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New Member

CEF vs. EIGRP for load balancing

Some of our sites have dual PVCs pointing to two different FR T1's on our head-end router, the PVC's have the same port speed, CIR, and the bandwidth command is set to the same for both PVC's, so from EIGRP the paths are equal. My question is how does EIGRP and CEF differentiate in terms of load balancing on a per-packet basis?

I have enabled per-packet on one of the sub-interfaces here is the output for 'sh ip cef detail' for that destination network:

10.0.1.0/24, version 358, per-packet sharing

0 packets, 0 bytes

via 10.75.3.10, Serial0/0.2, 0 dependencies

traffic share 1, current path

next hop 10.75.3.10, Serial0/0.2

valid adjacency

via 10.75.4.10, Serial0/1.2, 0 dependencies

traffic share 1

next hop 10.75.4.10, Serial0/1.2

valid adjacency

25500 packets, 17030273 bytes switched through the prefix

30 second output rate 78 Kbits/sec

My question is why does Serial0/0.2 say 'traffic share 1, current path' but for s0/1.2 it says 'traffic share 1'? What does the current path mean? Does it mean that traffic is only going out the S0/0.2 interface?

One last question... Does CEF need to be enabled on both ends of the Frame to work efficiently?

  • Other Network Infrastructure Subjects
16 REPLIES
New Member

Re: CEF vs. EIGRP for load balancing

Hi.

You should configure "ip load-sharing per-packet" on both interfaces.

CEF is local to the router, the other end of the link doesn't need to have CEF in order to load balance traffic that is going out of your router.

Rgds.

NM

New Member

Re: CEF vs. EIGRP for load balancing

Thanks. 'ip load-sharing per-packet' is configured on both interfaces. I was also wondering... Are there any MIB's or another way to extract the '30 second output rate'? I would like to use this to do bandwidth monitoring with trying to calculate the input and output rate the different serial interfaces. Thanks!!

-jason

Silver

Re: CEF vs. EIGRP for load balancing

New Member

Re: CEF vs. EIGRP for load balancing

For the interactive transactions, the data are not only transferred out from local, but also received from remote - the remote router also need to config CEF to ensure real balance.

The per-packet sharing will increase the CPU load to reassemble the packets from different routes.

But I don't know what's the situation when one line is more busy than another line, what's the detail scenario for the packet transferring.

Anyone has more information on it?

Gold

Re: CEF vs. EIGRP for load balancing

Current path means this is the path which will be used for the next packet transmitted to this destination. Of course, it should change as packets are actually switched. :-)

You probably won't ever get 100% equal amounts of traffic on the two links, just because of different packet sizes and statistical discrepancies. I've never seen it work that way, anyway.... CEF accounting might help you see what the traffic levels are better, as well as the interface level counters.

Finally, none of the routing protocols really determine load sharing, pretty much. Think of the rib as a meeting place for the switching path and the routing protocols. The rp's install stuff there that the switching code uses to determine how to switch packets. The routing protocols may install one or more paths to a given destination, with thei associated metrics. Once this is done, the rib figures out, from the metrics, what the ratio of packets between the two links should be. CEF then takes this information and fills in several tables based on it, and switches the packets from these tables.

I think that Inside Cisco IOS Software has some explanation of this, but I don't remember how much. The slides fro PS-540 at networkers should have some explanation as well, along with the slides from the two hour arch session there, if you can get your hands on them.

Russ

New Member

Re: CEF vs. EIGRP for load balancing

That sound good, but if there are several hops between destination and source terminal, the lines adjacent to the source router is busy or not does not mean the next lines adjacent to next hops will be busy or not. So the rib calculation is based on source-destination pairs or just the physical lines? do all these routers need to use CEF or just some routers need? thanks!

New Member

Re: CEF vs. EIGRP for load balancing

That is what I thought the current path meant... However on all of the interfaces that I have CEF per-packet enabled on it always shows that the serial 0/0.x is the 'current path'. Here is a copy of what the config looks like

interface Serial0/0.4 point-to-point

bandwidth 128

ip address XXX.75.3.17 255.255.255.252

no ip directed-broadcast

ip load-sharing per-packet

frame-relay interface-dlci 43 IETF

end

RTRAKR01#sh run int s0/1.4

Building configuration...

Current configuration:

!

interface Serial0/1.4 point-to-point

bandwidth 128

ip address XXX.75.4.17 255.255.255.252

no ip directed-broadcast

ip load-sharing per-packet

frame-relay interface-dlci 43 IETF

end

The 'sh ip cef' output for that remote subnet looks like this:

XXX.75.141.0/24, version 509, per-packet sharing

17806 packets, 5134468 bytes

via XXX.75.3.18, Serial0/0.4, 0 dependencies

traffic share 1, current path

next hop XXX.75.3.18, Serial0/0.4

valid adjacency

via XXX.75.4.18, Serial0/1.4, 0 dependencies

traffic share 1

next hop XXX.75.4.18, Serial0/1.4

valid adjacency

17806 packets, 5134468 bytes switched through the prefix

30 second output rate 32 Kbits/sec

Thanks!!

-jason

Gold

Re: CEF vs. EIGRP for load balancing

I don't know that this counter is telling you what the real current path is. :-) Have you tried turning on cef accounting to see what it says?

Russ

Gold

Re: CEF vs. EIGRP for load balancing

I've thought about my last response a bit more, and have another suggestion for you. :-)

What interface is this traffic inbound on? What sort of a box is this? Try enabling per packet on the inbound interface.

:-)

Russ

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