From: Pavel Bykov (slidersv@gmail.com)
Date: Tue Jan 20 2009 - 04:12:37 ARST
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
> of
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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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