Sri Rudram hymn is a part of YajurVeda Samhita (which I discuss here) In the Krishna Yajurveda Taittariya Samhita it is present from 4.5.1 to 4.5.11, thus forming total 11 Anuvakams:
The sixth mantra of 1st Anuvaka states:
अध्यवोचदधिवक्ता प्रथमो दैव्यो भिषक् ।
अहींश्च सर्वान् जम्भयन् सर्वाश्च यातुधान्यः ॥
The lord who speaks in favour of devotees, may he speak in favour of me (AdhyavochadAdhivakta), the foremost (Prathamo), present as godliness in Gods (Daivyo), the Doctor who treats all troubles (Bhisak). May he destroy (Jambhayan), seen enemies like serpents scorpions... (Sarvãn Ahinscha) and unseen enemies like ghosts, spritis, rakshasas (Sarvãscha yãtudhãnya:)
From the meaning of the mantra itself it is clear that the mantra is asking for protection from everything from Lord Shiva, so it itself is like a Kavacham (Armour).
The commentary on Sri Rudram given by Kamakoti site states this:
Chanting this mantra is capable of completely destroying miseries from Rakshasas, spirits, poison, fever etc. Hence this mantra has been hailed as ‘kavacham’ (armour) by Maharishi Bodhayana.
So, I'm interested, Where does Maharshi Bodhayana calls this mantra as Kavacham or Armour ?