Module 6 - Understanding EIGRP Flashcards
What type of routing protocol is EIGRP?
EIGRP is an advanced distance vector protocol. It is the only protocol which has a ‘plan B’, this is a backup route known as a feasible route. It immediatley is installed into the RIB should any issue occur.
Along with BGP it is also the only routing protocol which does unequal cost multipathing via variance.
It also supports upto 4 equal cost multi paths.
Features of EIGRP
It provides the following features/benefits:
- Rapid convergence (uses the DUAL algorithim)
- Load Balancing
- Higher Bandwidth
- Loop Free
- Multi-address family (supports IPV4 /IPV6)
EIGRP Traffic
EIGRP uses unicast and multicast rather than broadcast traffic. As a result of using multicast traffic end stations are unaffected by the routing updates.
The multicast address it uses is
224.0.0.10
Q.
What are three key characteristics of EIGRP? (Choose three.)
- classful routing protocol
- no support for IPv6
- classless routing protocol
- supports IPv4 only
- supports IPv4 and IPv6
- solution to distribute dial-plan information for VoIP networks
A.
Classless routing protocol, supports IPV4 and IPV6 and it is also a solution to distribute dial-plan information for VOIP networks.
EIGRP Reliable Transport
What protocol does EIGRP Run over?
EIGRP runs above the IP layer in it’s own protocol, numbered 88. This is RTP which guarantees the delivery of packets to other devices running EIGRP over multicast address 224.0.0.10.
EIGRP Operation Overview
The operation of EIGRP protocol is based on the information in which following 3 tables?
The routing table, the topology table and the neighbor table.
Successor and Feasible Successor
Successor - the route to the destination with the best metric. The successor routes are the ones which are placed in the routing table.
Feasible Successor (FS) - This is the best alternative route to a destination, not entered into the RIB due to not been the lowest cost or metric, however it is stored in case the primary route fails this one becomes available.
EIGRP Packet Types.
There are 6 EIGRP packet types these are:
He
Up
Que
Req
Rep
Ackn
Hello - sent before a neighbourship is formed, hellos sent to the multicast address.
Update - These update packets are sent when a new route is discovered.
Query - used to query neighbors when a feasible successor no longer exists.
Request - Request packets are used to get specific information from one or more neighbors.
Reply - Sent in response to a query packet.
Acknowledge - These are hello packets which ack the update, query, request and reply packets.
EIGRP Passive and Active
A route is considered passive when the router is not performing re-computation for that route;
while a route is considered active when the router is performing re-computation to seek for a new successor when the existing successor has become invalid.
EIGRP Neighbor Adjacency
What 4 things must match for neighbors to form??
The following 4 broad parameters must match for an EIGRP Adjacency to form, these are:
- AS Number
- K Values ( bandwidth and delay by default - K1 and K3)
- Common subnet
- Authentication method
Also note that the passive-interface cmd can be used to suppress Hello’s and routing information.
AS Number is very important to EIGRP. Also hello timers etc are not significant in EIGRP like they are in OSPF, they don’t have to match.
All of the 4 parameters listed above are sent within the EIGRP Hello packet.
Q.
Which two EIGRP parameters need to match for EIGRP neighbor relationships to establish? (Choose two)
- Router-ID
- Area ID
- AS number
- Hello timers
- K values
A.
K values and AS number must match.
EIGRP Metrics
What are the 5 K Values and which ones are set to 1 by default?
show ip protocols - can be used to view EIGRP metrics.
Unlike RIP or OSPF which use one attribute to determine metric, EIGRP uses a composite metric of 5 values:
K1 - Bandwidth
K2 - Load
K3 - Delay
K4 - Reliablity
K5 - MTU
Q.
In EIGRP, a successor route is which one of the following options?
- An alternative path
- A possible path
- A suboptimal path
- The best path
A.
The Best Path.
EIGRP Path Selection
The topology depicted shows two routers, R1 on the left has a network reachable off of it 10.1.1.0/24, this has a metric or advertised distance of 1000. The link between R1 and R2 also has a reported distance of 1500, the feasible distance then between R2 and the 10.1.1.0/24 network is 2500
The successor is the best advertised distanced + reported distance = lowest/best feasible distance , this is installed in the RIB.
The feasible successor is the next best loop free route…
The successor and feasible successor routes can be viewed in the eigrp topology table.
EIGRP ECMP Load Balancing
*remember maximum-paths cmd is required here and run within the router configuration mode. 16 paths are supported. If adjusted then the same must match on the other side.
Equal Cost Multi-Path (ECMP) is supported by routing in general via the maximum-paths command. It is a routing strategy to provide multiple paths to a destination.
*Remember good network design requires redundancy and path utilisation, where multiple devices and paths exist. It improves network bandwidth and is generally a customer expectation.
4 paths are supported by default with ECMP. If the value is set to 1 then ECMP would be disabled.
EIGRP Unequal Cost Load Balancing
The default variance is set to 1 on unequal cost load balancing. EIGRP will only install additional routes if there is 0 variance difference - meaning the same.
The example screenshot has a variance difference of almost 4… setting the variance to 4 will allow a previously unused link to be used. Potentially useful scenario with a WAN with a fast link and a backup link.
To control how traffic is distributed among routes when there are multiple routes for the same destination network that have different costs, use the traffic-share balanced command. With the balanced keyword, the router distributes traffic proportionately to the ratios of the metrics that are associated with different routes. Traffic-share balanced is the default balancing technique.
