From: Sidda6 (sidda6@gmail.com)
Date: Wed Jan 21 2009 - 09:43:53 ARST
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
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