EIGRP can only be used within CISCO routers, and all Cisco routers support MD5 authentication. But the routes are not encrypted, so a sniffer can easily see the password/s.
• Multiple Network Layer Protocol Support: EIGRP can support IP, IPX, and AppleTalk, whereas the other routing protocols support only one routed protocol. EIGRP will also perform auto-redistribution with NLSP, IPXRIP, RTMP. EIGRP supports incremental SAP and RIP updates, 224 HOPS, and it uses bandwidth + delay which is far more better than just Ticks and Hops used by IPXRIP. For RTMP it supports event driven updates, but it must run in a clientless networks(WAN), and also a better metric calculation.
• Use Of Multicast Instead Of Broadcast: EIGRP uses multicast address of 224.0.0.10 instead of broadcast.
• Unequal and Equal Cost Path Load-Balancing: This feature will enable the administrators to distribute traffic flow in the network. By default EIGRP will use up to 4 paths and this can be increased to 6.
• OSI and EIGRP: Like all TCP/IP routing protocols EIGRP relies in IP to deliver the packets, EIGRP maps to the transport layer of OSI and uses protocol number 88.
• Support Of Different Topology: EIGRP can support broadcast multi-access topologies such as Token-Ring, and Ethernet. Point to point topology such as HDLC. NBMA topology such as Frame-Relay.
• Easy configuration: The configuration of EIGRP is very similar to IGRP which is very simple.
• Support of hierarchical addressing scheme: Eigrp supports FLSM, VLSM, CIDR/Supernetting.
• 100% Loop Free: EIGRP uses DUAL to attain fast convergence while maintaining a totally loop free topology at every instance.
• Metrics: EIGRP uses 2 step metric: 1. VECTOR 2. COMPOSITE
Vector metric is: Min MTU, MAX Load, Min Reliability, Total delay, Min Bandwidth and Hop count.
The vector metric of a route received from a neighbor is computed from the received vector metric and the metric of the interface through which the route was received.
After the vector is received and calculated it is stored in the topology table.
The vector metric is never adjusted in the outgoing updates, the router always reports the values it has in its topology table and relies on the receiving router to adjust the values.
In the above diagram, the minute the Ethernet port on R-A comes active, it notifies R-B, and R-D with its own vector metric, R-D, and R-B will adjust these values based on the parameters of their interface to R-A, and then they will advertise that cost to R-C.
EIGRP uses the same formula as IGRP to calculate its composite metric, with one difference and that is EIGRP scales the metric component by 256 to achieve a finer metric granularity. This metric is calculated using Bandwidth, Delay, Reliability, Load, and MTU. The formula that it uses is as follows:
You can view the detailed vector and composite metric of a single EIGRP route from the topology table with the following command:
“ sh ip eigrp top <ip-address> “
• EIGRP Metric Calculation uses the following formula:
Metric = [107/Bandwidth(min))+(Delay(Sum)]/10)]*256 R-B
Bandwidth = the smallest of all bandwidths in the path to a given destination divided by 10,000,000.
Delay = the sum of all the delay values assigned to the interfaces along the path to a given destination divided by 10.
• To find out the value of bandwidth and the delay associated to a given interface, “ sh interface < the interface type > x “ where x is the interface number.
These values can be changed with the following interface mode commands:
“ bandwidth < bandwidth in Kbps> “
“ delay < delay in tens of microseconds > “
• Feasible Distance: FD is equal to advertised distance of a neighbor plus the cost of the link to that neighbor. In some cases we may have multiple routes to the same destination, in situation like that FD will be based on the lowest metric.
• Feasibility Condition: It is a condition that is met if a neighbor’s advertised distance to a destination is lower than the router’s FD to that same destination.
o FC states, that the route must be advertised by a downstream neighbor (with respect to the destination), and the cost of the advertising routes to the destination must be less than or equal to the cost of the route that is currently being used by the router receiving the advertisement.
• Successor: A directly connected neighboring router that has the best route to a given destination. These routers are always downstream routers.
o In order for a neighbor to become the successor, that neighbor must firstmeet the FC. Successors are entries that are kept in the routing table.
• Feasible Successor: FS are downstream neighboring router/s through which a destination can be reached. FS are nothing but backup routes to a given destination, or second best route to a given destination.
o FS s are kept in the topology table, and there may be more than one FS per destination.
o If a neighbor’s advertising distance to a destination meets the FC, the neighbor becomes a FS for that destination.
• Active State: When a router loses its route to a destination and no FS is available in the topology table, the router goes into active state, in this state the router sends out queries to all neighbors in order to find a route to that destination. It is possible for the routers that are receiving the queries to send queries to their neighbor, this can create a ripple effect.
• Passive State: When there is no change in the internetwork, there is no need to do a computation or convergence, so the routers are all in passive state. Even when a router loses its successor, as long as that router has a FS in the topology table, the router will remain in the passive state (normal state), and it will place the FS in the routing table, and no computation will be performed.
Terminology
• Topology Table: This includes route entries for all the destinations that the router has learned. FS are kept in this table for rapid convergence.
• Neighbor table: Each Eigrp router has a neighbor table that has a list of adjacent routers. Neighbor relationships ensure a bi-directional communication between each of the directly connected neighbor.
• Routing Table: Eigrp uses the best path to a given destination (the Successor/s) from the topology table and places it into the routing table.
• Downstream: A router which is closer to the destination than the local router.
• Upstream: This router is further away from the destination than the local router. This router will use the local router to get to the destination.
• Advertised Distance: Is a distance reported to the current router, by a neighbor. Sometimes its referred to as Reported Distance.
• Hello: Used for neighbor discovery process. Hello packets are sent as multicasts, and they use unreliable delivery meaning that they do not need an ACK, as long as these packets are received the routers can determine that the neighbor is up.
• Update: Update packets convey route information, these are transferred when necessary, and are sent only to the routers that require the
information. When updates are requested by a single router, the sending router will use unicast to convey the route information’s, but if an up date is requested by more than one router, then the updates are
multicast out to 224.0.0.10 address. The updates require ACK s. These packets are used when a router comes up for the first time, or when there is a topology change, or the metric of a route is changed for better or worst.
• Acknowledgements or ACK s: These packets are sent by the routers to acknowledge the receipt of an update. Acknowledgement packets use unicast and use unreliable delivery method.
• Queries: When a router looses its successor and has no feasible successor in the topology table, it will send a query to all neighbors in the neighbor table. Queries will always use multicast and requires an ACK.
• Replies: These packets are sent in response to queries, these packets will always use unicast and require an ACK.