EIGRP Equal and UnEqual cost load balancing

By default, EIGRP load balances across up to 4 equal cost paths. EIGRP can also load balance over unequal cost paths as well.

1. How do I turn off EIGRP load balancing?

A. maximum-paths 1

2. How do I load balance over 16 paths

A. maximum-paths 16

3. I set up load balancing and am trace-routing the destination from my console, but it is only using one path

A. Load balancing is performed only on traffic passing through the router, not router generated traffic

4. I want to set up unequal cost load balancing, what do I do?

A. First you must determine the variance you wish to use. Take your FD (Feasible Distance) of your best route. Now what number do you need to multiply it by to include the FD of all other routes you wish to load balance across?

For instance, if your successor, or best route, has a FD of 10, and you want to load balance across paths with costs of 15 and 20, choose a variant of 2. Then, all FDs 20 or under will be included in unequal cost load balancing. If you wanted to load balance across a 25, choose a variance of 3 to include all FDs under 30. (*Note* to avoid loops, the FD of routes must also be less than the advertised distance of your successor route. Read my last post for more on this)

traffic-share balanced

variance [#]

5. The load balancing command also has a minimum command, when would I use that?

A. The traffic-share min across-interfaces command is used for OSPF and other protocols to turn on equal-cost load balancing. If you have changed to unequal cost load balancing you could use this to return to equal cost in EIGRP. This command will leave all of your variance chosen paths in the routing table (see #4) but packets will only be routed out the paths that are the least cost (minimum) route(s).

EIGRP BASICS - Split Horizon/Route Poisoning

Split horizon, in EIGRP, affects the sending of update and query type packets.

Update packets contain route change information (sent reliably, needs ACK).

Query packets are sent when a router does not have a feasible successor which is a next hop router for a backup path (sent reliably, needs ACK).

Split Horizon is a way to prevent (minimize) routing loops. The logic is that if a router uses interface A to get to a destination, there is no reason to advertise that route out intA.

For example, if network 10.1.1.0/24's next hop router is located out fa0/0, then the router will not send update packets containing 10.1.1.0/24 out of fa0/0, only out fa0/1, fa0/2, etc.

If you think about it, if router A is your next hop to a network, why would you need to tell router A how to get to itself?

Split horizon is on by default on all interfaces.

If a router changes the interface it uses to get to a destination, it momentarily turns off split horizon and commits route poisoning. Route poisoning is when the router, in order to stop other routers from using its old route/cost that it just changed, floods out route unreachable packets. This causes other routers to flush that old route out of their routing tables so that the new route can now be successfully added.

EIGRP Basics - Hello Packets

Vocab:
IGRP: Interior Gateway Routing Protocol
EIGRP: Enhanced Interior Gateway Routing Protocol
AS: Autonomous System - a group of routers share an AS so they know they are part of a routing group or system and should share routes and form relationships within that group. A router can be a member of multiple AS's but must inject routes between them if the two groups wish to share common routes.

Cisco loves to ask questions on it's own proprietary protocols, so don't skip over something just because it isn't the most common protocol out there. (although it looks like IGRP is getting phased out)

EIGRP Hello Packets:

EIGRP sends hello packets to the multicast address 224.0.0.10 in order to create and maintain Neighbor relationships. These are not "reliable" packets and so do not expect an acknowledgement. When a router receives an EIGRP hello packet, it first checks three items. Assuming it is configured with:

1. The same AS
2. The Primary IP address is in the same subnet
3. The metric-calculation constants match (how it determines route cost/metrics)

Then a neighbor adjacency relationship will be established.

Over a fast link (greater than T1) Hello packets are sent every 5 seconds (default). Over a slow link (T1 or slower) Hello packets are sent every 60 seconds (default).

Hello packets include the hold time. This is the time a router waits, and if it doesn't receive any EIGRP packets from the neighbor switch, flushes the routes and attempts to re-converge deciding the neighbor is down. Hold time, by default is 3 times the hello interval. This means fast links have a hold time of 15 seconds, while slow links are 180 seconds. Remember that changing the hello time does NOT automatically change the hold time, you need to remember to change both.

Issue the command show ip eigrp neighbors to see if your hello packets have formed an adjacency.