INVITE without Supported 100 Rel is not being routed

Hi Fellows,

I know this is not a specific query related to the SBC, but I am sending traffic through my SBC with the following characteristics and I would like your recommendations and answers.

I would like you can help me to be more clear with the following.

Why a INVITE arrive to a Class4 Control Switch could not be able to route a INVITE that ingress without a 100rel supported parameter?

Example:

This is INVITE is not being routed by the C4.

Supported: X-cisco-srtp-fallback,X-cisco-original-called,timer,replaces,resource-priority

This INVITE is routed correctly by the C4.

Supported: em,100rel,timer,replaces,path,resource-priority,sdp-anat