigabitEthernet 0/1 has been established. This new adjacency happens automatically because both R1 and R2 use the same eigrp 1 autonomous system number, and the auto summarization is enabled on EIGRP, so, both routers now can send updates on their interfaces in their network.
The Network Command and Wildcard Mask
When we use the networkcommand for IPv4 network address, such as 10.0.0.0, all interfaces on the router that belong to that classful network address are enabled for EIGRP. So, there are possibilities when you do not want to incorporate all interfaces within a network when enabling EIGRP. For example, in Figure 1, assume that you want to enable EIGRP on R2, but only for the subnet 10.10.10.0 255.255.255.252, on the G0/1 interface.
To configure EIGRP to advertise specific subnets only we will use the wild card mask with the network command. The command syntax for using wildcard mask is as under :
Wildcard mask as the opposite of a subnet mask. For example, the inverse of the subnet mask 255.255.255.252 is 0.0.0.3. To calculate the wildcard mask of the subnet mask, deduct the subnet mask from 255.255.255.255 as shown in figure 3:
Thenetwork <network-address> <wildcard-mask> command particularly enables EIGRP on the particular interfaces. The configurations of the above topology using wildcard mask are as under.
Router R1
R1(config)#router eigrp 1
R1(config-router)# eigrp router-id 1.1.1.1
R1(config-router)# network 10.10.10.0 0.0.0.3
R1(config-router)# network 10.10.10.4 0.0.0.3
R1(config-router)# 192.168.0.0 0.0.0.255
Router R2
R2(config)#router eigrp 1
R2(config-router)# eigrp router-id 2.2.2.2
R2(config-router)# network 10.10.10.4 0.0.0.3
R2(config-router)# network 10.10.10.8 0.0.0.3
R2(config-router)# 192.168.1.0 0.0.0.255
Router R3
R3(config)#router eigrp 1
R3(config-router)# eigrp router-id 3.3.3.3
R3(config-router)# network 10.10.10.8 0.0.0.3
R3(config-router)# network 10.10.10.0 0.0.0.3
R3(config-router)# 192.168.2.0 0.0.0.255
Some versions of the Cisco IOS support the subnet mask instead of a wildcard mask. if the subnet mask is used, the Cisco IOS converts the command to thewildcard-mask format within the configuration. We can verify this using theshow running -configcommand.
EIGRP router ID is a 32-bit unique identifier identifying the router in the EIGRP domain. The router ID (RID) is represented in the same way as the IPv4 address. The router ID is used both in EIGRP and OSPF, while the role of the router ID is more significant in OSPF. EIGRP automatically selects the highest IP address on any active loopback interface as the EIGRP router ID. If there is no loopback interface, the highest IP address on any active interface is used. The criteria for EIGRP router ID in Cisco routers are the following:
Configure the router-ID using the command “eigrp router-id <IPv4 address>in router configuration mode.
If the router ID is not configured with the above command, it selects the highest IPv4 address of any of its loopback interfaces.
If no loopback interfaces are configured, the router selects any physical interface’s highest active IPv4 address.
A loopback address is a virtual interface automatically automatically changing the state to up when configured. The interface is not required to be included in the EIGRP network using network commands. It only requires the loopback interface in the up/up state.
Usually, we configure router ID using the “eigrp router-id” command. However, some versions of IOS will accept the command router-id without specifying eigrp.
Explore the basics of EIGRP router ID and its role in network routing.
Configuring the EIGRP Router ID
We can configure the eigrp router-id command in router configuration mode. When configuring EIGRP router ID with this command, it takes priority over any loopback or physical interface IPv4 addresses. Now, look at Figure 1, which shows the same topology we used for the router command. Let’s see the configuration of EIGRP router ID on all three routers.
Router R1R1(config)#router eigrp 1
R1(config-router)# eigrp router-id 1.1.1.1
Router R2
R2(config)#router eigrp 1
R2(config-router)# eigrp router-id 2.2.2.2
Router R3
R3(config)#router eigrp 1
R3(config-router)# eigrp router-id 3.3.3.3
The IPv4 address configured as router ID is any 32-bit number displayed in dotted-decimal notation. We can configure any IPv4 address as a router ID except the 0.0.0.0 and 255.255.255.255. The router ID must be unique in the EIGRP routing domain.
Loopback Address Used as the Router ID
We can use the IPv4 loopback address and the IPv4 address of the physical interface as router IDs. However, the loopback address has an advantage over the IPv4 address of the physical interface. The loopback interface cannot fail like the physical interface because there are no cables or adjacent devices on which the loopback interface depends on being in the upstate. So, using a loopback address for the router ID can provide a more reliable router ID than using an interface address.
If the router ID is not configured manually and loopback interfaces are configured, then EIGRP chooses the highest IPv4 address from the loopback interfaces as the router ID. To enable and configure the loopback interface, use the following command.
Router(config)# interface loopback <number>Router(config-if)# ip address <ipv4-addresssubnet-mask
We can remove the router-id using the “no router eigrp” command if the router ID is manually configured with the eigrp router-id command. If the router dynamically selects the router-id, we cannot change it.
Verifying the EIGRP Process
We can verify the EIGRP process and router-id using the show ip protocols command. Figure 2 illustrates the output for R1. The show ip protocols command also displays the parameters and current state of active routing protocol processes, including EIGRP and OSPF. The command also displays different types of output specific to each routing protocol.
FAQs
What is an EIGRP router ID?
An EIGRP router ID is a unique 32-bit identifier assigned to each EIGRP-speaking router, represented in a dotted decimal format like an IPv4 address.
How is an EIGRP router ID determined?
EIGRP automatically selects the highest IP address on any active loopback interface as the router ID. If no loopback interface is configured, the highest IP address on any active interface is used.
Why is the EIGRP router ID necessary?
The router ID is crucial for identifying routers within an EIGRP autonomous system and plays a role in neighborship and routing decisions.
Can two EIGRP routers have the same router ID?
While EIGRP routers can form neighbor adjacencies with duplicate router IDs, assigning unique IDs is best practice to avoid potential issues with redistributed routes.
How do you manually configure an EIGRP router ID?
You can manually set the router ID using the eigrp router-id command in EIGRP configuration mode.
