EIGRP SRTT AND RTO TIMER

[surendrasinghtanwar667]

Hey everyone recently i was studying out the EIGRP protocol and i have a few doubt that what exactly is RTO and SRTT timer and when router switch from multicast to unicast and send 16 update packet to synchronize please let me know

Thanks 

[M02@rt37]

Hello @surendrasinghtanwar667 

SRTT  is a key metric that represents the average time it takes for a packet to be sent to a neighboring router and for the acknowledgment to return. This value is measured in milliseconds and is dynamically adjusted based on network conditions. It allows the router to gauge how long it typically takes for a round-trip communication with its neighbors.

RTO ,on the other hand, is the amount of time the router waits for an acknowledgment before retransmitting a packet. Initially, EIGRP uses multicast to send updates to all neighbors. If no acknowledgment is received within the RTO period, EIGRP will switch to unicast and retransmit the update directly to the unresponsive router. The RTO is calculated using the SRTT value to ensure it is appropriate for the network's current latency.

The combination of SRTT and RTO ensures reliable delivery of updates, with retransmissions occurring only when necessary, helping EIGRP maintain efficient and stable communication between routers.

[surendrasinghtanwar667]

so what exactly is 16 unicast update when no acknowledgment arrives

[David Ruess]

The 16 packets are sent in rapid succession as unicast because it's likely that a specific neighbor hasn't replied to a packet that requires a response. Some packets in EIGRP need a response, so when that doesn't happen the packets become unicast. You wouldn't send it multicast because then everyone on the link would get it. It used for efficiency. 

[M02@rt37]

@surendrasinghtanwar667 

In details, in EIGRP, the term "16 unicast updates" refers to a specific behavior that occurs when a router fails to receive an acknowledgment for a multicast update sent to its neighbors. Initially, EIGRP utilizes multicast addresses (specifically, 224.0.0.10) to send routing updates to all its neighboring routers simultaneously. This approach allows for efficient communication, enabling the router to disseminate routing information quickly. However, after sending a multicast update, the sending router expects to receive acknowledgments from each neighbor, confirming that they have received the update.

If a router does not receive an acknowledgment within the designated RTO period, it assumes that the update was lost or that the neighbor is unresponsive. In response to this situation, EIGRP switches from multicast to unicast communication, sending the updates specifically to the neighbor that failed to respond. This transition enhances the reliability of the communication process because unicast packets are directed to a single router, reducing the chances of lost packets and ensuring that critical routing information is delivered.

The protocol is designed to send a total of 16 unicast updates to the non-responsive neighbor. This means that after the initial multicast update is unsuccessful, EIGRP will attempt to resend the update using unicast up to 16 times. This mechanism ensures that routing information is effectively communicated, especially in cases where multicast delivery may have failed. If, after these 16 attempts, the router still does not receive an acknowledgment, it may conclude that there is a more significant issue affecting the neighbor's availability or connectivity, potentially marking the neighbor as down. This process underscores EIGRP's emphasis on reliable communication in maintaining accurate and up-to-date routing information within the network.

[surendrasinghtanwar667]

Actually i have read out this doc https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/118974-technote-eigrp-00.html and it should that EIGRP will increate the RTO time 200 miliseconds to 5 seconds so so as you said after expiring the RTO time then it switch from multicast to unicast but if the rto time expires so what is the use case of this 80 second retries please let me know 

[M02@rt37 · Accepted solution]

@surendrasinghtanwar667 

In EIGRP, RTO and the retry mechanism are essential for maintaining reliable communication between routers. The RTO is the time EIGRP waits for an acknowledgment after sending a multicast update, typically starting at 200 milliseconds and potentially increasing to a maximum of 5 seconds. If an acknowledgment isn’t received within this timeframe, EIGRP switches to unicast for retransmission to improve the chances of successful communication.

Additionally, the 80-second timer is critical for neighbor relationship maintenance; if a router doesn’t receive EIGRP messages, such as Hello packets, from a neighbor within this period, it considers that neighbor down. This mechanism allows EIGRP to attempt recovery of communication before declaring a neighbor as inactive, which is particularly valuable in environments with fluctuating link reliability.

Overall, the combination of RTO and the 80-second timer enhances EIGRP's robustness by allowing it to adapt to network conditions while minimizing unnecessary disruptions in routing...

[surendrasinghtanwar667]

Thank you man Now i got the actual point which i was missing out really thank you

[David Ruess]

Hello,

Taken from a Cisco Press article: EIGRP Terminology and Operation > Configuring the Enhanced Interior Gateway Routing Protocol | Cisco Press

• Smooth Round Trip Timer (SRTT)—The average number of milliseconds it takes for an EIGRP packet to be sent to this neighbor and for the local router to receive an acknowledgment of that packet. This timer is used to determine the retransmit interval, also known as the retransmit timeout (RTO).
• RTO—The amount of time, in milliseconds, that the router waits for an acknowledgment before retransmitting a reliable packet from the retransmission queue to a neighbor.

-David

[MHM Cisco World]

 I run this lab for you 
it show you eigrp use both multicast and unicast 

first the R1 send Hello multicast check if there are any eigrp neighbor in same subnet of link same is done by R2 and R3
Now each router knows neighbor and it IP 

each router then send update as unicast and neighbor must reply by ACK 

if there is no ACK then router start to
1- retransmit the unicast packet, sure it not for infinite but until the retransmit count reach 16 
2- increase the RTO, and max is 5000