Re: Load balancing

To answer you question in brief
Different routes to the same destination will follow this selection criteria
to choose the best route:

1- Longest, or most specific match will always be chosen first
2- If masks equal in length, AD will decide which route get selected
3- If AD are equal, (routes from the same routing protocol), the IGP route
type would be considered first. (with OSPF O>O*IA>E1/E2) etc.
4- Else the metrics of best IGP will determine the best route for insertion
into the Routing Table.

Up to this point CEF will only show one exit interface = single best route.

If you have two routes that are equal up to step 4 with equal metrics, then
both will be candidate to be inserted into the routing table, provided the
IGP allows/supports it, most defaults does. (maximum-paths) OR
If you have two routes that are equal up to step 4 but different metrics,
protocols like EIGRP (not default behavior though) does allow you to
configure a "variance" for unequal load-sharing.

Now that your IGP's has done its work and put multiple routes in the
routing-table, in what manner does the traffic leave the router if multiple
exits existsin the routing table, even or un-even

The forwarding switching method used will determine the order and what
interfaces packets leaves the router:
 ie CEF-Switching, Fast Switching, Process Switching, etc.

With Process-Switching it is per-packet load-balancing, compliments of your
CPU. 1 here, 1 there, 1 here, too much info CPU starts hurting....

With Fast-Switching it is Per-Destination load-sharing. Not load-balancing,
but load-sharing. Since traffic to 1 destination will always use the same
link, the two links will never be balanced.

Then came CEF, and offered kinda both.
CEF Per-Packet load-balancing and CEF Per-Destination load-sharing, where
Per-Destination is actually per-source-destination hashed, bla bla bla.
Although more balanced than fast-switching, it will still never be balanced.

CEF Per-Packet is the Real Deal, besides heavy process switching, CEF
Per-Packet does load-balance traffic leaving two interfaces, provided the
metrics are the same, by using the 16 hash buckets. 2 paths = each 8
buckets, 12121212121 etc bla bla bla

But what if their is two EIGRP routes in the routing table with different
metrics? CEF is clever enough to maintain the relative ratio between the two
metrics as close as possible, 16 buckets, & 2 Paths (say ratio 2:3) =
112221122211222, etc

All of this pertains to 1 ROUTER, MULTIPLE LINKS.

Sharing the load when 2+ ROUTERS, with multiple links, becomes a different
ballgame.

HTH
<ruhann>

On Tue, Jul 7, 2009 at 9:47 AM, jack daniels <jckdaniels12_at_gmail.com> wrote:

> Dear All,
>
> I'm very confused on the diffrent ways for load balancing the traffic if I
> have more than one route in my routing table.
>
> Also please suggest diffrence between load sharing and load balancing
>
>
> Regards
> J.Daniels
>
>
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-- 
<ruhann>
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