You cannot configure filters, policies, or services on the fabric interface. Fragmentation is not supported on the fabric link. The maximum transmission unit (MTU) size is 8,980 bytes. We recommend that no interface in the cluster exceed this MTU. Jumbo frame support on the member links is enabled by default.
http://www.juniper.net/techpubs/software/junos-security/junos-security10.2/junos-security-swconfig-security/topic-43702.html
You cannot configure filters, policies, or services on the fabric interface. Fragmentation is not supported on the fabric link. The maximum transmission unit (MTU) size is 8,980 bytes. We recommend that no interface in the cluster exceed this MTU. Jumbo frame support on the member links is enabled by default.