Load-balancing in MPLS Core

Question

How is load-balancing achieved in MPLS L3 vpns and equal cost multiple links exist to reach egress PE along with per-destination load-balancing enabled on interfaces.
I have tried to simulate the network below
Ingress PE--->P1--->>P2--->Egress PE
Multiple equal cost links exist between P1 and P2, cisco platform,LDP, IGP-ospf being used.

Answer 1

Hi,
Destination based load balancing in MPLS L3VPNs can be categorized into two scenarios:
1) multiple pathes between two PE routers
2) multiple access links to a single CE or site
Your question as I understand it was about the first scenario. So let me first quickly review how customer traffic is forwarded between VRFs on two different PE routers.
The VRF routing table will have BGP entries for the routes learned from the remote PE usually with next hop addresses being the remote PE loopback IP used for PE-to-PE BGP peering.
The traffic will be forwarded across P routers using the label for the BGP next hop.
Thus the load balancing accross the MPLS core in a first step is decided by the IGP, which has to insert several equal cost pathes into the global routing table for the BGP next hop networks (PE loopbacks).
Side note: MPLS traffic engineering in the core would allow for unequal cost load balancing.
The decision, which labeled packet to send across which path in the core is done by CEF using a hash algorithm. To achieve the same load balancing as with unlabeled IP traffic, a Cisco MPLS enabled router will look for the bottom label - the one with bottom-of-stack bit set to 1 - and try to determine, if the transported packet behind the bottom label is IP. If so, the hash is calculated for the customer IP header like for normal IP traffic. This ensures all traffic for a certain customer destination will always go through the same path. No unwanted packet reordering will occur.
Be aware, that the customer IP packet header will only be used for CEF hash calculation, no IP lookup will be performed, as core routers in MPLS L3VPNs do not have any knowledge about customer addresses.
As a side note: if the traffic transported is not IP (e.g. Ethernet over MPLS), the bottom label will be used for the CEF load balancing (e.g. the VC label).
For the second scenario - CE load balancing with multihomed CE/sites - it is first required to have two equal cost entries in the VRF routing tables. The difference will be the two different PE BGP next hop addresses. The first load balancing decision is the performed by CEF based on the IP packet received by the CE and the VRF routing table entries. Once CEF decided, which VRF entry to use, the required BGP next hop label (and the VPN label) is applied and the packet is transported across the MPLS core. load balancing there is done as described above.
Hope this helps! Please rate all posts.
Regards, Martin