What is the 'political firewall' against Germany's far-right AfD?

Germany's centrist parties reject any cooperation with the Alternative for Germany (AfD), parts of which are classified as right-wing extremist. However, this policy is coming under increasing pressure.

source https://www.dw.com/en/what-is-the-political-firewall-against-germany-s-far-right-afd/a-78431554?maca=en-rss-en-ger-1023-rdf