From: Pavel Bykov (slidersv@gmail.com)
Date: Tue Jan 20 2009 - 08:47:47 ARST
Thanks Joshua. Good info.
On Tue, Jan 20, 2009 at 11:28 AM, joshua atterbury <
joshuaatterbury@gmail.com> wrote:
> Hi Pavel,
>
> Correct, The output of your command shows that cef is using the "per
> destination" load balancing mechanism, What the router is not telling you,
> is that the hash process for per destination uses the source and destination
> address of the packet, This allows the device to perform a "fairer" load
> balancing process.
>
> Check this link,
> http://www.cisco.com/en/US/products/hw/modules/ps2033/prod_technical_reference09186a00800afeb7.htmlWith specific attention to this statement
>
> "For per destination load balancing a hash is computed out of the source
> and destination IP address. This hash points to exactly one of the adjacency
> entries in the adjacency table, providing that the same path is used for all
> packets with this source/destination address pair"
>
> So as you mentioned, the source interface does not play a part, But the
> source address does infact influence the load-balancing process.
> This all came to light when I had issues with traffic being black-holed and
> had to investigate how/why
>
> Cheers.
> Josh
> On Tue, Jan 20, 2009 at 9:07 PM, Pavel Bykov <slidersv@gmail.com> wrote:
>
>> Also:
>> Rack1R4(config-if)#ip load-sharing ?
>> per-destination Deterministic distribution
>> per-packet Random distribution
>>
>>
>> Where did you get that information? Aren't you confusing CEF with
>> Etherchannel?
>>
>>
>> On Tue, Jan 20, 2009 at 9:21 AM, joshua atterbury <
>> joshuaatterbury@gmail.com> wrote:
>>
>>> Take care to remember that cef uses the source ip and destination ip for
>>> the hashing process, To view the route that cef would take for a particular
>>> Src/Dst group use the following command
>>>
>>> show ip cef exact-route SRC DST
>>>
>>> Cheers
>>> Josh
>>>
>>> On Tue, Jan 20, 2009 at 5:12 PM, Pavel Bykov <slidersv@gmail.com> wrote:
>>>
>>>> Well, router chooses not source interface, but output interface. When
>>>> the
>>>> packet came into interface, it can either drop it ( e.g. based on uRPF)
>>>> or
>>>> process it.
>>>> In any case, output interface selection on not per-packet basis
>>>> (per-packet
>>>> is round-robin) is usually done per hash. I.E. you input something in,
>>>> and
>>>> the output is hash.
>>>>
>>>> This is really a CEF thing, which you should read in to if you're
>>>> interested.
>>>> For example:
>>>>
>>>> OSPF finds out that there are 3 equal paths to a destination network
>>>> 150.1.146.0, so this is programmed into CEF. Output interfaces are
>>>> Serial
>>>> 0/1, 0/2 and 0/3
>>>> CEF has 16 table entries, which CEF fills in the following fasion:
>>>> 00: Serial 0/1
>>>> 01: Serial 0/2
>>>> 02: Serial 0/3
>>>> 03: Serial 0/1
>>>> 04: Serial 0/2
>>>> 05: Serial 0/3
>>>> 06: Serial 0/1
>>>> 07: Serial 0/2
>>>> 08: Serial 0/3
>>>> 09: Serial 0/1
>>>> 10: Serial 0/2
>>>> 11: Serial 0/3
>>>> 12: Serial 0/1
>>>> 13: Serial 0/2
>>>> 14: Serial 0/3
>>>> 15: Serial 0/1
>>>>
>>>>
>>>> The packet comes into a router (through any interface, no uRPF).
>>>> Destination
>>>> is 150.1.146.56. Hash is run (I'm don't exactly know the algorithm).
>>>> Hash
>>>> outptut is always 4 bits, therefore output will be from 00 to 15, and it
>>>> will correspond to CEF table entry. Let's say for 150.1.146.56 hash
>>>> outputs
>>>> 09.
>>>> 09 is looked up, and it corresponds to output interface Serial 0/1, so
>>>> packet is sent out using interface Serial 0/1.
>>>>
>>>> Next packet is let's say destined to 150.1.146.182. Hash for that
>>>> destination is 03. Output interface is Serial 0/1... again! So no load
>>>> balancing this time.
>>>> So you can see that load balancing really depends on quality of hash
>>>> algorithm.
