EIGRP (Enhanced Interior Gateway Routing Protocol)
EIGRP is a Cisco-proprietary Hybrid routing protocol
EIGRP the following characteristics:
•EIGRP uses Diffusing Update Algorithm (DUAL) to determine the best path among all “feasible” paths. DUAL also helps ensure a loopfree routing environment.
• EIGRP will form neighbor relationships with adjacent routers in the same Autonomous System (AS).
• EIGRP traffic is either sent as unicasts, or as multicasts on address 224.0.0.10, depending on the EIGRP packet type.
• Reliable Transport Protocol (RTP) is used to ensure delivery of most EIGRP packets.
• EIGRP routers do not send periodic, full-table routing updates. Updates are sent when a change occurs, and include only the change
• EIGRP is a classless protocol, and thus supports VLSMs.
• EIGRP applies an Administrative Distance of 90 for routes originating within the local Autonomous System.
• EIGRP applies an Administrative Distance of 170 for external routes coming from outside the local Autonomous System
• EIGRP uses Bandwidth and Delay of the Line, by default, to calculate its distance metric. It also supports three other parameters to calculate its metric: Reliability, Load, and MTU.
• EIGRP has a maximum hop-count of 224, though the default maximum hop-count is set to 100.
EIGRP table :
• Neighbor table – list of all neighboring routers. Neighbors must belong to the same Autonomous System
• Topology table – list of all routes in the Autonomous System
• Routing table – contains the best route for each known network
EIGRP Neighbors
EIGRP forms neighbor relationships is called adjacencies,
after an adjacency is formed can routers share routing information. Hello packets are sent as multicasts to address 224.0.0.10.
EIGRP Hellos are sent every 5 seconds
The EIGRP Hello timer can be adjusted on a per interface basis:
Router(config-if)# ip hello-interval eigrp 10 7
The above command allows us to change the hello timer to 7 seconds for Autonomous System 10.
By default, the Hold timer is three times the Hello timer. Thus, on highspeed links the timer is set to 15 seconds,
The Hold timer can also be adjusted on a per interface basis:
Router(config-if)# ip hold-interval eigrp 10 21
A neighbor table is constructed from the EIGRP Hello packets, which includes the following information:
• The IP address of the neighboring router.
• The local interface that received the neighbor’s Hello packet.
• The Hold timer.
• A sequence number indicating the order neighbors were learned
EIGRP Topology Table
Once EIGRP neighbors form adjacencies, they will begin to share routing information. Each router’s update contains a list of all routes known by that router, and the respective metrics for those routes. All such routes are added to an EIGRP router’s topology table. The route with the lowest metric to each network will become the Feasible Distance (FD). The Feasible Distance for each network will be installed into the routing table. The Feasible Distance is derived from the Advertised Distance of the router sending the update, and the local router’s metric to the advertising router.
To allow convergence to occur quickly if a link fails, EIGRP includes backup routes in the topology table called Feasible Successors (FS). A route will only become a Successor if its Advertised Distance is less than the current Feasible Distance. This is known as a Feasible Condition (FC)
EIGRP Packet Types
• Hello packets - multicast
• Update packets – unicast or multicast
• Query packets – multicast
• Reply packets – unicast
• Acknowledgement packets - unicast
Hello packets are used to form neighbor relationships, and were explained in detail previously. Hello packets are always multicast to address 224.0.0.10.
Update packets are sent between neighbors to build the topology and routing tables. Updates sent to new neighbors are sent as unicasts. However, if a route’s metric is changed, the update is sent out as a multicast to address 224.0.0.10.
Query packets are sent by a router when a Successor route fails, and there are no Feasible Successors in the topology table. The router places the route in an Active state, and queries its neighbors for an alternative route. Query packets are sent as a multicast to address 224.0.0.10.
Reply packets are sent in response to Query packets, assuming the responding router has an alternative route (feasible successor). Reply packets are sent as a unicast to the querying router.
Recall that EIGRP utilizes the Reliable Transport Protocol (RTP) to ensure reliable delivery of most EIGRP packets. Delivery is guaranteed by having packets acknowledged using…..Acknowledgment packets!
