Enhanced Interior Gateway Routing Protocol - (EIGRP) is a Cisco proprietary routing protocol loosely based on their original IGRP. EIGRP is an advanced distance-vector routing protocol, with optimizations to minimize both the routing instability incurred after topology changes, as well as the use of bandwidth and processing power in the router. Routers that support EIGRP will automatically redistribute route information to IGRP neighbors by converting the 32 bit EIGRP metric to the 24 bit IGRP metric. Most of the routing optimizations are based on the Diffusing Update Algorithm (DUAL) work from SRI, which guarantees loop-free operation and provides a mechanism for fastconvergence.
EIGRP stores data in three tables:
Neighbor Table: Stores data about the neighboring routers, i.e. those directly accessible through directly connected interfaces.
Topology Table: Confusingly named, this table does not store an overview of the complete network topology; rather, it effectively contains only the aggregation of the routing tablesgathered from all directly connected neighbors. This table contains a list of destination networks in the EIGRP-routed network together with their respective metrics. Also for every destination, a successor and a feasible successor are identified and stored in the table if they exist. Every destination in the topology table can be marked either as "Passive", which is the state when the routing has stabilized and the router knows the route to the destination, or "Active" when the topology has changed and the router is in the process of (actively) updating its route to that destination.
Routing table: Stores the actual routes to all destinations; the routing table is populated from the topology table with every destination network that has its successor and optionallyfeasible successor identified (if unequal-cost load-balancing is enabled using the variance command). The successors and feasible successors serve as the next hop routers for these destinations.
Unlike most other distance vector protocols, EIGRP does not rely on periodic route dumps in order to maintain its topology table. Routing information is exchanged only upon the establishment of new neighbor adjacency after which only changes are sent. Also, it uses route tagging.
Bandwidth
Minimum Bandwidth (in kilobits per second) along the path from router to destination network
Load
Load (number in range 1 to 255; 255 being saturated)
Delay
Total Delay (in 10s of microseconds) along the path from router to destination network
Reliability
Reliability (number in range 1 to 255; 255 being the most reliable)
MTU
Minimum path Maximum Transmission Unit (MTU) (never used in the metric calculation)
Hop Count
Number of routers a packet passes through when routing to a remote network, used to limit the EIGRP AS.
The K Values There are five (5) K values used in the Composite metric calculation - K1 through K5. The K values only act as multipliers or modifiers in the composite metric calculation. K1 is not equal to Bandwidth, etc.
By default, only total delay and minimum bandwidth are considered when EIGRP is started on a router, but an administrator can enable or disable all the K values as needed to consider the other Vector metrics.
For the purposes of comparing routes, these are combined together in a weighted formula to produce a single overall metric:
where the various constants (K1 through K5) can be set by the user to produce varying behaviors. An important and totally non-obvious fact is that if K5 is set to zero, the term
is not used (i.e. taken as 1).
The default is for K1 and K3 to be set to 1, and the rest to zero, effectively reducing the above formula to (Bandwidth + Delay) * 256.
Obviously, these constants must be set to the same value on all routers in an EIGRP system, or permanent routing loops will probably result. Cisco routers running EIGRP will not form an EIGRP adjacency and will complain about K-values mismatch until these values are identical on these routers.
EIGRP scales Bandwidth and Delay metrics with following calculations:Bandwidth for EIGRP = 107 / Interface BandwidthDelay for EIGRP = Interface Delay / 10
On Cisco routers, the interface bandwidth is a configurable static parameter expressed in kilobits per second (setting this only affects metric calculation and not actual line bandwidth). Dividing a value of 107 kbit/s (i.e. 10 Gbit/s) by the interface bandwidth statement yields a value that is used in the weighted formula. Analogously, the interface delay is a configurable static parameter expressed in microseconds. Dividing this interface delay value by 10 yields a delay in units of tens of microseconds that is used in the weighted formula.
IGRP uses the same basic formula for computing the overall metric, the only difference is that in IGRP, the formula does not contain the scaling factor of 256. In fact, this scaling factor was introduced as a simple means to facilitate backward compatility between EIGRP and IGRP: In IGRP, the overall metric is a 24-bit value while EIGRP uses a 32-bit value to express this metric. By multiplying a 24-bit value with the factor of 256 (effectively bit-shifting it 8 bits to the left), the value is extended into 32 bits, and vice versa. This way, redistributing information between EIGRP and IGRP involves simply dividing or multiplying the metric value by a factor of 256, which is done automatically.
