Nov 22, 2011

Abstract of CCNA study guide-26 - EIGRP 2

Configuring EIGRP
To start an EIGRP session on a router, use the router eigrp command followed by the autonomous system number of your network.
Then use the network command followed by the network numbers connected to the router.

Example of enabling EIGRP for autonomous system 20 on a router connected to two networks, with the network numbers being and
Router#config t
Router(config)#router eigrp 20

Remember—as with RIP, you use the classful network address, which is all subnet and host bits turned off.
Understand that the AS number is the same in  all routers , You can use any number from 1 to 65,535.

If you need to stop EIGRP from working on a specific interface, such as a BRI interface or a serial connection to the Internet.  you would flag the interface as passive using the passive-interface interface command:

Router(config)#router eigrp 20
Router(config-router)#passive-interface serial 0/1

Doing this will prohibit the interface from sending or receiving Hello packets and, This means that it won’t send or receive route information on this interface.

Corp#config t
Corp(config)#router eigrp ?
<1-65535> Autonomous system number
Corp(config)#router eigrp 10

As with RIPv1,you still need to add the classful network numbers you want to advertise. But unlike RIP,
EIGRP uses classless routing—but you still configure it as classful. Classless, which I’m sure
you remember, means that the subnet mask information is sent along with routing protocol
updates (RIPv2 is classless).

Configuring Discontiguous Networks
Remember first Figure and how it demonstrated how EIGRP would auto-summarize the boundaries on a discontiguous network? Take a look at that figure again, Both routers, by default, would automatically summarize the classful boundaries and routing would not work. Here’s the configuration that would make this network work:
Lab_A#config t
Lab_A(config)#router eigrp 100
Lab_A(config-router)#no auto-summary
Lab_B#config t
Lab_B(config)#router eigrp 100
Lab_B(config-router)#no auto-summary
Because I used the no auto-summary command, EIGRP will advertise all the subnets between the two routers.
Load Balancing with EIGRP
By default, EIGRP can load-balance up to four equal-cost links. And we can configure EIGRP to load-balance across up to six equal-/unequal cost.

First, let’s take a look at the R1 routing table and make sure that EIGRP has already found both links between the routers:
R1#sh ip route is subnetted, 12 subnets
D            [90/2684416] via, 00:50:37, Serial0/0/1
    [90/2684416] via, 00:50:37, Serial0/0/0
D             [90/2707456] via, 01:24:09, Serial0/0/1
  [90/2707456] via, 01:24:09, Serial0/0/0
D             [90/2707456] via, 01:24:09, Serial0/0/1
  [90/2707456] via, 01:24:09, Serial0/0/0
D              [90/2684416] via, 00:10:10, Serial0/0/1
     [90/2684416] via, 00:10:10, Serial0/0/0
C              is directly connected, Serial0/0/1
C             is directly connected, Serial0/0/0

You can see that we have two links to every route in our internetwork, and again, EIGRP will load balance across the s0/0/0 and s0/0/1 links by default because they’re the same metric.
EIGRP is automatic load balancing. But how about bundling links? Well, EIGRP can allow us to do this too—even with no extra configuration! Let me show you how this works. I’m going to configure the links between our Corp and R1 routers with the same subnet, meaning both links will have all interfaces within the same subnet. Check out my configuration:

Corp#config t
Corp(config)#int s0/0/1
Corp(config-if)#ip address
R1#config t
R1(config)#int s0/0/1
R1(config-if)#ip address
R1(config-if)#do show run | begin interface
interface Serial0/0/0
description 1st Connection to Corp Router
ip address
interface Serial0/0/1
description 2nd connection to Corp Router
ip address
Now both links have all four interfaces in the same subnet.
R1(config-if)#do show ip route is subnetted, 12 subnets
D [90/2684416] via, 00:04:44, Serial0/0/1
   [90/2684416] via, 00:04:44, Serial0/0/0
D [90/2707456] via, 00:04:44, Serial0/0/1
   [90/2707456] via, 00:04:44, Serial0/0/0
D [90/2707456] via, 00:04:44, Serial0/0/1
 [90/2707456] via, 00:04:44, Serial0/0/0
D [90/2707456] via, 00:04:44, Serial0/0/1
 [90/2707456] via, 00:04:44, Serial0/0/0
D [90/2684416] via, 00:04:44, Serial0/0/1
  [90/2684416] via, 00:04:44, Serial0/0/0
C is directly connected, Serial0/0/0
 is directly connected, Serial0/0/1
D [90/2172416] via, 00:03:56, Serial0/0/1
[90/2172416] via, 00:03:56, Serial0/0/0
C is directly connected, FastEthernet0/1

There are  two changes  in the routing table now? Networks and used to show up as individual, directly connected interfaces, but not anymore. Now only the network shows up as two directly connected interfaces, and the router now has a 3MB pipe through that line instead of just two 1.5Mbps T1 links.