Acknowledgment packets (also known as ACK’s) are simply Hello packets with no data, other than an acknowledgment number. ACK’s are always sent as unicasts. The following packet types employ RTP to ensure reliable delivery via ACK’s:
EIGRP Route States• Active state
• Passive StateA Passive state indicates that a route is reachable, and that EIGRP is fully converged. A stable EIGRP network will have all routes in a Passive state.
A route is placed in an Active state when the Successor and any Feasible Successors fail, forcing the EIGRP to send out Query packets and reconverge. Multiple routes in an Active state indicate an unstable EIGRP network. If a Feasible Successor exists, a route should never enter an Active state.
EIGRP Metrics• Bandwidth (K1) – Slowest link in the route path, measured in kilobits
• Load (K2) – Cumulative load of all outgoing interfaces in the path, given as a fraction of 255
• Delay of the Line (K3) – Cumulative delay of all outgoing interfaces in the path in tens of microseconds
• Reliability (K4) – Average reliability of all outgoing interfaces in the path, given as a fraction of 255
• MTU (K5) – The smallest Maximum Transmission Unit in the path. The MTU value is actually never used to calculate the metric
formula for determining the EIGRP metric is:
[10000000/bandwidth + delay] * 256
Configuring EIGRP Metrics
Router(config)# router eigrp 10
Router(config-router)# metric weights 0 1 1 1 0 0
The first command enables the EIGRP process for Autonomous System 10. The second actually identifies which EIGRP metrics to use. The first number (0) is for Type of Service, and should always be zero. The next numbers, in order, are K1 (1), K2 (1), K3 (1), K4 (0), and K5 (0)
configure of bandwidth and delay
Router(config)# int s0/0
Router(config-if)# bandwidth 64
Router(config-if)# ip bandwidth-percent eigrp 10 30
serial interface bandwidth 1.54 Mbps
By default, EIGRP will use up to 50% of the bandwidth of an interface.
ip bandwidth-percent eigrp this command to use set bandwidth on interface
the delay metric (in tens of microseconds) on an interface
Router(config)# int s0/0
Router(config-if)# delay 10000
Autonomous System number can range from 1 to 65535
The network statements serve two purposes in EIGRP:
• First, they identify which networks you wish to advertise to other EIGRP routers (similar to RIP).
• Second, they identify which interfaces on the local router to attempt to form neighbor relationships out of (similar to OSPF).
EIGRP is a Cisco-proprietary Hybrid routing protocol
EIGRP the following characteristics:
•EIGRP uses Diffusing Update Algorithm (DUAL) to determine the best path among all “feasible” paths. DUAL also helps ensure a loopfree routing environment.
• EIGRP will form neighbor relationships with adjacent routers in the same Autonomous System (AS).
• EIGRP traffic is either sent as unicasts, or as multicasts on address 224.0.0.10, depending on the EIGRP packet type.
• Reliable Transport Protocol (RTP) is used to ensure delivery of most EIGRP packets.
• EIGRP routers do not send periodic, full-table routing updates. Updates are sent when a change occurs, and include only the change
• EIGRP is a classless protocol, and thus supports VLSMs.
• EIGRP applies an Administrative Distance of 90 for routes originating within the local Autonomous System.
• EIGRP applies an Administrative Distance of 170 for external routes coming from outside the local Autonomous System
• EIGRP uses Bandwidth and Delay of the Line, by default, to calculate its distance metric. It also supports three other parameters to calculate its metric: Reliability, Load, and MTU.
• EIGRP has a maximum hop-count of 224, though the default maximum hop-count is set to 100.
EIGRP table :
• Neighbor table – list of all neighboring routers. Neighbors must belong to the same Autonomous System
• Topology table – list of all routes in the Autonomous System
• Routing table – contains the best route for each known network
EIGRP Neighbors
EIGRP forms neighbor relationships is called adjacencies,
after an adjacency is formed can routers share routing information. Hello packets are sent as multicasts to address 224.0.0.10.
EIGRP Hellos are sent every 5 seconds
The EIGRP Hello timer can be adjusted on a per interface basis:
Router(config-if)# ip hello-interval eigrp 10 7
The above command allows us to change the hello timer to 7 seconds for Autonomous System 10.
By default, the Hold timer is three times the Hello timer. Thus, on highspeed links the timer is set to 15 seconds,
The Hold timer can also be adjusted on a per interface basis:
Router(config-if)# ip hold-interval eigrp 10 21
A neighbor table is constructed from the EIGRP Hello packets, which includes the following information:
• The IP address of the neighboring router.
