EIGRP Maximum Delay pitfall

July 24, 2008 at 5:55 pm | Posted in EIGRP, Routing | Leave a comment

Setting interface delay to a maximum number will have undesired effect on the routing. You would normally expect that setting it to the max value, EIGRP will unfavor the path via  that link. It is not always the case. I think this is because when exceeding a certain integer number, the metric will be offset back to a small number, and is actually preferred.

Similar weid behavior is observed in OSPF virtual link path cost too. If you set the the “exiting” interface of the router with virtual link the to max of 65535, virtual link wont come up. However, it is OK to set ANY transit link to this maximum value, and the virtual link is fine. That weid things make IOS so beautifull, and attractive ;)

Here again is example of the weird EIGRP behaviour when setting the link delay to max.

IE WB – VOL1 – ver5, Task 5.14

Default Interface Delay

Rack1SW1#sh int vlan 67 | in DLY
MTU 1504 bytes, BW 1000000 Kbit, DLY 10 usec,
Rack1SW1#sh ip eigrp topology 150.1.6.0/24
EIGRP-IPv4 (AS 100): Topology Default-IP-Routing-Table(0) entry for 150.1.6.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 128256
Routing Descriptor Blocks:
155.1.67.6 (Vlan67), from 155.1.67.6, Send flag is 0x0
Composite metric is (128256/128000), Route is Internal
Vector metric:
Minimum bandwidth is 1000000 Kbit
Total delay is 5010 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1504
Hop count is 1

EIGRP is well behaving when Interface Delay is set to 1sec.

Rack1SW1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Rack1SW1(config)#int vlan 67
Rack1SW1(config-if)#delay ?
<1-16777215>  Throughput delay (tens of microseconds)

Rack1SW1(config-if)#delay 100000
Rack1SW1(config-if)#end
Rack1SW1#sh ip eigrp topology 150.1.6.0/24
02:09:19: %SYS-5-CONFIG_I: Configured from console by console
Rack1SW1#sh int vlan 67 | in DLY
MTU 1504 bytes, BW 1000000 Kbit, DLY 1000000 usec,
Rack1SW1#sh ip eigrp topology 150.1.6.0/24
EIGRP-IPv4 (AS 100): Topology Default-IP-Routing-Table(0) entry for 150.1.6.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 668160
Routing Descriptor Blocks:
155.1.37.3 (FastEthernet0/3), from 155.1.37.3, Send flag is 0x0
Composite metric is (668160/642560), Route is Internal
Vector metric:
Minimum bandwidth is 1544 Kbit

Total delay is 26100 microseconds ! via other path

Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 3
155.1.67.6 (Vlan67), from 155.1.67.6, Send flag is 0x0
Composite metric is (25728000/128000), Route is Internal
Vector metric:
Minimum bandwidth is 1000000 Kbit
Total delay is 1005000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1504
Hop count is 1
Rack1SW1#sir 150.1.6.6
Routing entry for 150.1.6.0/24
Known via “eigrp 100”, distance 90, metric 668160, type internal
Redistributing via eigrp 100
Last update from 155.1.37.3 on FastEthernet0/3, 00:00:52 ago
Routing Descriptor Blocks:
* 155.1.37.3, from 155.1.37.3, 00:00:52 ago, via FastEthernet0/3
Route metric is 668160, traffic share count is 1
Total delay is 26100 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 3

EIGRP (or IOS??) is a bad boy when Interface Delay is set to Maximum.


Rack1SW1#
Rack1SW1#
Rack1SW1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Rack1SW1(config)#int vlan 67
Rack1SW1(config-if)#delay ?
<1-16777215>  Throughput delay (tens of microseconds)

Rack1SW1(config-if)#delay 16777215
Rack1SW1(config-if)#
Rack1SW1#conf t
02:10:40: %SYS-5-CONFIG_I: Configured from console by console
Rack1SW1#sh int vlan 67 | in DLY
MTU 1504 bytes, BW 1000000 Kbit, DLY 167772150 usec,
Rack1SW1#sh ip eigrp topology 150.1.6.0/24
EIGRP-IPv4 (AS 100): Topology Default-IP-Routing-Table(0) entry for 150.1.6.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 127744
Routing Descriptor Blocks:
155.1.67.6 (Vlan67), from 155.1.67.6, Send flag is 0x0
Composite metric is (127744/128000), Route is Internal
Vector metric:
Minimum bandwidth is 1000000 Kbit
Total delay is 4990 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1504
Hop count is 1
Rack1SW1#sir 150.1.6.6
Routing entry for 150.1.6.0/24
Known via “eigrp 100”, distance 90, metric 127744, type internal
Redistributing via eigrp 100
Last update from 155.1.67.6 on Vlan67, 00:00:19 ago
Routing Descriptor Blocks:
* 155.1.67.6, from 155.1.67.6, 00:00:19 ago, via Vlan67
Route metric is 127744, traffic share count is 1
Total delay is 4990 microseconds, minimum bandwidth is 1000000 Kbit
Reliability 255/255, minimum MTU 1504 bytes
Loading 1/255, Hops 1

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