This tutorial explains Reliable Transport Protocol (RTP) and five EIGRP packet types: Update, Query, Reply, Hello, and Acknowledgment. Learn the meaning of EIGRP packet types and how EIGRP uses RTP to communicate with other EIGRP speaking routers.
Like other routing protocols, the EIGRP routing protocol does not use TCP or UDP for communication. An EIGRP router uses RTP protocol to communicate with other EIGRP speaking routers.
RTP stands for Reliable Transport Protocol. It is a Cisco proprietary protocol. Cisco designed it to provide reliable communication between two devices. It uses multicasts and unicasts to ensure that packets are delivered quickly and reliably.
RTP sends only one packet at a time. Since it sends only one packet at a time, it does not use windowing or any congestion control mechanism. It supports multicast and unicast methods of transmission.
How does EIGRP use RTP?
EIGRP uses RTP to communicate with other EIGRP speaking routers on the network. In EIGRP implementation, RTP is responsible for guaranteed and ordered delivery of EIGRP packets with the use of sequence and acknowledge numbers.
Each EIGRP router knows who its neighbors are. When an EIGRP router sends a multicast, it builds a list and uses it to track the neighbors who have replied. If the EIGRP router doesn't get a reply from a neighbor via the multicast, the EIGRP router uses unicasts to resend the same data. If it does not get a reply from the neighbor after 16 unicast attempts, it declares the router dead. This process is known as reliable multicast.
To track each packet, EIGRP assigns a sequence number to each packet. Sequencing allows routers to identify old and redundant packets and discard packets that are out of sequence.
RTP maintains a retransmission table for each neighbor. It uses this table to track all the reliable packets that were sent but not acknowledged within the Retransmission Time Out (RTO). When RTP sends a reliable packet, it creates an entry for the packet in the table. The entry contains the RTO timer. If the RTO timer expires before an ACK packet is received, RTP will transmit the same copy of the reliable packet again. RTP will repeat this process until the hold time of the neighbor expires in the neighbor table. If the hold time of a neighbor expires in the neighbor table, EIGRP removes the neighbor from the neighbor table.
EIGRP packet types
EIGRP uses five types of packets. These types are Update, Query, Reply, Hello, and Acknowledgment. Let's discuss these types.
Update
An Update packet contains the routing update or route information. When two EIGRP routers build a neighbor relation, they use update packets to exchange routing information. After building a neighbor relation, they use update packets only to exchange information about a change.
EIGRP always uses reliable multicast or unicast to send to an update packet. EIGRP uses multicast to send the same information to all neighbors and a unicast to send the information only to a specific neighbor.
Regardless of which method EIGRP uses to send an update packet, EIGRP always requires an acknowledgment for each update packet.
Query
EIGRP uses a query packet to find an alternate path to a particular destination when it has lost the exiting path to the destination. EIGRP always uses the reliable multicast method to send query packets and requires an acknowledgment for each query packet.
Reply
EIGRP uses a reply packet to respond to a query packet. EIGRP uses the reliable unicast method to send a reply packet. A reply packet either includes information about a specific route or a message indicating that there is no known route to the specific route. EIGRP also needs an acknowledgment for a reply packet.
Hello
EIGRP uses hello packets to discover potential EIGRP neighbors. EIGRP sends hello packets via the unreliable multicast method. It does not need an acknowledgment for a hello packet.
Ack
EIGRP sends an ack packet in response to an update, query, or reply packet. An ack packet confirms that the destination device received the packet. Ack packets are always sent via unicast and never require an acknowledgment. An ack packet is itself sent as acknowledgment. An acknowledgment for an acknowledgment packet makes no sense.
Key points
- EIGRP uses RTP to exchange packets with other EIGRP speaking routers.
- EIGRP exchanges five types of packets. These packet types are Update, query, reply, hello, and acknowledgment.
- Update, query, and reply packets require acknowledgment.
- Hello, and ack packets do not require acknowledgment. Since they don't require acknowledgment, they do not have sequence numbers.
- EIGRP multicast address is 224.0.0.10. A packet sent to this address is heard by all connected EIGRP routers.
- EIGRP Hello packets are sent every 5 seconds on LANs and point-to-point links. On T1 or low-speed interfaces, they are sent every 60 seconds.
That's all for this tutorial. In this tutorial, we discussed EIGRP packet types and how EIGRP uses the RTP protocol.