From: shiran guez (shiranp3@gmail.com)
Date: Thu Jan 22 2009 - 05:29:19 ARST
it is not exactly that the the router is not telling you, it is a simple
miss in the definition as the load sharing is done by default session based
and you can change it to packet based.
so the command should have been:
ROUTER(config-if)#ip load-sharing ?
per-session Deterministic distribution
per-packet Random distribution
:-)
On Tue, Jan 20, 2009 at 12:28 PM, 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
> >>> > >>
> >>> > >>
> >>> _______________________________________________________________________
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> >>> > >>
> >>> > >>
> >>> > >>
> >>> > >>
> >>> > >>
> >>> > >>
> >>> > >>
> >>> > >
> >>> > >
> >>> > >
> >>> > > --
> >>> > > Pavel Bykov
> >>> > > ----------------
> >>> > > Don't forget to help stopping the braindumps, use of which reduces
> >>> value
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> >>> > >
> >>> >
> >>>
> >>>
> >>>
> >>> --
> >>> 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:
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> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>
> >
> >
> > --
> > 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
>
> _______________________________________________________________________
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>
>
>
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>
-- Shiran Guez MCSE CCNP NCE1 JNCIA-ER CCIE #20572 http://cciep3.blogspot.com http://www.linkedin.com/in/cciep3Blogs and organic groups at http://www.ccie.net
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