Re: L3 Port Channel Question.

My .02 ..., a few great comments so far and so I hope this is helpful.

When you have multiple L3 links between devices, then you have multiple OSPF
adj, multiple networks, spf processing, etc ... What happens when one link
goes down, and how does this affect OSPF?

When you have multiple links within a port-channel, then the throughput is
greater and this link should be your preferred uplink / trunk. How about
multiple L3 port-channels between devices? How about more and then more and
then more ... lol ... so much for a simple network design.

A L3 port-channel only requires one network, one OSPF Adj, etc ... what
happens when one of these links fails? Does the port-channel stay up? Does
OSPF even know about it? Apart from a metric change ...

I have not tested this ... but doing a debug ip routing on a pair of
switches and failing one link within a L3 port-channel should be a nice
test. Perhaps some ping traffic as well ... If I get some time this week I
will try it.

My mind is all over the place at the moment, ... sorry about this, but when
I write this I think of how iSPF would help in any environment and how this
might help you in whichever design you prefer. Without iSPF, any change in
the network can have a rippling affect.

HTH some and sorry about being scatter-brained,

Andrew Lee Lissitz

On Mon, Jan 4, 2010 at 1:17 PM, Ruchir Jain (rucjain) <rucjain_at_cisco.com>wrote:

> To add to Marko's point: some platforms give the ability to load balance
> based on layer 4 port numbers and in some scenarios using them would give
> better load distribution rather than using the other method. In my opinion
> it would all boil down to the network requirements. One more thing that I
> could think of where using port channel would be a benefit is QOS.
>
> Also, regarding the hash values each platform has a separate way of
> calculating it like 6500 does not use the vlan number in computing hash
> while Nexus has the support for the same. I am not sure about the routers
> though.
>
> /Ruchir
>
> -----Original Message-----
> From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
> Marko Milivojevic
> Sent: Monday, January 04, 2010 11:30 PM
> To: Radioactive Frog
> Cc: Poplawski, James; Cisco certification
> Subject: Re: L3 Port Channel Question.
>
> On Mon, Jan 4, 2010 at 18:43, Radioactive Frog <pbhatkoti_at_gmail.com>
> wrote:
> > Good question!
> >
> > AFAIK the tie breaker for chosing OSPF over a L3 port channeling would be
> > "OSPF provides MULTIPLE path load balancing".
> > Also checks multiple other network related parameters (latency bandwidth
> > etc). With L3 port channeling we can't enforce based on bandwidth/load,
> > etc).
>
> Sorry but you are not correct here. OSPF does not take anything other
> than cost to the destination as the basis for load sharing. If more
> than one link shares the the same cost to the destination, it will be
> load shared. Where packet is sent to depends on the hash the CEF uses.
> I'm sort of tip toeing around defining the hash, because I'm not 100%
> sure what it consists of by default...
>
> --
> Marko Milivojevic - CCIE #18427
> Senior Technical Instructor - IPexpert
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