Sequence Number Present. Strict source route present. Acknowledgment sequence number present. Must be set to zero. Protocol type Set to hex 880B. Key Use of the Key field is up to the implementation.
Sequence number Contains the sequence number of the payload.
Acknowledgment number Contains the sequence number of the highest numbered GRE packet received by the sending peer for this user session. In particular, the protocol protects against the failure of the first hop router when the source host cannot learn the IP address of the first hop router dynamically. A large class of legacy host implementations that do not support dynamic discovery are capable of configuring a default router. HSRP provides failover services to those hosts. HSRP runs on top of UDP, and uses port number 1985.