Enhanced Interior Gateway Routing Protocol
From Wikipedia, the free encyclopedia
Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary routing protocol based on their original IGRP. EIGRP is a balanced hybrid IP 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.
Some of the routing optimizations are based on the Diffusing Update Algorithm (DUAL) work from SRI, which guarantees loop-free operation. In particular, DUAL avoids the "count to infinity" behaviour of RIP when a destination becomes completely unreachable. The maximum hop count of EIGRP-routed packets is 224. EIGRP has a lower maximum hop count than IGRP, 224 for EIGRP and 255 for IGRP. This is due to EIGRP having a larger routing overhead.
Contents |
[edit] Basic operation
The data EIGRP collects is stored in three tables:
- Neighbor Table: Stores data about the neighbouring 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 the aggregation of the routing tables gathered from all the neighbours. This table actually contains only those routes which are considered not to potentially be part of routing loops (part of the DUAL algorithm); in EIGRP terminology, they are considered "Feasible Successors".
- Routing table: Stores the actual routes to all destinations; routes may be marked either as "Passive", which is the normal state when the routing has stabilized, or "Active" when the topology has changed, and the router is in the process of updating its route to that destination.
[edit] Multiple metrics
EIGRP associates five different metrics with each route:
- Delay
- Bandwidth
- Reliability
- Load
- MTU (though not actually used in the calculation)
For the purposes of comparing routes, these are combined together in a weighted formula to produce a single metric:
- [(K1*Bandwidth) + ((K2*Bandwidth)/(256-Load)) + (K3*Delay)] * [(K5/(Reliability + K4))] * 256
where the various constants (K1 through K5) can be set by the user to produce varying behaviours. If K5 is set to zero, the K4/K5 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.
EIGRP scales Bandwidth and Delay metrics with following calculations:
- Bandwidth for EIGRP = (10000000/Bandwidth)
- Delay for EIGRP = Delay/10
These calculations are done because EIGRP unlike IGRP uses 32-bit metric, not 24-bit as in IGRP.
[edit] Successor
A successor (or next hop) is a primary route that is used to reach a destination. There is a limit of four successor routes for any specific route. The successor routes are placed in the topology table as well.
[edit] Feasible Successor
A feasible successor(FS) is a backup route. These routes are used in case a successor route dies. A feasible successor has to have a lower advertised cost then the current successors cost to a route destination.
[edit] Advertised Distance and Feasible Distance
Advertised Distance is a distance to a particular destination as reported by a router to its neighbors. This distance is sometimes also called a Reported Distance.
After a router receives information about a destination network and the associated Reported Distance from its neighbor, it calculates its own distance to the network via this neighbor by adding the cost of the link to this neighbor to the Reported Distance. If more neighbors advertise the same destination network (with respective Reported Distances), the distance via every such neighbor will be calculated. The lowest calculated distance available is called a Feasible Distance.
A Feasible Distance is thus the lowest known distance from a router to a particular destination. This distance will be advertised to neighboring routers and from their point of view, it will be called the Advertised or the Reported Distance.
[edit] Other details
EIGRP is able to deal with Classless Inter-Domain Routing (CIDR), allowing the use of variable-length subnet masks - the protocol's main advantage over its predecessor. Its main disadvantage is that it runs only on Cisco equipment, which may lead to an organization being locked in to this vendor.
EIGRP can run separate routing processes for IP, IPX and AppleTalk. However, this does not facilitate translation between protocols.
