Some doubts about eigrp

[ciscolover]

Hi all... I'm studing about eigrp and i have some doubts.

1º)Wich is the goal to etablish ASN? Its to change EIGRP information only with the routers in the same ASN, this its the objetive?

2º)In the topology of the image, i dont understand why it appears 10.0.0.0/8 is a summary in R1?? Why it appears this?in all the network commands of eigrp i put the wildcard mask.

     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C       10.10.10.0/24 is directly connected, Serial0/0
D       10.0.0.0/8 is a summary, 00:05:41, Null0
D       10.127.0.0/24 [90/2681856] via 10.10.10.2, 00:02:01, Serial0/0
C    192.168.1.0/24 is directly connected, FastEthernet0/0

3º)And the chan keys for authentication conatisn more keys... If all the keys have life time configured and the lifetimes finished...¿you need reconfigure another time the lifetimes of the keys contained in the key chain??

4º)Finally i not understand very well the use of the hold timer..

Thanks ¡¡¡¡¡¡

[Ricardo Prado Rueda]

1º)Wich is the goal to etablish ASN? Its to change EIGRP information  only with the routers in the same ASN, this its the objetive?

  More or less. An autonomous system number is defined as the collection of prefixes under a common administration. This is a general definition in networking not a concept exclusive to EIGRP (BGP also uses the concept of ASN). Based on this, the same ASN number should be configured on all routers running EIGRP that belong to the same company (administration).

2º)In the topology of the image, i dont understand why it appears  10.0.0.0/8 is a summary in R1?? Why it appears this?in all the network  commands of eigrp i put the wildcard mask.

   Most likely, this is caused by the "auto-summary" option that is enabled by default on some IOS versions for EIGRP. Although EIGRP is a classless protocol and will still recognize prefixes with subnets, when this option is enabled it will summarize your subnets into a classful network (Class A, B, and C). When you are working with discontiguous networks (several non-consecutive subnets of a Class A, B, or C network) it is always recommended to disable this option with the command "no auto-summary" under the router process.

3º)Finally i not understand very well the use of the hold timer..

   The hold-time parameter, tells the router the maximum time that it should wait before considering a neighbor as unreachable. In other words, when the router stops receiving the hello packets from its neighbor, it will wait all this hold-time before assuming the neighbor is no longer there and try to re-start the neighborship. The default value is 3 thimes the hello timer.

Regards,

Rick.

[Peter Paluch]

Hello,

1º)Wich is the goal to etablish ASN? Its to change EIGRP information 
only with the routers in the same ASN, this its the objetive?

Yes, you are correct. The EIGRP is by its nature an internal gateway protocol (IGP), i.e. it is supposed to be run in a single autonomous system only. Using the ASN as a part of EIGRP's configuration allows for a simple test whether two routers belong to the same AS. If their ASN does not match, they will not establish an EIGRP adjacency.

2º)In the topology of the image, i dont understand why it appears 10.0.0.0/8 is a summary in R1??

It is because you have automatic summarization activated and the R1 sits between two so-called major networks - it has interfaces in two classful networks or their subnets, 10.0.0.0/8 and 192.168.1.0/24. For more discussion about automatic summarization and the Null0 route that is added automatically, please see the following discussions here on CSC:

https://supportforums.cisco.com/message/829837#829837

https://supportforums.cisco.com/message/829838#829838

https://supportforums.cisco.com/message/3206453#3206453

3º)Finally i not understand very well the use of the hold timer..

The Hold timer advertised by a router defines how long should its neighbors wait for any valid packet from that router before declaring it dead. If a router A advertises a hold time of 15 seconds, all its directly attached neighbors expect that the router A sends at least one valid EIGRP packet every 15 seconds. Usually, the Hello packets are used to meet this requirement as they are sent each 5 seconds. However, any other packet would also do (Update, Query, Reply, Ack). So in essence, the hold time is the maximum time between receiving two valid EIGRP packets before declaring that router dead.

Feel welcome to ask further!

Best regards,

Peter

[ciscolover]

Hi and thanks for the replyes¡¡¡

I had not forgotten the thread, was waiting to finish study about eigrp to make a new questions ¡¡¡

1º)You can annonce a default route using eigrp... But, it talk about creating a loopback interface and annonce it. Can anyone explain me this?Its not better use a real network¿?why the people use this loopback interface. Maybe for networks  without routing to internet, in one router that knows all the routes of the network?

2º)Why when you create a summary route manually this route asigns him to the interface null0...It talks about loops but i dont understadn very well this.

3º)In the same chapter it talk booth prefix list and acl to filter eigrp routes(using with distribute list).I have read prefix list is best than ACL because it matches the mask and the acl not...I dont understand... the acl is used with a wildcard ¡¡¡ It uses mask ¡¡

"Using IP prefix lists for route filtering has several advantages. First, IP prefix lists allow
matching of the prefix length, whereas the ACLs used by the EIGRP distribute-list command
cannot.""

Thanks alls for your help and sorry, i speak bad english¡

[davy.timmermans · Accepted solution]

1º)You can annonce a default route using eigrp... But, it talk about creating a loopback interface and annonce it. Can anyone explain me this?Its not better use a real network¿?why the people use this loopback interface. Maybe for networks  without routing to internet, in one router that knows all the routes of the network?

A loopback interface is a logical interface and remains always up (unless the router is down). As long as there is an interface up to an EIGRP neighbor it will be valid and announced. If it's the best solution depends on the design of the network.

2º)Why when you create a summary route manually this route asigns him to the interface null0...It talks about loops but i dont understadn very well this.

If you have for example a network 10.0.0.0/25  (10.0.0.0-127)  and you summarize and advertize this route as 10.0.0.0/24 (10.0.0.0-255)

it will send 10.0.0.0/24 to the rest of the network and install locally the route 10.0.0.0/24 to the null interface and 10.0.0.0/25 as connected.

If someone in the network sends a packet to 10.0.0.134 (does not exist in the network):

The packet arrive at his default gateway router

This router will see a route 10.0.0.0/24 and forward it in that direction

Finally it will arrive on the router who advertized the route.

If it didn't install 10.0.0.0/24 to null interface the following would happen:

10.0.0.134? lookup in routing table --> I don't know a route, but I know a default route --> sent it in that direction

This will happen until the packet's TTL is 0

By installing a route to the null interface, the packet will be dropped at the advertizing router.

[ciscolover]

Wow, excelent answer ¡¡¡ I understand perfect now ¡¡¡

Now i have only the doubt about the distribute list....