>>>>
>>>>
>>>> On Tue, Jan 20, 2009 at 6:47 AM, Sidda6 <sidda6@gmail.com> wrote:
>>>>
>>>> > From your explanation I understood my scenario is by default using
>>>> > per-destination load balancing. So, how does router determines source
>>>> > interface for the packet here ( with per-destination ) .. does it pick
>>>> > randomly any one of two interfaces or is there there any specific
>>>> > criteria. ( Is there anything ARP cache plays role here with respect
>>>> > to picking up of source interface of packet)
>>>> >
>>>> > On Mon, Jan 19, 2009 at 3:30 PM, Pavel Bykov <slidersv@gmail.com>
>>>> wrote:
>>>> > > Default max-paths is 4 for all except BGP, (which is 1) so command
>>>> is not
>>>> > > needed.
>>>> > > If IP route outputs two destinations, the load balancing is taking
>>>> place.
>>>> > > Usually, CEF will do per destination load balancing.
>>>> > > Setting on the interface defines the method and can be per packet
>>>> > (depends
>>>> > > on platform)
>>>> > >
>>>> > > for information about CEF tables, use:
>>>> > >
>>>> > > "show ip cef 155.1.146.0 internal"
>>>> > >
>>>> > > So, to be exact and answer your questions:
>>>> > > 1. Depends on the method, per packet or per destination. Packets are
>>>> load
>>>> > > balanced
>>>> > > 2. Not required.
>>>> > >
>>>> > > On Mon, Jan 19, 2009 at 8:24 AM, Sidda6 <sidda6@gmail.com> wrote:
>>>> > >>
>>>> > >> I have route to 155.1.146.6 from my route table: route table looks
>>>> like
>>>> > >> this->
>>>> > >>
>>>> > >> R5# show ip route
>>>> > >> O IA 155.1.146.0 [110/65] via 150.1.0.4, 00:01:26, Serial0/0/0
>>>> > >> [110/65] via 150.1.0.1, 00:01:26, Serial0/0/0
>>>> > >>
>>>> > >> R5#show ip route 155.1.146.6
>>>> > >> Routing entry for 155.1.146.0/24
>>>> > >> Known via "ospf 1", distance 110, metric 65, type inter area
>>>> > >> Last update from 150.1.0.4 on Serial0/0/0, 00:04:06 ago
>>>> > >> Routing Descriptor Blocks:
>>>> > >> 150.1.0.4, from 160.1.4.4, 00:04:06 ago, via Serial0/0/0
>>>> > >> Route metric is 65, traffic share count is 1
>>>> > >> * 150.1.0.1, from 160.1.1.1, 00:04:06 ago, via Serial0/0/1
>>>> > >> Route metric is 65, traffic share count is 1
>>>> > >>
>>>> > >> 1. what happens with OSPF path selection to destination network
>>>> when
>>>> > >> we have multiple equal costs...which path it selects in my case? if
>>>> it
>>>> > >> selected Serial0/0/1.. why it did not picked even Serial0/0/0 has
>>>> > >> equal cost.
>>>> > >> 2. Do i need to use max-paths command to load maximum-paths to
>>>> > >> loadbalance between two interfaces?
>>>> > >>
>>>> > >> Regards,
>>>> > >> Sidda
>>>> > >>
>>>> > >>
>>>> > >> Blogs and organic groups at http://www.ccie.net
>>>> > >>
>>>> > >>
>>>> _______________________________________________________________________
>>>> > >> Subscription information may be found at:
>>>> > >> http://www.groupstudy.com/list/CCIELab.html
>>>> > >>
>>>> > >>
>>>> > >>
>>>> > >>
>>>> > >>
>>>> > >>
>>>> > >>
>>>> > >
>>>> > >
>>>> > >
>>>> > > --
>>>> > > Pavel Bykov
>>>> > > ----------------
>>>> > > Don't forget to help stopping the braindumps, use of which reduces
>>>> value
>>>> > of
>>>> > > your certifications. Sign the petition at
>>>> http://www.stopbraindumps.com/
>>>> > >
>>>> >
>>>>
>>>>
>>>>
>>>> --
>>>> Pavel Bykov
>>>> ----------------
>>>> Don't forget to help stopping the braindumps, use of which reduces value
>>>> of
>>>> your certifications. Sign the petition at
>>>> http://www.stopbraindumps.com/
>>>>
>>>>
>>>> Blogs and organic groups at http://www.ccie.net
>>>>
>>>> _______________________________________________________________________
>>>> Subscription information may be found at:
>>>> http://www.groupstudy.com/list/CCIELab.html
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>
>>
>> --
>> Pavel Bykov
>> ----------------
>> Don't forget to help stopping the braindumps, use of which reduces value
>> of your certifications. Sign the petition at
>> http://www.stopbraindumps.com/
>>
>
>
-- Pavel Bykov ---------------- Don't forget to help stopping the braindumps, use of which reduces value of your certifications. Sign the petition at http://www.stopbraindumps.com/Blogs and organic groups at http://www.ccie.net
This archive was generated by hypermail 2.1.4 : Sun Mar 01 2009 - 09:43:39 ARST