Verifying EIGRP
There are several commands can be used  to help you troubleshoot and verify the EIGRP configuration:

Next TABLE  EIGRP Troubleshooting Commands
show ip route
Shows the entire routing table
show ip route eigrp
Shows only EIGRP entries in the routing table
show ip eigrp neighbors
Shows all EIGRP neighbors
show ip eigrp topology
Shows entries in the EIGRP topology table
debug eigrp packet
Shows Hello packets sent/received between adjacent routers
Debug ip eigrp notification
Shows EIGRP changes and updates

The following router output is from the Corp router in our example:
Show ip eigrp neighbors command:
Corp#sh ip eigrp neighbors
IP-EIGRP neighbors for process 10
H         Address           Interface          Hold Uptime    SRTT   RTO     Q         Seq
   (sec)               (ms)                Cnt       Num
1           Se0/0/1            14 00:35:10        1        200      0          81
2           Se0/1/0            13 03:17:20        1         200     0          20
0           Se0/0/0            10 03:19:37        1        200      0          80

We read the information in this output like this:
-    The H field: the order in which the neighbor was discovered.
-    The hold time: how long this router will wait for a Hello packet to arrive from a specific neighbor.
-    The uptime: how long the neighborship has been established.
-    The SRTT field: the smooth round-trip timer—an indication of the time it takes for a round-trip from this router to its neighbor and back.

The time between multicast attempts is specified by…
-    The Retransmission Time Out (RTO) field: the amount of time EIGRP waits before retransmitting a packet to a neighbor.
-    The Q value: indicates whether there are any outstanding messages in the queue.
-    The Seq field: indicates the sequence number of the last update from that neighbor.

The show ip eigrp neighbors command allows you to check the IP addresses ,the retransmit interval and queue counts for the neighbors that have established an adjacency.

show ip eigrp topology command :
Corp#sh ip eigrp topology
IP-EIGRP Topology Table for AS(10)/ID(
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P, 1 successors, FD is 2172416 via (2172416/28160), Serial0/2/0
P, 1 successors, FD is 2172416 via (2172416/28160), Serial0/2/0
P, 1 successors, FD is 76839936 via Connected, Serial0/0/1
   via (9849856/7719936), Serial0/0/0
P, 1 successors, FD is 2169856 via Connected, Serial0/0/0
             via (2681856/551936), Serial0/0/0
P, 1 successors, FD is 28160 via Connected, FastEthernet0/1

Notice that every route is preceded by a P. This means that the route is in the passive state, which is a good thing because routes in the active state (A) indicate that the router has lost its path to this network and is searching for a replacement.
Each entry indicates the feasible distance (FD), to each remote network plus the next-hop neighbor.
Plus, each entry also has two numbers in parentheses. The first indicates the feasible distance, and the second the advertised distance to a remote network.
Now here’s where things get interesting—notice that under the and outputs there are two links to each network and that the feasible distance and advertised distance are different. What this means is that we have one successor to the networks and one feasible successor (a backup route)remember that even though both routes to network and are in the topology table, only the successor route (the one
with the lowest metrics) will be copied and placed into the routing table.

EIGRP will load-balance across both links automatically when they are of equal variance (equal
cost), but EIGRP can also load-balance across unequal-cost links as well if we use the variance
command. The variance metric is set to 1 by default you can change the metric anywhere up to 128.

if the variance is set to 1, only routes with the same metric as the successor will be installed in the local routing table. And, if the variance is set to 2, any EIGRP learned route with a metric less than two times the successor metric will be installed in the local routing table (if it is already a feasible successor).

Debug eigrp packet command :that will show our Hello packets being sent between neighbor routers:
Corp#debug eigrp packet
EIGRP Packets debugging is on
*Mar 21 23:17:35.050: EIGRP: Sending HELLO on FastEthernet0/1
*Mar 21 23:17:35.050: AS 10, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0

debug ip eigrp notification command :this command’s output shouldn’t show you anything at all! That’s right—the only time you’ll see output from this command is if there’s a problem on your network or you’ve added or deleted a network from a router

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