Packet flow in 7600 when it arrives to Sup 720

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Jun 8th, 2009
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Dear All,

Can someone explain what exactly happens when packet comes to Sup 720 of Cisco 7600 to reach the destination which is extended on an access switch.


Many Thanks,

Saran.

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Giuseppe Larosa Mon, 06/08/2009 - 12:57
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Hello Saran,

a detailed answer depends on the type of linecards and of submodules installed on the linecards.


The sup720 with all its components PFC 3 Y and MSFC 3 build a topology based CEF table for all the known IP prefixes.


Depending on the linecards capabilities and on the submodule installed (DFC or CFC) the sup720 is consulted or not.


More specifically if the packets of a flow are destined for a host connected to an access switch connected to the C7600:

the ARP table tells what MAC address is associated to the host ip address

the L2 CAM table tells out what port (the uplink for the access-layer switch is connected to)the MAC address is learned.

This is enough and it is built with ARP request for host ip address triggered by first packet destinated to it


Hope to help

Giuseppe

Jon Marshall Mon, 06/08/2009 - 13:23
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Saran


The 7600 architecture is very similar to the 6500 architecture. Have a look at this white paper on the 6500 architecture. If you scroll to the end of the paper there are 2 examples of how packets are processed, one for centralized forwarding and one for distributed forwarding -


http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/prod_white_paper0900aecd80673385.html


Jon

saran_chn Mon, 06/08/2009 - 21:40
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Thanks Jhon,

It was really helpful to know more abt the architecture. Let me explain you the issue we had actually faced, we create a /29 subnet for our customers in 7600. In which 3 IP will be used for HSRP, Pri-Router and Sec- Router. Next two IP's will be given to the customer's firewall. Where in we were able to reach the IP's assigned routers from internet but not the IP's on firewall that too from particular location, example firewall IP will be reachable from india but not from singapore. So we raised a tac and got it sorted out. They said that packet coming to Sup will generate an RHV value with index number which decides the egress port for the packet to flow. In our case it was deciding the wrong exit so the packet was not able to reach at some time. That was the reason which made me to know more about behind the scene of the packet flow architecture. Please help me out to get an exact answer for this.


Many thanks,

Saran.

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