• The local interface that received the neighbor’s Hello packet.
• The Hold timer.
• A sequence number indicating the order neighbors were learned
EIGRP Topology Table
Once EIGRP neighbors form adjacencies, they will begin to share routing information. Each router’s update contains a list of all routes known by that router, and the respective metrics for those routes. All such routes are added to an EIGRP router’s topology table. The route with the lowest metric to each network will become the Feasible Distance (FD). The Feasible Distance for each network will be installed into the routing table. The Feasible Distance is derived from the Advertised Distance of the router sending the update, and the local router’s metric to the advertising router.
To allow convergence to occur quickly if a link fails, EIGRP includes backup routes in the topology table called Feasible Successors (FS). A route will only become a Successor if its Advertised Distance is less than the current Feasible Distance. This is known as a Feasible Condition (FC)
EIGRP Packet Types
• Hello packets - multicast
• Update packets – unicast or multicast
• Query packets – multicast
• Reply packets – unicast
• Acknowledgement packets - unicast
Hello packets are used to form neighbor relationships, and were explained in detail previously. Hello packets are always multicast to address 224.0.0.10.
Update packets are sent between neighbors to build the topology and routing tables. Updates sent to new neighbors are sent as unicasts. However, if a route’s metric is changed, the update is sent out as a multicast to address 224.0.0.10.
Query packets are sent by a router when a Successor route fails, and there are no Feasible Successors in the topology table. The router places the route in an Active state, and queries its neighbors for an alternative route. Query packets are sent as a multicast to address 224.0.0.10.
Reply packets are sent in response to Query packets, assuming the responding router has an alternative route (feasible successor). Reply packets are sent as a unicast to the querying router.
Recall that EIGRP utilizes the Reliable Transport Protocol (RTP) to ensure reliable delivery of most EIGRP packets. Delivery is guaranteed by having packets acknowledged using…..Acknowledgment packets!
Acknowledgment packets (also known as ACK’s) are simply Hello packets with no data, other than an acknowledgment number. ACK’s are always sent as unicasts. The following packet types employ RTP to ensure reliable delivery via ACK’s:
EIGRP Route States• Active state
• Passive StateA Passive state indicates that a route is reachable, and that EIGRP is fully converged. A stable EIGRP network will have all routes in a Passive state.
A route is placed in an Active state when the Successor and any Feasible Successors fail, forcing the EIGRP to send out Query packets and reconverge. Multiple routes in an Active state indicate an unstable EIGRP network. If a Feasible Successor exists, a route should never enter an Active state.
EIGRP Metrics• Bandwidth (K1) – Slowest link in the route path, measured in kilobits
• Load (K2) – Cumulative load of all outgoing interfaces in the path, given as a fraction of 255
• Delay of the Line (K3) – Cumulative delay of all outgoing interfaces in the path in tens of microseconds
• Reliability (K4) – Average reliability of all outgoing interfaces in the path, given as a fraction of 255
• MTU (K5) – The smallest Maximum Transmission Unit in the path. The MTU value is actually never used to calculate the metric
formula for determining the EIGRP metric is:
[10000000/bandwidth + delay] * 256
Configuring EIGRP Metrics
Router(config)# router eigrp 10
Router(config-router)# metric weights 0 1 1 1 0 0
The first command enables the EIGRP process for Autonomous System 10. The second actually identifies which EIGRP metrics to use. The first number (0) is for Type of Service, and should always be zero. The next numbers, in order, are K1 (1), K2 (1), K3 (1), K4 (0), and K5 (0)
configure of bandwidth and delay
Router(config)# int s0/0
Router(config-if)# bandwidth 64
Router(config-if)# ip bandwidth-percent eigrp 10 30
serial interface bandwidth 1.54 Mbps
By default, EIGRP will use up to 50% of the bandwidth of an interface.
ip bandwidth-percent eigrp this command to use set bandwidth on interface
the delay metric (in tens of microseconds) on an interface
Router(config)# int s0/0
Router(config-if)# delay 10000
Autonomous System number can range from 1 to 65535
The network statements serve two purposes in EIGRP:
• First, they identify which networks you wish to advertise to other EIGRP routers (similar to RIP).
• Second, they identify which interfaces on the local router to attempt to form neighbor relationships out of (similar to OSPF).
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