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New Member

OSPF Routing

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.

21 REPLIES
New Member

Re: OSPF Routing

hi,

As per me,

OSPF supports equal cost path load balancing.U can set the cost manually. so if u have more than one paths with equal cost they will load balance .

default it supports 4 paths for load balancing and maximum it supports upto 6 paths.

load balancing is per packet basis.

sham.

Re: OSPF Routing

Will it really be per packet? I thought if you have CEF enabled it will be "per destination". Incidentally, "per destination" is a bit of a misnomer. The path depends on both source and destination addresses, and should better be called "per flow".

Kevin Dorrell

Luxembourg

New Member

Re: OSPF Routing

Hi all,

thanks all of you for ur inputs but i think Kevin is right when i have parellel routes to two other Layer 3 switches running ECMP (OSPF) then by default it will be flow based balancing but if you have CEF then per-packet is possible....

Again topology is

Switch B(3750) --------- Switch C(3750)

Switch A ( 6500)

two paths --- A to B other A to C

now node on A is talking to Node on B then will it follow parallel path in case costs are equal from two routes or not

But thanks all of you once again for your inputs i will request you guys again to think about it basically i m looking for Per - Packet routing if there is any change need to be made let me know....

Bronze

Re: OSPF Routing

Hi,

If you have two connections in parallel (e.g. two ports on the 6500 to two ports on switch A), have you looked at setting them up as an etherchannel? You can configure the various choices for load distribution with much more control than ECMP.

Or am I misunderstanding, and you are trying to do multipath with A to B as the first path and A to C to B as the second? If that's the case, I think ECMP will not help since that additional hop on C will definitely not be equal cost from a routing protocol standpoint. (EIGRP will do "variance" and you can adjust the variance to permit sharing the two paths).

John

Bronze

Re: OSPF Routing

Hi.

OSPF support support loadbalance in equal path. you need to put " maximum path 2" command in ospf, it will work in round robin fashion for same destionation.

I hope this will solve your problem, and if yes, rate this post.

-Minu

New Member

Re: OSPF Routing

Hi all,

thanks all of you for ur inputs but i think Kevin is right when i have parellel routes to two other Layer 3 switches running ECMP (OSPF) then by default it will be flow based balancing but if you have CEF then per-packet is possible....

Again topology is

Switch B(3750) --------- Switch C(3750)

Switch A ( 6500)

two paths --- A to B other A to C

now node on A is talking to Node on B then will it follow parallel path in case costs are equal from two routes or not

But thanks all of you once again for your inputs i will request you guys again to think about it basically i m looking for Per - Packet routing if there is any change need to be made let me know....

Silver

Re: OSPF Routing

New Member

Re: OSPF Routing

Separate the idea of multiple forwarding entries in the route table for the same prefix, and how the router forwards traffic according to data in the route table.

Of course OSPF allows multiple forwarding entries in the route table for the same prefix. But, how does the router treat traffic according to those multiple entries? That is a question of proprietory implementation.

Cisco uses CEF, which is a src/dst pairing model. If you turn CEF off, the forwarding engine will revert to packet-by-packet. The "load-balancing" is a function of forwarding, not the routing protocol.

peter

New Member

Re: OSPF Routing

Hi all,

It does not matter to me whether this Per packet load balancing is properiatry or not i just wanted to know when i have three layer 3 switches 1 uplink to each switch connecting each other as i have mention in my previous diagram... then if node (x) on Switch A (CAT 6000) talks to Node (y) on Switch B(Cat 3750) and ther is one more path from Switch A to Switch C then Node x will use SRc/dest or per packet

thanks in advance...

New Member

Re: OSPF Routing

Well, actually, it does matter to you. Because the answer to your question is dependent on the hardware and software in the routers. Not the routing protocol.

Most Cisco routers will use src/dst pair matching to spread traffic across multiple paths (as per ECMP and CEF).

But if you turn CEF off - or use a router that does not run CEF - then the behaviour may be different. Different in what way? Who knows, because there is no prescribed standard on how to deal with that situation.

Needless to say, because this is a Cisco forum, you can be assured that your Cisco hardware will run CEF, and all traffic between two nodes across multiple routed paths will traverse the same link - ie. no load balancing at all.

Remember that the path used could be different in each direction.

peter

New Member

Re: OSPF Routing

If your scenario is as simple as you describe... multiple links between 'switches', then you may want to look at ether-channels... bundle parallel ethernet links into a single logical link. Load-sharing across each of the links is then performed by the port-channel rather than L3 forwarding.

peter

New Member

Re: OSPF Routing

may be i have not explained but actually i wanted to achieve this with the help of OSPF ( Equal Cost Multiple Path).....

New Member

Re: OSPF Routing

Yes. The issue is that routing protocols (such as OSPF) do NOT perform load balancing (ignoring EIGRP).

ECMP allows multiple paths to be entered into the routing table from the same routing protocol process.

How the forwarding process regards these multiple paths is a separate function. As already mentioned, CEF on Cisco routers will use src/dst loading. So all traffic from/to the same pair of addresses will always traverse the same path (depending on frequency of transmission and CEF cahce clearing timers).

peter

New Member

Re: OSPF Routing

thanks a lot peter, please correct me if i m wrong OSPF cannt do load balancing like Per-packet but its possible via CEF, n i believe we need to enable CEF to achieve per-packet instead of turning it off because Src/dest flow based is already enable by default.

Thanks again man !!

New Member

Re: OSPF Routing

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

New Member

Re: OSPF Routing

thanks Peter for all UR support.....

New Member

Re: OSPF Routing

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

New Member

Re: OSPF Routing

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.

New Member

Re: OSPF Routing

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

New Member

Re: OSPF Routing

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

New Member

Re: OSPF Routing

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

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