RTP in this circumstance has nothing to do with Real-time Transport Protocol in VoIP networks. No, this is Reliable Transport Protocol and is used exclusively with EIGRP for reliable routing updates.
The reason EIGRP does not use UDP or TCP for its updates can be traced back to a time when IP didn’t own the monopoly on layer 4 protocols, and other options like AppleTalk and IPX were still very much alive. Cisco decided to create its own protocol for the purpose of supporting multiple layer 3 protocols.
An EIGRP packet will use protocol no. 88 in its header field, since it cannot use TCP or UDP. It runs above the IP layer, and uses protocol number 88.
When using multicast for routing updates, EIGRP will use the address 224.0.0.10. It will also sometimes use unicast for certain communication types. The characteristics of a given RTP/EIGRP communication depend on the EIGRP Header and TLV values in the 8-bit Opcode field. Some communications require acknowledgement, some require sequence numbers, some require unicast and become multicast, others multicast, and still others start as multicast but become unicast.
There are several types of EIGRP packets:
- Hello – Unacknowledged/Multicast. Used in neighbor discovery
- Acknowledgement – Unacknowledged/Unicast. Acknowledges EIGRP update, query , and reply packet types
- Update – Reliable/Multicast. This is how routing table updates occur. These are always handled as multicast UNLESS it is the first exchange of routing information during neighbor formation, in which case it is sent as a unicast.
- Query – Reliable/Multicast. The first query for routes is sent via multicast, and if there is no reply, subsequent attempts are sent as unicast up to a limit of 16, in which case the route is marked as unreachable.
- Reply – Reliable/Unicast. This is a reply to a query
Stuck-in-Active: Journal of an IT-Network Administrator 