Polycarp endorses Ignatius's letters. This is relevant because Ignatius would not endorse Polycarp's letters unless Polycarp's teachings agreed with it.
There are two instances where Polycarp endorses Ignatius's letters:
• Polycarp (writting in c. AD 110-140): “I appeal now to every one of you to hear and obey the call of holiness, and to exercise the same perfect fortitude that you have seen with your own eyes in the blessed Ignatius, and Zosimus, and Rufus; and not in them alone, but in a number of your own townsmen as well – to say nothing of Paul himself and the other Apostles.” [Polycarp to the Phillipians Section 9]
• Polycarp (writting in c. AD 110-140): “You and Ignatius have both written to me to ask whether anyone who may be going to Syria could deliver a letter from you there along with ours. I will see that this is done; perhaps by myself personally if I can find a suitable opportunity, or else by someone whom I will send to act for both of us. I am sending you Ignatius’s letters, as you requested; the ones he wrote to us, and some others that we had in our possession. They are enclosed herewith; you will be able to derive a great deal of benefit from them, for they tell you all about faith, and perseverance, and all the ways of self-improvement that involve our Lord.” [Polycarp to the Phillipians Section 13]