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Cisco GLBP Doubt

Posted on 2010-09-05
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Last Modified: 2012-05-10
I have a doubt with glbp

In the figure that i have attached,
let's assume that the client1 issued an arp broadcast request, so the switch will have the source mac add of client1 in the mac address table and the port no , the avg will send an arp  reply with the virtual mac address and the switch will record the virtual mac and the port no of the source, which in this case is the virtual mac address of Router A.

i found no doubt in the above but,

when the client2 issues an arp, the avg(RA) will receive it and will send the virtual mac address of the avf(Router B), so now the swich will record the mac address of the sender which in this case is virtual mac addr of Router B (but given by Router A), so In the switch cam table, mac addr will be the virtual mac addr of routerb, but the port no recorded by the switch would be that of the port on which the routerA is present as it was the replier to the arp query,

So how does a switch know that it has to forward the frames to routerB as the replier to the arp query fro client B was Router A and its port is recorded by the switch.  


GLBP.png
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Question by:tpat
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LVL 24

Expert Comment

by:rfc1180
ID: 33608938
>So how does a switch know that it has to forward the frames to routerB as the replier to the arp query fro client B was Router A and its port is recorded by the switch.

The ARP reply information for the destination MAC and IP (Router B) is contained in the data payload of the Ethernet fram (ARP data is encapsulated into an Ethernet frame) and not in the source and destination of the Ethernet Frame as you think it is, these are 2 different pieces of information.

http://book.chinaunix.net/special/ebook/oreilly/Understanding_Linux_Network_Internals/0596002556/understandlni-CHP-28-SECT-1.html


Billy
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Author Comment

by:tpat
ID: 33609129
So what would be the layer 2 information in the ARP Reply from router a  to client b.
would it be:

SMAC: 0007.b400.0101
DMAC:  client B's mac address.

ARP REPLY:
SMAC: 0007.b400.0102
DMAC: client 2's mac address
SIP: 10.21.8.10
DIP: client 2's ip address
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Accepted Solution

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rfc1180 earned 1000 total points
ID: 33609303
not sure who Client B is, as you only have client 2 on the map (Maybe a typo):

SMAC: 0007.b400.0101
DMAC:  client 2's mac address.  <----- this is how the switch will make the forwarding decision on

ARP REPLY:
SMAC: 0007.b400.0102
DMAC: client 2's mac address
SIP: 10.21.8.10
DIP: client 2's ip address


Billy
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by:rochey2009
rochey2009 earned 1000 total points
ID: 33611294

No.     Time        Source                Destination           Protocol Info
     32 0.094000    c2:00:07:d4:00:00     c2:03:07:d4:00:00     ARP      192.168.0.1 is at 00:07:b4:00:00:01

Frame 32 (60 bytes on wire, 60 bytes captured)
Ethernet II, Src: c2:00:07:d4:00:00 (c2:00:07:d4:00:00), Dst: c2:03:07:d4:00:00 (c2:03:07:d4:00:00)
Address Resolution Protocol (reply)
    Hardware type: Ethernet (0x0001)
    Protocol type: IP (0x0800)
    Hardware size: 6
    Protocol size: 4
    Opcode: reply (0x0002)
    Sender MAC address: Cisco_00:00:01 (00:07:b4:00:00:01)
    Sender IP address: 192.168.0.1 (192.168.0.1)
    Target MAC address: c2:03:07:d4:00:00 (c2:03:07:d4:00:00)
    Target IP address: 192.168.0.100 (192.168.0.100)

Hi,
Here's a trace of an arp response from my GNS3 lab. The source of the ethernet frame is  c2:00:07:d4:00:00 which is the mac address of the ethernet interface on the GLBP router. Encapsulated in the arp request is the virtual mac-address 00:07:b4:00:00:01, so the switch doesn't see the virtual mac-address at this point and records the routers mac-address in it's mac table and not the virtual mac-address.
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Author Comment

by:tpat
ID: 33612490
So the trace shows the ARP reply has the Source Mac address of the physical interface of the router, and the ARP payload has the source mac adress  as the virtual mac address. Am i right?
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LVL 24

Expert Comment

by:rfc1180
ID: 33612511
>Am i right?
correct, no different that what was discussed prior
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LVL 17

Expert Comment

by:rochey2009
ID: 33616501
your correct.
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Author Closing Comment

by:tpat
ID: 33738594
Understood it!
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