EIGRP also maintains a hop count for every route, however, the hop count is not used in metric calculation. It is only verified against a predefined maximum on an EIGRP router (by default it is set to 100 and can be changed to any value between 1 and 255). Routes having a hop count higher than the maximum will be advertised as unreachable by an EIGRP router.
EIGRP stores data in three tables:
Neighbor Table: Stores data about the neighboring routers, i.e. those directly accessible through directly connected interfaces.
Topology Table: Confusingly named, this table does not store an overview of the complete network topology; rather, it effectively contains only the aggregation of the routing tablesgathered from all directly connected neighbors. This table contains a list of destination networks in the EIGRP-routed network together with their respective metrics. Also for every destination, a successor and a feasible successor are identified and stored in the table if they exist. Every destination in the topology table can be marked either as "Passive", which is the state when the routing has stabilized and the router knows the route to the destination, or "Active" when the topology has changed and the router is in the process of (actively) updating its route to that destination.
Routing table: Stores the actual routes to all destinations; the routing table is populated from the topology table with every destination network that has its successor and optionallyfeasible successor identified (if unequal-cost load-balancing is enabled using the variance command). The successors and feasible successors serve as the next hop routers for these destinations.
Unlike most other distance vector protocols, EIGRP does not rely on periodic route dumps in order to maintain its topology table. Routing information is exchanged only upon the establishment of new neighbor adjacency after which only changes are sent. Also, it uses route tagging.
Bandwidth
Minimum Bandwidth (in kilobits per second) along the path from router to destination network
Load
Load (number in range 1 to 255; 255 being saturated)
Delay
Total Delay (in 10s of microseconds) along the path from router to destination network
Reliability
Reliability (number in range 1 to 255; 255 being the most reliable)
MTU
Minimum path Maximum Transmission Unit (MTU) (never used in the metric calculation)
Hop Count
Number of routers a packet passes through when routing to a remote network, used to limit the EIGRP AS.
The K Values There are five (5) K values used in the Composite metric calculation - K1 through K5. The K values only act as multipliers or modifiers in the composite metric calculation. K1 is not equal to Bandwidth, etc.
By default, only total delay and minimum bandwidth are considered when EIGRP is started on a router, but an administrator can enable or disable all the K values as needed to consider the other Vector metrics.
For the purposes of comparing routes, these are combined together in a weighted formula to produce a single overall metric:
where the various constants (K1 through K5) can be set by the user to produce varying behaviors. An important and totally non-obvious fact is that if K5 is set to zero, the term
is not used (i.e. taken as 1).The default is for K1 and K3 to be set to 1, and the rest to zero, effectively reducing the above formula to (Bandwidth + Delay) * 256.
Obviously, these constants must be set to the same value on all routers in an EIGRP system, or permanent routing loops will probably result. Cisco routers running EIGRP will not form an EIGRP adjacency and will complain about K-values mismatch until these values are identical on these routers.
EIGRP scales Bandwidth and Delay metrics with following calculations:Bandwidth for EIGRP = 107 / Interface BandwidthDelay for EIGRP = Interface Delay / 10
On Cisco routers, the interface bandwidth is a configurable static parameter expressed in kilobits per second (setting this only affects metric calculation and not actual line bandwidth). Dividing a value of 107 kbit/s (i.e. 10 Gbit/s) by the interface bandwidth statement yields a value that is used in the weighted formula. Analogously, the interface delay is a configurable static parameter expressed in microseconds. Dividing this interface delay value by 10 yields a delay in units of tens of microseconds that is used in the weighted formula.
IGRP uses the same basic formula for computing the overall metric, the only difference is that in IGRP, the formula does not contain the scaling factor of 256. In fact, this scaling factor was introduced as a simple means to facilitate backward compatility between EIGRP and IGRP: In IGRP, the overall metric is a 24-bit value while EIGRP uses a 32-bit value to express this metric. By multiplying a 24-bit value with the factor of 256 (effectively bit-shifting it 8 bits to the left), the value is extended into 32 bits, and vice versa. This way, redistributing information between EIGRP and IGRP involves simply dividing or multiplying the metric value by a factor of 256, which is done automatically.
EIGRP also maintains a hop count for every route, however, the hop count is not used in metric calculation. It is only verified against a predefined maximum on an EIGRP router (by default it is set to 100 and can be changed to any value between 1 and 255). Routes having a hop count higher than the maximum will be advertised as unreachable by an EIGRP router.
