You are trying to establish an EIGRP adjacency but the neighbor won’t come up. When you do a show ip eigrp neighbor, you see an RTO of 5000, a low SRTT value, and a Q count of 1 or higher. Why is that?
Switch-02#show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(10)
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 192.168.201.5 Vl201 11 00:00:13 2 5000 1 4881
Switch-02#
You show the EIGRP neighbors again, and you see the RTO value creeping up high all the way to 5000 and then stops. Also, you check the logs and find that EIGRP resends its packets continuously and the EIGRP adjacency flaps over and over again.
Switch-02#show logging last 10 Syslog logging: enabled (0 messages dropped, 2 messages rate-limited, 0 flushes, 0 overruns, xml disabled, filtering disabled) <truncated> Showing last 10 lines Log Buffer (16384 bytes): 005835: *Mar 26 2022 17:36:21.712 EDT: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 192.168.201.5 (Vlan201) is down: retry limit exceeded 005836: *Mar 26 2022 17:36:26.159 EDT: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 192.168.201.5 (Vlan201) is up: new adjacency 005837: *Mar 26 2022 17:37:12.463 EDT: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 192.168.201.5 (Vlan201) is down: retry limit exceeded 005838: *Mar 26 2022 17:37:13.013 EDT: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 192.168.201.5 (Vlan201) is up: new adjacency 005839: *Mar 26 2022 17:37:59.314 EDT: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 192.168.201.5 (Vlan201) is down: retry limit exceeded 005840: *Mar 26 2022 17:37:59.832 EDT: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 192.168.201.5 (Vlan201) is up: new adjacency 005841: *Mar 26 2022 17:38:46.135 EDT: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 192.168.201.5 (Vlan201) is down: retry limit exceeded 005842: *Mar 26 2022 17:38:46.327 EDT: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 192.168.201.5 (Vlan201) is up: new adjacency 005843: *Mar 26 2022 17:39:32.632 EDT: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 192.168.201.5 (Vlan201) is down: retry limit exceeded 005844: *Mar 26 2022 17:39:34.160 EDT: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 192.168.201.5 (Vlan201) is up: new adjacency Switch-02#
Let’s take a look at why we have this EIGRP RTO of 5000.
Here’s a diagram of the topology.
Solution
To be clear, let’s make sure we understand SRTT and RTO. So, what are the SRTT and RTO values?
SRTT stands for Smooth Round Trip Timer. SRTT is the time taken by a packet to reach the neighboring router and the acknowledgment of the packet to reach the local router.
RTO stands for Retransmission Timeout. RTO is the time in which the local router waits for an acknowledgment of a unicast packet. If a multicast fails initially, unicast is then sent to the soon-to-be neighbor router.
In our case above, the SRTT value is low, 2 milliseconds, so you should be able to ping the remote end with no delay. Let’s ping from Switch-02 to Switch-01.
Switch-02#ping 192.168.201.5 repeat 100 Type escape sequence to abort. Sending 100, 100-byte ICMP Echos to 192.168.201.5, timeout is 2 seconds: !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Success rate is 100 percent (100/100), round-trip min/avg/max = 1/1/1 ms Switch-02#
Ping packets are small and make it through, so it may be that the problem is that bigger packets cannot make it to the other end. These EIGRP packets might be bigger because the number of routes is large. Let’s check the MTU values on Switch-01 and Switch-02.
Switch-01#show ip interface vlan 201 | i Internet|MTU Internet address is 192.168.201.5/24 MTU is 1500 bytes Switch-01# Switch-02#show ip interface vlan 201 | i Internet|MTU Internet address is 192.168.201.6/24 MTU is 9198 bytes Switch-02#
Switch-02’s MTU value on interface vlan 201 is 9198 bytes unlike Switch-01’s MTU of 1500 bytes.
Because the MTU value of Switch-02’s VLAN 201’s SVI is bigger than 1500 bytes and EIGRP has a large list of updates to send, it’s very likely that Switch-02’s EIGRP packets are bigger than 1500 bytes and that the Layer 2 service that’s connecting the two switches is only allowing frames with an MTU of up to 1500 bytes.
