The necessities of confining a forum to the limited and legitimate purposes for which it was created may justify the State in reserving it for certain groups or for the discussion of certain topics. See, e. g., Cornelius v. NAACP Legal Defense & Ed. Fund, Inc., 473 U. S. 788,806 (1985); Perry Ed. Assn. v. Perry Local Educators' Assn., 460 U. S. 37, 49 (1983). Once it has opened a limited forum, however, the State must respect the lawful boundaries it has itself set. The State may not exclude speech where its distinction is not 'reasonable in light of the purpose served by the forum,' Cornelius, supra, at 804-806; see also Perry Ed. Assn., supra, at 46, 49, nor may it discriminate against speech on the basis of its viewpoint, Lamb's Chapel v. Center Moriches Union Free School Dist., 508 U. S. 384, 390 (1993). ; see also Perry Ed. Assn., supra, at 46; R. A. V: v. St. Paul, 505 U. S. 377, 391 (1992) ; cf. Texas v. Johnson, 491 U. S. 397, 414-415 (1989). In determining whether the State is acting to preserve the limits of the forum it has created so that ...