EIGRP runs straight over IP. How does it deliver updates reliably, and what do SRTT, RTO and Q Cnt tell you?
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It uses its own Reliable Transport Protocol. Each reliable packet carries a sequence number and must be acknowledged by every neighbor. The router multicasts it, then retransmits by unicast to any neighbor that has not acknowledged within that neighbor's retransmission timeout. After 16 retries the neighbor is reset. SRTT is the smoothed round trip time to the neighbor, RTO is the retransmit timer derived from it, and Q Cnt is the number of packets waiting to go to that neighbor.
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The neighbor table, column by column
R1# show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(100)
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
1 10.0.13.3 Gi0/0/1 11 00:42:10 2 100 0 41
0 10.0.12.2 Gi0/0/0 13 01:02:33 1 100 0 27- H: order in which neighbors came up.
- Hold: seconds left before the neighbor is declared dead.
- SRTT: average time for this neighbor to acknowledge a reliable packet.
- RTO: how long to wait before retransmitting to this neighbor.
- Q Cnt: reliable packets queued for this neighbor. It should be 0.
- Seq Num: last sequence number received from the neighbor.
Signs of trouble
- Q Cnt staying above 0 and RTO near its ceiling of several seconds: the neighbor is not acknowledging. Look for a one-way link, a congested WAN or a firewall dropping unicast EIGRP.
- Uptime resetting every few minutes with
retry limit exceededin the log: same cause, worse.
Pacing
EIGRP limits itself to 50 percent of the interface bandwidth by default
(ip bandwidth-percent eigrp 100 50). On a WAN link with a wrong, low bandwidth value, that pacing
slows large updates and can make convergence look broken.
They will ask next
Why does it not just multicast the retransmission again?
Only one or two neighbors are usually missing. Unicasting to them avoids making every other neighbor process a duplicate, and lets a slow neighbor be handled without holding up the fast ones.
Which interface counter would you check next if Q Cnt is stuck?
Output drops and errors on the link with
show interfaces, andshow ip eigrp interfaces detailfor retransmissions and pending routes. Then I test unicast reachability to the neighbor's address.
Traps
- Ignoring Q Cnt. A nonzero queue that never drains is the earliest warning before a neighbor reset.