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Load-balancing method on 3750 switches?

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May 3rd, 2010
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I've configured a Catalyst 3750 switch with equal-cost paths, learned via OSPF, to a remote destination.  I know that by default, OSPF will load-balance over the two paths, and I've confirmed this with the following output:

/* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;}


RR-A#sh ip route 192.168.250.1

Routing entry for 192.168.250.1/32

  Known via "ospf 100", distance 110, metric 102, type intra area

  Last update from 172.16.10.6 on Tunnel2, 00:21:44 ago

  Routing Descriptor Blocks:

  * 172.16.10.6, from 192.168.250.1, 00:21:44 ago, via Tunnel2

      Route metric is 102, traffic share count is 1

    172.16.10.2, from 192.168.250.1, 00:21:44 ago, via Tunnel1

      Route metric is 102, traffic share count is 1


My question is, how do I know if the switch is load-balancing using per-packet or per-destination?  From what I've read, it sounds like per-destination is the default, but I'm wondering if there's some command that will confirm that for me.


Thanks.

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Jon Marshall Mon, 05/03/2010 - 13:30
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It's doing per-destination load-balancing because the 3750 does not support per-packet load-balancing - see the unsupported interface commands in this link ie. the "ip load-sharing per-packet" command is not supported -


3750 interface unsupported commands


Jon

pweinhold Mon, 05/03/2010 - 13:35
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Very well, that's all I wanted to know.  Thanks for the quick reply.

Giuseppe Larosa Mon, 05/03/2010 - 14:37
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Hello Pweinhold,

be aware that using GRE tunnels on a C3750 is acceptable in a lab, but not in a production environment: packets routed over GRE tunnels are processed in software.


The end result could even be similar to per packet destination for the fact of using GRE tunnels (process switching), it would be flow based destination if using other types of L3 interfaces like SVI or routed ports.


Hope to help

Giuseppe

pweinhold Mon, 05/03/2010 - 15:28
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Yes, this is just a lab environment, just to test out a proof-of-concept, but thanks for the heads-up.

pughaz1330 Mon, 05/03/2010 - 23:08
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Hi

The below command shows that the "  Per packet load-sharing is disabled " so per session is working by default and if you enable per packet load sharing in the interface level  then you can able to see the flow on the sh ip cef command ; try the same !!!!


Router#sh cef interface
XXXXXXXXXXXXXXXXXXXXXXXX

  ICMP redirects are never sent

Per packet load-sharing is disabled

  IP unicast RPF check is disabled

  Inbound access list is not set

  Outbound access list is not set

  Hardware idb is GigabitEthernet0/0

  Fast switching type 1, interface type 27

  IP CEF switching enabled

  IP CEF Feature Fast switching turbo vector

  Input fast flags 0x400040, Input fast flags2 0x0, Output fast flags 0x10100, Output fast flags2 0x0

  ifindex 1(1)

  Slot 0 Slot unit 0 Unit 0 VC -1

  Transmit limit accumulator 0x0 (0x0)

  IP MTU 1500

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