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Load-sharing in BGP

If the router receives and installs multiple paths with the same administrative distance and cost to a destination, load−balancing can occur. The number of paths used is limited by the number of entries the routing protocol puts in the routing table. Four entries is the default in IOS for most IP

routing protocols with the exception of Border Gateway Protocol (BGP), where one entry is the default. Six different paths configured is the maximum number.

My question here is, i am studying for the BGP CCIP exam and i wanted to discuss the different sceinarios of load-sharing (2 colocated links to a single provider, 2 links on differnet PEs and Multihomed customer with 2 providers) and how to configure load-sharing in BGP (for both iBGP and eBGP)

Thanks in advance.

7 REPLIES
Silver

Re: Load-sharing in BGP

You can configure load-sharing in BGP using maximum-paths command inside the router BGP configuration.

Here the link clarifies your doubt,

http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a00801a7f79.html

Re: Load-sharing in BGP

Hi,

I have read this pdf, but i needed to discuss it.

is it effective only when the both links are colocated on the same PE ??? What about the other scienarios ???

I just needed to discuss it.

Thanks in advance

Re: Load-sharing in BGP

Hi,

the "maximum-paths 2" command is effective only for BGP table entries which are received via eBGP. Once you learn BGP prefixes via iBGP the above mentioned command does not work.

There is a "maximum-paths ibgp 2" command for load-sharing prefixes learned via iBGP.

And you have "maximum-paths eibgp 2" under address-family ipv4 vrf in an MPLS VPN environment, where loadsharing between iBGP and eBGP makes sense in some scenarios.

Hope this helps

Martin

New Member

Re: Load-sharing in BGP

Hi all,

What about as-path ignor command in BGP will it do load sharing in iBGP and eBGP.

Not sure just a thought

KAmlesh

Re: Load-sharing in BGP

Dear MARTIN,

Really, thanks very very very much

When issuing "maximum-paths 2" Is it balanced load-sharing (per-packet --> depending on the switching mode of the router)?

Does it depend on whether the 2 paths are colocated on the same PE or CE?

Thanks in advance.

Re: Load-sharing in BGP

Hi Mohammed,

Yes load-sharing depends on switching mode. With CEF you have per-session as default.

When you do not have the two paths colocated at the PE/CE you will not learn them both through eBGP ...

one will be through eBGP and one through iBGP.

Regards

Martin

Re: Load-sharing in BGP

Is there a way to do further BGP balancing in the the last case (2 paths one through eBGP and the other through iBGP) ?

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