OSPF Routing

[gajolly]

Hi Team,

Say we have three Layer 3 Switches (A(cat 6500),B & C (Cat 3750))connected to each other and OSPF,ECMP routing is going on between them.Now i have 2 paths from A to B and A to C i would like to know if node on A will talk to Node on B.Will there be a utilization of both links A to B or A to C and if yes, will that be Per destination or Per-packet.If Per Destination then how can we change it to Per-packet.

Thanks in Advance.

[pnschurr]

Well, it's a matter of terminology. ECMP in OSPF allows multiple paths to go into the route table.

Typically, CEF will do the load balancing across those paths.

CEF by default spreads flows across multiple paths. A flow is src/dst pair. So if this is happening on your router already, then you may already have CEF turned on. Do a 'show ip cef' command to see its status.

peter

[gajolly]

thanks Peter for all UR support.....

[dave_miller]

Just jumping on the back of this conversation as it is relevant to a situation I have ...

We have multiple point to point serial links set up to load balance using cef, however, we setup initially using static routes to load over the 5 links to the destination (multiple IP's at the destination so load balancing works well).

However, this then (hindsight is a wonderful thing) causes us severe problems when any of the circuits drop as there is no longer any path to the destination and the packets intermittently drop as they try to find their way via the static routes.

Is OSPF our answer - or are any of the other routing protocols better/worse for redistributing on a link state change ?

More prevelant because we are passing voice over the circuits

[kamlesh.sharma]

Q 1 - : Will there be a utilization of both links A to B or A to C and if yes,

A 1 - : No both the link will not be used

Reason1 - OSPF will not install second route bcos of higher Metric via path C.

Q 2 - : will that be Per destination or Per-packet.

A 2 - : It will be per destination always if you din't configure anything.

Q 3 - : If Per Destination then how can we change it to Per-packet.

A 3 - : Use no ip route-cache comand on the interface from where the data is coming means where the packet is coming. no need to set destination post with command ip load-sharing per-packet also.

but is CEF is enable than you can use no ip route-cache cef and so on..

If it helps please rate

Kamlesh

I think now it is more clearer than other answer.

[gajolly]

KAMLESH,it will use both paths if we will configure the same matrix

[gajolly]

Dave,

May be i havent undertstood the requirment but just wanted to put some light here when you say you are thinking of Sending Voice using Per-packet it wil not be good idea many people have tried it before and that didnt work so per destination wil be good idea in case of voice reason being packet may lose order or there can be delay while sending packets across different links...

let me know your thoughts

[dave_miller]

agreed .. per packet for voice does not work .. we use per destination .. however, as we use static routes, we encounter problems when any of the configured circuits changes state to down .. and have to manually remove batches of routes to clear up communication.

Looking for an automated solution that is a much smarter config than what we have in place at present.

Thought OSPF between the voip routers would possibly give me that .. so I'm looking for confirmation that this may be the case.

Thanks