Hi Bern,
Which version of Junos are you running? You should go for at least 15.1X49-D45 and if you are using irb-interfaces then add "set protocols l2-learning global-mode switching"
Read this regarding layer2 on the new SRX branch platforms: http://www.juniper.net/techpubs/en_US/junos15.1x49-d40/information-products/pathway-pages/security/security-layer2-bridging-transparent-mode.html
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Best regards,
Jonas