Why we need iBGP-multipath

[Volodymyr Khrystenko]

Hi. My question is do we really need iBGP Multipath? Ok, we have many iBGP routes on PE (I am not asking about eBGP), and many equal IGP paths. BGP by default selects one route to remote PE. But according to IGP we have many routes and CEF table gets all of them and starts load balancing traffic by its altorithms. And what if iBGP Multipath is on? There would be the same paths and the same result in CEF.

[anand kumar]

Hi Volodymyr,

A special extension to BGP known as “Add Paths” which allows BGP speaker to propagate and accept multiple paths for the same prefix.

The “Add Paths” capability allows peering BGP speakers to negotiate whether they support advertising/receiving multiple paths per prefix  and actually advertise such paths.

A special 4-byte path-identifier is added to NLRIs to differentiate multiple paths for the same prefix  sent across a peering session.

Notice that BGP still considers all paths as comparable from the viewpoint of best-path selection process – all paths are stored in the BGP RIB and only one is selected as the best-path

We can enable the same n mpls as below

router bgp 100

address-family ipv4 unicast vrf site2

maximum-paths ibgp 3

Regards

anand

[Volodymyr Khrystenko]

Thank for info. But my question was not what  iBGP multipath is but for what reason do we need it. Cos CEf is the one who balance traffic and for it there is no difference which procotol contains CEF table: OSPF with multiple outgoing interfaces or BGP with the same interfaces. So where is the difference? For eBGP and for ieBGP it is clear, but what about iBGP multipath?

[Harold Ritter]

Hi Volodymyr,

Here are two basic scenarios.

1. Load balancing traffic across multiple core paths to one egress PE.

2. Load balancing traffic across multiple PE CE links when CE is connected to more than one PE.

ibgp multipath is needed for the second scenario.

Regards

[Volodymyr Khrystenko]

As I understood for the second scenario we can use eBGP or iBGP multipath, right? So it means that we can avoid using iBGP always?

[Harold Ritter]

For scenario 2 you absolutely need iBGP. eBGP wont help between the ingress and the egress PEs.

There is another scenario where the destination CE is connected to a remote PE and to the ingress PE as well, you would need ieBGP multipath in order to load balance between the two PE CE links.

Regards

[Volodymyr Khrystenko]

I got it))) Thanks. There is a big difference between OSPF loadbalancing and iBGP loadbalancing. With iBGP MPLS network is loaded more smoothly then with OSPF pointing only to one PE. Thanks.