Interface Aggregation

Unanswered Question
Oct 8th, 2008

Hi guys,

I have a problem.I know interface aggregation works when the interfaces you are aggregating are going to same device.Is it feasible to create an aggregate interface when interfaces are connected to different devices.Lets say I have 3 routers ,A,B,C.RTA has two gige interfaces.Gigi0/1 is connected to RTB and gigi0/2 is connected to RTC.i tried creating aggregate interface on RTA.But this created lots of degradation.Is is that traffic gets split between this two interfaces or loop ? Am not using any switch here.

I have this problem too.
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Giuseppe Larosa Wed, 10/08/2008 - 12:48

Hello Peter,

between routers you can use parallel L3 links and take advantage of load balancing

Using etherchannels between routers even the same two routers is something I don't recommend.

With IGP protocols like OSPF,EIGRP, IS-IS you can use 4 parallel links by default and up to 6, 8 or more depending on platform

load balancing can be flow based and with CEF is an exor of source and destination IP addresses and a seed hash

So my suggestion is use L3 routing

Hope to help

Giuseppe

Istvan_Rabai Wed, 10/08/2008 - 21:06

Hi Peter,

An aggregate inteface is a Port-channel interface and is part of the Ether-channeling feature. On a multilayer switch, you can configure a Port-channel interface for Layer3 operation and assign it an ip address. Load balancing will be implemented using an algorithm specific to Etherchannel, but the next hop will always be the same router.

When you connect 2 interfaces of RouterA to 2 different routers, it is already not an aggregate interface.

In this case you need to put them into different ip subnets on Router A and it maybe called an alternate route to some remote subnets if it really leads to the same subnets.

As Giuseppe says, this solution can be used for load balancing between alternate routes (usually alternate next hops). In this case load balancing is accomplished by the routing protocol according to the protocol configuration.

Cheers:

Istvan

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