Monday, September 1, 2008

Here is a short tutorial on EIGRP for the ccna.Can use it to revise >Covers most of the stuff on the exam


1.Fast convergence
2.Loop free topology
3.VLSM support
4.Multicast update
5.Multiple routed protocols

********IGRP v/s EIGRP*****
1.Load balancing across 6 paths (min 4 paths)
2.Similar metric structure
3.Automatic redistribution occurs when the same AS number is used for EIGRP and IGRP

1.Classful Classless

2.Metric=10^7/BW Metric=10^7/BW *256
Delay=delay/10 delay=delay/10 *256
24 bit metric for BW &dly 32 bit metric

3.Max hop cnt=255 Max hop cnt=224,default=100

4.No diff bet int and Outside routes redistributed and tagged
ext routes

5.AD=100 AD=90 for IEGRP,=170 for Ext EIGRP


k1 for bandwidth=1
k2 for load=0
k3 for delay=1
k4 and k5 for Reliability=0


*************EIGRP terminology**************
1.Neighbor table – Each EIGRP router maintains a neighbor table that lists adjacent routers. This table is comparable to the adjacency database used by OSPF. There is a neighbor table for each protocol that EIGRP supports.

2.Topology table – Every EIGRP router maintains a topology table for each configured network protocol. This table includes route entries for all destinations that the router has learned. All learned routes to a destination are maintained in the topology table.

3.Routing table – EIGRP chooses the best routes to a destination from the topology table and places these routes in the routing table. Each EIGRP router maintains a routing table for each network protocol.

4. Successor – A successor is a route selected as the primary route to use to reach a destination. Successors are the entries kept in the routing table. Multiple successors for a destination can be retained in the routing table.

5.Feasible successor – A feasible successor is a backup route. These routes are selected at the same time the successors are identified, but are kept in the topology table. Multiple feasible successors for a destination can be retained in the topology table.

***************EIGRP Neighbor adjacency***********
EIGRP routers establish adjacencies with neighbor routers by using small hello packets.
Hellos are sent every 5 seconds by default
K values must be the same between neighbors.
An EIGRP router assumes that, as long as it is receiving hello packets from known neighbors, those neighbors (and their routes) remain viable.
Hold time tells the router how long it should consider the neighbor alive if it has not received any EIGRP packets (Hello, EIGRP updates, etc.)
Hold time is normally three times the configured Hello interval.
Both the Hello and Hold time intervals are configurable on a per interface basis, and do not have to match neighbor.
EIGRP routers exchange routing information the same way as other distance vector routing protocols, but do not send periodic updates.
EIGRP updates are only sent when a network is added or removed from the topology database, when the successor for a given network changes, or when the locally used metric is updated. (later)
EIGRP, like any other distance-vector routing protocol uses split-horizon.
By forming adjacencies, EIGRP routers do the following:
Dynamically learn of new routes that join their network
Identify routers that become either unreachable or inoperable
Rediscover routers that had previously been unreachable

A neighbor meets the feasible condition(FC) if the reported distance by the neighbor is smaller than or equal to the current feasible distance (FD) of this router.

***********IF successor fails****************
Feasible Successor exists:
If current successor route fails, feasible successor becomes the current successor, i.e. the current route.
Routing of packets continue with little delay.

No Feasible Successor exists
This may be because the Reported Distance is greater than the Feasible Distance.
Before this route can be installed, it must be placed in the active state and recomputed. (later)
Routing of packets continue but with more of a delay.
If a feasible successor does not exist:
1. The router flags the route as active.
2. The router looks for an alternate path by sending out a query packet to all neighbors to learn if they have a path to the given destination.
The query packets are multicast out every interface except the one which the dead link was learned, adhering to the split horizon rule.
4. When the query router receives replies, it reacts based on the answer in the reply:
If the reply included a successor or feasible successor, the information is put into its topology table, and the querying router waits until all replies are received. It then recalculates the topology table, and adds the successr(s) to the routing table. The route returns to a passive state in the topolgy table and routing can continue.
If none of the replies includes a successor or feasible successor, the querying router removes the active route from its topology table and routing tables.
If a neighbor router to which a query is sent does not reply within the active time of 180 seconds, EIGRP tears down the neighbor relationship with the offending router and puts routes learned from that router into an active state

***************EIGRP packet types************

********Config EIGRP***********
Router(config)#router eigrp autonomous-system-number
This value must match all routers within the internetwork.

Router(config-router)#network network-number
The network command configures only connected networks.

Router(config-if)#bandwidth kilobits
When configuring serial links using EIGRP it is important to configure the bandwidth setting on the interface. If the bandwidth setting is not changed for these interfaces EIGRP assumes the default bandwidth on the link instead of the true bandwidth.

EIGRP automatically summarizes routes at the classful boundary, the boundary where the network address ends as defined by class-based addressing.
In the presence of discontiguous subnetworks, automatic summarization must be disabled for routing to work properly.
To turn off auto-summarization, use the following command:
Router(config-router)#no auto-summary

Manual summary per interface
Router(config-if)#ip summary-address eigrp autonomous-system-number ip-address mask administrative-distance

************EIGRP show and debug commands*********************

1.show ip protocols:
shows metrics;maximum hopcount:default 100;metric variance:1;AS no,autosummarization if in effectInterfaces participating in eigrp and AD

2.show ip route:eigrp routes marked with D;AD/metric;neighbor and interface on which update was recd

3.show ip eigrp neighbor:Shows AS no
Neighbor address:Provides address
Interface:local interface on which update is being recd
Hello:Time left for next hello
SRTT:time to sent request and receive update
RTO:time to wait before re-transmit
Q count:No of hellos for which reply is awaited
Seq:Sequence no of packet sent

4.show ip eigrp topology
shows if routes learnt are in active or passive state
no of successor and feasible successor routes for a particular network
the feasible distance and the reported or advertised distance of each route

5.show ip eigrp traffic
shows no of hellos sent and recd
no of acks sent and recd
no of replies sent and recd
no of updates sent and recd
no of queries sent and recd

6.debug ip eigrp
shows all updates sent and recd

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