From: Nitin Venugopal (nitinsworld@gmail.com)
Date: Wed Jan 21 2009 - 10:19:41 ARST
Is there any command in IOS to view table build by "route-cache" = show ip
cache flow
Regds
Nitin
On Wed, Jan 21, 2009 at 3:43 PM, Sidda6 <sidda6@gmail.com> wrote:
> Thank you experts, one last small question related to this, By default
> my router ( with recent IOS versions ) is enabled "ip cef" ( which i
> can see from running config ). What happens if I disable "ip cef" with
> - > NO IP CEF command. After disabling IP CEF, does router falls back
> to traditional "Route-Cache" mechanism. Is there any command in IOS to
> view table build by "route-cache" .
>
> Regards,
> Krishna
>
> On Tue, Jan 20, 2009 at 4:17 PM, Pavel Bykov <slidersv@gmail.com> wrote:
> > 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.html
> >> With 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
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
Blogs and organic groups at http://www.ccie.net
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