show ip eigrp topolgy output

[sarahr202]

Hi every body

1)When  we use the command Show ip eigrp topology, the output also shows seq among other things.  Seq  indicates the last eigrp packet received from the neighbor.  My  question  is  when router receives a hello, should it increase the seq counter as hello is also a eigrp packet?  I noticed it does not.

2)  l  r1-------------------r2

both routers are running eigrp,  my book says  when  right after neighbor relationship is formed,  reach router will sends its entire routing table in update  which will be unicast.  After that if there is any change, router just  update listing the change  at multicast address.

The problem  is when i performed the lab,  all updates were unicast.  Even i introduced changes in the network,  router just update at unicast address.  Is is the  normal ?

thanks and have a nice day

[Luc De Ghein · Accepted solution]

Hi,

1) Only reliable EIGRP packet (e.g. Query, Update, Reply) have sequence numbers. Unreliable EIGRP pacets (Hello, Ack) do not have sequence numbers.

2) EIGRP uses multicast and unicast to send Updates and Queries. Usually, indeed multicast is used, but there are conditions when unicast can be used, this is the case when retransmitting EIGRP packets, or when using the neighbor command.

Could you post you config?

Thanks,

Luc

[sarahr202]

Thanks for your reply.

I have attached the debug output and show ip eigrp topology in the file

Lab set -up:

        199.199.199.0/24                        10.0.0.08

R1 s0-------------------------------------s0 R2 f0------------------------l2switch

Not shown, R1 is also connected to R3 via s0/1.  R  3  is connected to l2 switch  via f0/0  .  The result is  R1  is learning about 10.0.0.0/8 via R2 and R3, next  I changed  the delay setting under f0//0  on R2 to manipulate metric. as a result  R1 chose R3 as successor.,

The output of show ip eigrp topology on R1 looks like as shown below.

show ip eigrp topology

10.0.0.0/8 via R3  (200.200.200.3, successor )

                via  R2  (  199.199.199.2 , feasible successor).

Next i simlute the link failure  by shutting down f0/0 on R3 which  is directly connected to 10.0.0.0 via l2 switch.

thanks  and have a great day

[Luc De Ghein · Accepted solution]

Hi,

During EIGRP neighbor up process, all updates are sent unicast.

When EIGRP neighborship is across point-to-point link as in your case, all Update and Query packets are always send unicast.

So, what you are seeing is normal.

Thanks,

Luc

[Mohamed Sobair · Accepted solution]

Hi Sarah,

1) Rliable Eigrp Packets have Sequence number like (Query ,update , reply) as indicated by Luc, un reliable Eigrp Packets doesnt have sequence numbers.

2) NO , this is not normal.  In normal circumstances, The update packet is multicast, Only ( Acknowledgment and Query Reply) packets are sent as Unicast. Unless you have configured the (neighbor statemnet) which also includes the Hello packets to be sent as unicast and update as unicast as well.

HTH

Mohamed

[Thotsaphon Lueangwattanaphong · Accepted solution]

Hi Sarahr,

1)When  we use the command Show ip eigrp topology, the output also shows seq among other things.  Seq  indicates the last eigrp packet received from the neighbor.  My  question  is  when router receives a hello, should it increase the seq counter as hello is also a eigrp packet?  I noticed it does not.

           You mean the output of  "show ip eigrp neighbor". Right?  It's SeqNum. Yes, Hello won't affect to increse the number. The following information is excerpted from cisco website.

            "The neighbor table entry also includes information required by the reliable transport mechanism. Sequence numbers are employed to match acknowledgments with data packets. The last sequence number received from the neighbor is recorded so out of order packets can be detected. A transmission list is used to queue packets for possible retransmission on a per neighbor basis. Round trip timers are kept in the neighbor data structure to estimate an optimal retransmission interval."

2)  l  r1-------------------r2
both routers are running eigrp,  my book says  when  right after neighbor relationship is formed,  reach router will sends its entire routing table in update  which will be unicast.  After that if there is any change, router just  update listing the change  at multicast address.
The problem  is when i performed the lab,  all updates were unicast.  Even i introduced changes in the network,  router just update at unicast address.  Is is the  normal ?
thanks and have a nice day

      It depends. The following information is excerpted from cisco website.

          "Updates are used to convey reachability of destinations. When a new neighbor is discovered, update packets are sent so the neighbor can  build up its topology table. In this case, update packets are unicast. In other cases,such as a link cost change, updates are multicast. Updates are always transmitted reliably. "

     Ref:  http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f07.shtml

HTH,

Toshi