Cisco Tips & Tricks

July 6, 2006

CCIE notes for EIGRP

Filed under: IP Routing, Router, [EIGRP] — ciscotips @ 6:20 pm

At last i have made up my mind to sit for ccie R&S written by early August. I am starting a tips & tricks section for dynamic routing protocols. I will start with EIGRP and then gradually move towards OSPF & BGP.

EIGRP uses DUAL ( Diffused update algorithm). Now what is DUAL? This fancy name is being used by cisco as a kind of marketing tool. Cisco calls EIGRP as a hybrid protocol. It has features of both distance vector & link state protocol. EIGRP topology table will always keep a feasible successor. Feasible successor is nothing but a backup route.

Lets say we have two links. One is 56k & other is a T1 link. Now EIGRP topology table will contain t1 as a primary route and 56k link as a feasible route. The difference between EIGRP and other dynamic routing protocols is that due to the feaible successor already in the topology table,Convergence time is very fast.

Incase of T1 link failiure, in fraction of seconds 56k link will become a primary route for EIGRP. This is the magic of DUAL, in other protocols like OSPF it will take atleast 50 secs to converge as it has to do everything from the scratch.

-EIGRP route metrics :- All of us have known about k values in EIGRP. but what are ‘k’ values?K values are the meterics being used in EIGRP to influence routes. routes in EIGRP can be influenced with 5 different ‘k’ values. k1,k2,k3,k4 & k5. Bandwidth and delay are the default metrics in EIGRP but we have load, reliability and MTU which can also influence routes in EIGRP. Cisco doesn’t recommend enabling other ‘k’ values like load. the reason behind that is that it can increase cpu cycles and thus can slow down the whole process.

IF we have two links with equal cost (bandwidth), both eigrp & ospf will automatically load balance between the two links. Incase we need to prefer one link over other, we will have to change the bandwidth statement ( i.e. we ‘ll lie to the router) to a lower bandwidth so that the other link will be preffered over this link. This practice will definitely resolve our routing problem but it can lead to other problems. bandwidth statement is not only used by routing protocols but also used by qos for evaluating/priortizing traffic. changing bandwidth can have adverse effects.

In eigrp we have a feature called delay we can simply go under interface configuration mode and change delay. its a cool feature as even if bandwidth is same for 2 interfaces, delay will help us in making one link as primary and other secondary by simply increasing value of delay.
Now its a time for EIGRP tip. Not many people know about this command but using this command we can really do wonders in EIGRP.

metric weights tos k1 k2 k3 k4 k5

where k1 is bandwidth, k2 is load, k3 is delay, k4 is reliability and k5 is MTU. This command is used under router configuration mode.
default usage of this commmand is meteric weights 0 1 0 1 0 0. by default only bandwidth and delay is accounted for metric calculation.

Use this command to alter the default behavior of EIGRP routing and metric computation and allow the tuning of the EIGRP metric calculation for a particular type of service (ToS).

If k5 equals 0, the composite EIGRP metric is computed according to the following formula:

metric = [k1 * bandwidth + (k2 * bandwidth)/(256 – load) + k3 * delay]

If k5 does not equal zero, an additional operation is performed:

metric = metric * [k5/(reliability + k4)]

Bandwidth is inverse minimum bandwidth of the path in BPS scaled by a factor of 2.56 * 1012. The range is from a 1200-bps line to 10 terabits per second.

Delay is in units of 10 microseconds. The range of delay is from 10 microseconds to 168 seconds. A delay of all ones indicates that the network is unreachable.

The delay parameter is stored in a 32-bit field, in increments of 39.1 nanoseconds. The range of delay is from 1 (39.1 nanoseconds) to hexadecimal FFFFFFFF (decimal 4,294,967,040 nanoseconds). A delay of all ones (that is, a delay of hexadecimal FFFFFFFF) indicates that the network is unreachable.

Troubleshooting EIGRP

Cisco has set of flow charts which I have personally found the best way to troubleshoot EIGRP.

Summarising EIGRP routes:-

We can use following command to summarise EIGRP routes. Under interface config mode, issue following command

ip summary address eigrp x.x.x.x subnet-mask.

EIGRP or OSPF

EIGRP has a faster convergence time as compare to OSPF. Administrative distance for internal EIGRP is 90 and that of external EIGRP is 170.

OSPF is still widely used dynamic protocol as compare to EIGRP, this is because EIGRP is cisco properietory and thus runs only on cisco routers where as OSPF is vendor independent routing protocol.

 

 

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Subnet Mask Cheat Sheet

Filed under: Router — ciscotips @ 3:25 pm

Subnet Cheat Sheet

 


Hosts

Netmask

Amount of a Class C

/30

4

255.255.255.252

1/64

/29

8

255.255.255.248

1/32

/28

16

255.255.255.240

1/16

/27

32

255.255.255.224

1/8

/26

64

255.255.255.192

1/4

/25

128

255.255.255.128

1/2

/24

256

255.255.255.0

1

/23

512

255.255.254.0

2

/22

1024

255.255.252.0

4

/21

2048

255.255.248.0

8

/20

4096

255.255.240.0

16

/19

8192

255.255.224.0

32

/18

16384

255.255.192.0

64

/17

32768

255.255.128.0

128

/16

65536

255.255.0.0

256

 


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