A BIG-IP cipher string controls which TLS cipher suites an SSL profile will offer and in what order. You will find it as the cipher field of an ltm cipher rule, or typed directly into an SSL profile. It looks cryptic at first, but it follows a small, consistent grammar.

Sets, separated and ordered

A cipher string is a list of sets, separated by a colon :, a comma ,, or whitespace. Order matters: a server walks its list top to bottom and picks the first suite the client also supports, so the sets near the front are the ones you prefer. ECDHE:RSA:AES is three sets, evaluated in that order.

Keywords combined with plus

Within a set, keywords are joined with a plus sign + to narrow the selection. Each keyword constrains a different part of a cipher suite: the protocol version (TLSv1_2), the key exchange and authentication (ECDHE, RSA, ECDHE_ECDSA), the bulk cipher (AES-GCM, 3DES), and the MAC or hash (SHA256). So TLSv1_2+ECDHE+AES-GCM+SHA384 means exactly the suites that are TLS 1.2, use , use (Advanced Encryption Standard)-, and use -384. A bare keyword like AES with no pluses is broad, matching every AES suite. The explainer reads keywords without regard to case, but none of the F5 pages read for this article says TMM does, and OpenSSL treated cipher lists as case-sensitive until version 4.0, so write them in the case F5's own articles use: TLSv1_2, ECDHE, AES-GCM.

Operators

A set can carry a leading operator that changes what BIG-IP does with it:

  • ! excludes the set permanently. !SSLv3 removes every SSL 3.0 suite, and nothing later in the string can add them back. This is the safe way to forbid weak ciphers.
  • - deletes the set from the list built so far, but a later set may re-add it. It is weaker than !.
  • A leading + lowers the priority of the set, moving its suites toward the end of the list rather than removing them. Note this is different from the + that joins keywords inside a set.
  • @STRENGTH is not a set but a directive: it re-sorts everything selected so far by key length, strongest first. @SPEED asks for the same re-sort by encryption speed, but do not rely on it: on BIG-IP 14 a user found DEFAULT:@SPEED returned the same list as DEFAULT, and an F5 engineer replied that sorting by speed is no longer supported.

So ECDHE:RSA:!SSLv3:!RC4:@STRENGTH reads as: prefer ECDHE, then , never allow SSL 3.0 or RC4 (Rivest Cipher 4), then sort what remains by strength.

What the string does not tell you directly

The grammar tells you the instructions, but not the final list of suites those instructions produce. That list depends on which suites a given TMOS version actually supports, so the same string can expand differently across BIG-IP versions. The F5 cipher-string explainer parses the grammar and explains every keyword and operator; to see the exact ordered suites, run tmm --clientciphers on the target box. For which keywords are safe and which are not, see TLS cipher security keywords; for how rules and groups assemble these strings, see F5 cipher rules and groups.

Sources

Parts of this grammar are documented by F5 and parts are not. F5's articles show the colon separator, the ! exclusion, keywords joined with +, and @STRENGTH. None of the F5 pages read here shows the - operator, a leading +, or the comma and whitespace separators, or says whether keywords ignore case: the operators and separators are the rules of OpenSSL's cipher-list format, which K15194 says F5's format resembles while warning that some strings differ, so confirm them on the target version with tmm --clientciphers. Every page below was read on 2 October 2026.