Enhanced Interior Gateway Protocol (EIGRP) – notes

• Cisco proprietary
• Uses IP protocol 88 as transport
• Support for MD5 authentication (no clear text)
• Sends updates to 224.0.0.10
• Distance vector but has some link state like features

Timers

Uses a hello and a hold timer. Neighbors discovered via hello protocol. Hold timer used for declaring when a neighbor is dead. EIGRP doesn’t use it own timers for keeping track of the neighbor, it uses the timers that the neighbor supplied in the hello packet. Retransmission TimeOut (RTO) timer used for knowing if to resend an update to a neighbor. Smoothed Round Trip Time (SRTT) keeps track of latency between neighbors and the RTO is derived from the SRTT timer. SRTT is the average time in ms between sending a packet to a neighbor and receiving an ACK. The default timer for hello is 5 seconds for most interfaces and a hold time of 15. NBMA interfaces with T1 or lower speeds use a 60 second hello timer and a 180 second hold time. Changing the hello timer does not automatically adjust the hold time.

Sending updates

Updates are sent as multicasts but resends are unicast to neighbors who didn’t ACK the update before the RTO timer expired. 16 resends using unicast will be used before declaring a neighbor dead. The multicast flow timer is used for knowing when to switch to unicast packets instead of multicast for a neighbor.

Metric

Based on cumulative delay and constraining bandwidth. Can factor in load, reliability and MTU if needed but not recommended by Cisco. To change what K values are used (constants) set them with the metric weights command. To calculate the metric use: 256*(10^7/bandwidth)+256(delay).

EIGRP measures delay in tens of microseconds, this needs to be considered when calculating the metric.
EIGRP uses Reported Distance (RD) and Feasible Distance (FD) for the metric. Reported distance is what the neighbor sending the update has calculated the metric to be. Feasible distance is the distance of the route with the lowest metric, it is the RD + the distance between the neighbor announcing the route and the local router. The route with the lowest metric that is entered into the routing table is called a successor route. A feasible successor route is a route that doesn’t have the lowest metric but meets the feasibility condition meaning it has a RD lower than what the current FD is.

Input events and local computation

When an input event occurs EIGRP needs to react, this could be an interface failing, a neighbor failing or an update for a new prefix. When the input event has occured EIGRP performs a local computation, EIGRP looks for a Feasible Successor (FS) route in its topology table and if it cannot find one it will actively query its neighbors for a route.

EIGRP algorithm

Uses the Diffusing Update ALgorithm (DUAL). Functioning routes are in a passive mode. Routes that no longer have a successor is in active mode since the route has to query its neighbors for a FS. The term Stuck In Active (SIA) means that an route has been active for too long, the active timer has expired. The active timer is set to 180 seconds by default, the active timer can also be disabled if needed.

Load balancing

EIGRP allows for up to 16 equal-metric routes to be installed in the routing table, the default is four.  EIGRP also has something called variance. Variance allows for non equal-metric load balancing.  The route still has to meet the feasibility condition to be considered for load balancing. The variance command is a multiplier,  if the FD is 10000 for the current succcessor and there is a FS with a RD of 5000 and FD of 200000, variance 2 would make the router load balance between these two routes, variance 2 means the FD of the second best route can be twice as high as the best.
The load balancing can be done in a few different ways, traffic-share balanced means that the traffic will be distributed according to the metric, routes with lower metrics will see more traffic on them. Traffic-share min, install multiple routes but send only traffic on the one with the lowest metric. Traffic-share min across-interfaces, if more than one route has the same metric choose different outgoing interfaces for a better load balancing. The no traffic-share command will balance evenly across routes no matter what the metric is.

Authentication

EIGRP has support for MD5 authentication, clear text is not supported. The keys are entered into a key-chain. A key can have a lifetime specified or use a lifetime that is always valid. Authentication is configured per interface.

Summarization

Uses auto-summary by default, turned off with no auto-summary. EIGRP has support for summarizing on every EIGRP interface compared to OSPF which can only summarize at area borders.

Split horizon

EIGRP is a distance vector protocol which means it uses split horizon. Split horizon means the router doesn’t send updates back out on the interface it received them. This can cause issues in non P2P networks. Split horizon can be turned off on an interface basis with the command no ip split-horizon eigrp asn command where asn is the AS-number specified.

Filtering

Has support for distribute lists and offset lists. Distribute lists are used for filtering inbound or outbound routing updates and what is allowed to enter the routing table. Offset lists are used to change the metric, only adding to the metric is supported, not removing from it.