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GLBP dest mac - not able to understand

Unanswered Question
Mar 20th, 2012
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Hi,


i configured a basic topology as following.


topo.jpg


F1/0 in R1 is 1.1.1.1/24 and F1/0 in R2 is 1.1.1.3/24. R2 is the AVG.

In R2, 0007.b400.0101 is the AVF and 0007.b400.0102 is in listen state. in R1 it is the vice versa.


Now from host C1, i pinged C2, for which i got one dest mac for arp reply 00:07:b4:00:01:01 ( which is the AVF in R2 )

from C4 i pinged C2, for which i got 00:07:b4:00:01:02 ( which is the AVF in R1 )

but what is happening is, for the arp and further icmp replies , it is using the dest mac in the ethernet frame and sending to the same router ( R2 ). It is not load balancing. What could be the problem here.


Thanks

Chidambaram

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Dan-Ciprian Cicioiu Tue, 03/20/2012 - 11:23
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Hi Chidambaram,


First of all the AVG is responsible for answering ARP requests for the virtual IP address. Load sharing is achieved by the AVG replying to the ARP requests with different virtual MAC addresses.


I can assume that if you are using GLBP on the other side , the AVF for C2 is R2


Regards

Dan

chid_1989 Tue, 03/20/2012 - 11:50
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Hi Dan,

Thanks for the reply.


Ya, the AVG replied for the ARP with 2 different dest macs. Also the AVF for C2 is R2. but my concern, why is it putting the dest mac as the mac of the i/f of r2. it should put the dest mac as the glbp mac right? Since it is putting the i/f mac, it is not going to both the routers. Or may be i can put it like this. I am not clear still


attached my pkt capture

if you see in the icmp req's , the dest mac is the interface mac opposed to what shuld have been glbp mac.


Thanks

Chidambaram

Dan-Ciprian Cicioiu Tue, 03/20/2012 - 13:27
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Chidambaram,


Could you please check the default route of the C4 , because If I'm not mistaken in your capture , just before transmiting ICMP request you have ARP request for 1.1.1.2 , not for 1.1.1.1 which is the Virtual Gateway. Correct my if I'm wrong the glbp ip is 1.1.1.2.



Regards

Dan

chid_1989 Wed, 03/21/2012 - 07:23
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Dan,


Yes, 1.1.1.2 is been configured as the default gateway in the hosts. I have a basic doubt. Shouldn't the dest mac be the glbp mac in the packets. Why is the dest mac, mac of the interface

Dan-Ciprian Cicioiu Wed, 03/21/2012 - 07:35
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Hi Chidambaram,


Yes you are right the destination mac must be the virtual mac address.



Regards

Dan

chid_1989 Wed, 03/21/2012 - 07:37
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Thank you for clearing that doubt. I think there should be some prob in the host ( sim ). Because in the repsonse from the routers, it is sending different macs. I guess the host picks the wrong mac. i will replace the hosts with routers and try. Thanks very much for replying.


Chidambaram

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