Switch-02#ping 192.168.201.5 size 1499 Type escape sequence to abort. Sending 5, 1499-byte ICMP Echos to 192.168.201.5, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms Switch-02#ping 192.168.201.5 size 1500 Type escape sequence to abort. Sending 5, 1500-byte ICMP Echos to 192.168.201.5, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms Switch-02#ping 192.168.201.5 size 1501 Type escape sequence to abort. Sending 5, 1501-byte ICMP Echos to 192.168.201.5, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) Switch-02#
When the provider of the Ethernet circuits sees an Ethernet frame with a payload bigger than 1500 bytes, the provider’s network drops the frames.
So what’s the solution? Change the MTU value to 1500 bytes on the interface over which you want the EIGRP neighbor to be established.
IP packets, including EIGRP packets, become the payload of Ethernet frames because IP packets are encapsulated in an Ethernet header and a trailer. When the IP MTU of an interface is 1500 bytes, the IP packets built before exiting that interface will be capped at 1500 bytes.
Switch-02#config terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch-02(config)#interface vlan 201
Switch-02(config-if)#ip mtu 1500
Switch-02(config-if)#end
Switch-02#
Switch-02#show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(10)
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 192.168.201.5 Vl201 14 00:00:37 1 100 0 5549
Switch-02#
After changing the MTU value of VLAN 201’s interface to 1500 bytes, the EIGRP neighbor came up. An EIGRP neighbor should always have a Q count of 0. Notice that when the EIGRP neighbor was failing, the Q count was 1.
Check for EIGRP routes on Switch-01.
Switch-01#show ip route eigrp
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, m - OMP
n - NAT, Ni - NAT inside, No - NAT outside, Nd - NAT DIA
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
H - NHRP, G - NHRP registered, g - NHRP registration summary
o - ODR, P - periodic downloaded static route, l - LISP
a - application route
+ - replicated route, % - next hop override, p - overrides from PfR
& - replicated local route overrides by connected
Gateway of last resort is not set
2.0.0.0/32 is subnetted, 40 subnets
D 2.2.2.1 [90/130816] via 192.168.201.6, 00:03:09, Vlan201
D 2.2.2.2 [90/130816] via 192.168.201.6, 00:03:09, Vlan201
D 2.2.2.3 [90/130816] via 192.168.201.6, 00:03:09, Vlan201
D 2.2.2.4 [90/130816] via 192.168.201.6, 00:03:09, Vlan201
D 2.2.2.5 [90/130816] via 192.168.201.6, 00:03:09, Vlan201
D 2.2.2.6 [90/130816] via 192.168.201.6, 00:03:09, Vlan201
D 2.2.2.7 [90/130816] via 192.168.201.6, 00:03:09, Vlan201
D 2.2.2.8 [90/130816] via 192.168.201.6, 00:03:09, Vlan201
D 2.2.2.9 [90/130816] via 192.168.201.6, 00:03:09, Vlan201
D 2.2.2.10 [90/130816] via 192.168.201.6, 00:03:09, Vlan201
D 2.2.2.11 [90/130816] via 192.168.201.6, 00:03:09, Vlan201
D 2.2.2.12 [90/130816] via 192.168.201.6, 00:03:09, Vlan201
D 2.2.2.13 [90/130816] via 192.168.201.6, 00:03:09, Vlan201
<truncated>
Switch-01#
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Summary
We had a scenario in which Switch-01 had an IP MTU of 1500 bytes; an Ethernet provider that delivered an Ethernet service with a max IP MTU of 1500 bytes; and Switch-02 with an IP MTU of 9198 bytes. The EIGRP adjacency between the two switches did not completely come up and flapped continuously.
The Q count was not 0 as it should be in a successfully built EIGRP neighborship; instead, the Q count was 1. And the RTO increased to 5000 because sent packets were not being acknowledged.
Based on the logs, we realized that Switch-02’s EIGRP process repeatedly tried sending its EIGRP packets. Because of a big enough number of subnets, those EIGRP packets were bigger than 1500 bytes and those packets were being filtered by the provider. Switch-01 was not receiving those EIGRP packets from Switch-02.
What did we do to solve the high RTO of 5000? We changed the IP MTU value on Switch-02’s interface from 9198 to 1500 bytes.
