[frr] 2 MPLS Questions

[David Ahern]

On 4/6/17 11:33 AM, Olivier Dugeon wrote:
> Well, I'm not sure to correctly understand.
> 
> Is this means that we could manage for example 136 segment paths of 30 labels each, 256 segment paths of 16 labels each, 1024 segment paths of 4 labels, ... In other words, the sum of all label stacks of all segment paths must fit under the 4k bytes limit. I understand that we must put a limit, but 4k bytes is short. Very short. At the maximum it only authorizes 4k connexions with only one label i.e. only 4K LDP, RSVP-TE or MP-BGP paths.
> 
> Or is this means that the total size of the packet + labels must not increase 4k bytes ? That's enough for standard 1500 bytes packets, but not sufficient for jumbo frames.
> 
> Or is this means that the total information per mpls_route must not exceed 4k bytes ? In this case, it is largely enough as label stacks in segment routing replace the number of nexhops. I means that, then you describe an mpls_route in segment routing, the stack of label is the path and nexthops are the label. So, no needs to have both nexhops and labels.
> 
> Can you precise which of my interpretation is correct, or if I'm completely wrong ?

The 4k limit is on a single mpls_route spec in the kernel. For example,
a route like this:

ip -f mpls route add 100 \
    nexthop as to 101/102/.../130 via inet 172.16.1.1 \
    nexthop as to 201/202/.../230 via inet 172.16.2.1 \
    ...
    nexthop as to 3001/3002/.../3030 via inet 172.16.30.1 \

can only consume 4096 bytes in the kernel. 30'ish nexthops each with 30
labels all in a single MPLS route should be rather excessive - with both
the number of labels and the number of nexthops.

It has no correlation to packet size, number of routes, number of
segment paths, etc.