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MPLS CEF loadbalancing confusion

Halil.Zakaria
Level 1
Level 1

Hello,

Thank you in advance for your precious support.

I have a question regarding CEF load balanving in MPLS, in MPLS fundamentals book chapter 6, it is said that :

The algorithm for load balancing the MPLS packets becomes as follows:

If the MPLS payload is an IPv4 packet, the load balancing is done by hashing the source and

destination IP address of the IPv4 header.

If the MPLS payload is an IPv6 packet, the load balancing is done by hashing the source and

destination IP address of the IPv6 header.

If the MPLS payload is not an IPv4 or IPv6 packet, the load balancing is done by looking at

the value of the bottom label.

I understand that by Simply looking to the Top label an MPLS P-router can determine if the mpls payload is IPv4 or IPv6 (because he itself advertises the label for the corresponding IPv4 or IPv6 Prefix), but the confusion comes from the CEF Per destination-load balancing method, the P router needs to hash ip source and destination in the IP packet in order to determine which path to take, is P router able to read IP header in this case?

Thnak you.

2 Replies 2

Hello.

P-router may look into payload and if it sees "4" or "6" as first 4 bits - it assumes it's IPv4/IPv6 payload.

PS: once MAC-addresses started using "4" and "6" in first 4 bits it became a small problem.

PS2: different platforms have different limitations on a number of labels it may examine, so sometimes P-router does not go deep to the IP-header, but leverage on 5th label.

PS3: load-balancing for L2VPN may be done with FAT-label.

Thank you for these clarifications, the question can be expressed as followed:

the P router needs to hash ip source and destination in the IP packet in order to determine which path to take, is MPLS-P router able to read IP header in this case when label swaping packets?

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