From: joshua atterbury (joshuaatterbury@gmail.com)
Date: Tue Jan 20 2009 - 06:21:34 ARST
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
> > > ----------------
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> >
>
>
>
> --
> Pavel Bykov
> ----------------
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