EIGRP Routing

thorsten-gadau · ‎09-07-2015

Dear all,

I am having a strange behaviour of a 3650 Switch, running IP-Services ( Router1).

This switch is connected via 2 cables to a second 3650 switch, running IP-Base ( Router2). I configured EIGRP between these two and both adjacencies are successfully established. Although, the 1st switch is only showing the route through the first connection. If I unplugg the first connection the route is switched to the second connection.

Could it be possible that this is a software bug, both switches are running IOS 3.3.5SE? Here is my configuration of both switches:

Router1:

interface GigabitEthernet1/0/1
description Uplink Bridge1_Werk1
no switchport
ip address 172.19.1.1 255.255.255.248
ip summary-address eigrp 1 0.0.0.0 0.0.0.0
auto qos trust dscp
service-policy input AutoQos-4.0-Trust-Dscp-Input-Policy
service-policy output AutoQos-4.0-Output-Policy

interface GigabitEthernet1/0/3
description Uplink Bridge2_Werk1
no switchport
ip address 172.19.1.9 255.255.255.248
ip summary-address eigrp 1 0.0.0.0 0.0.0.0
auto qos trust dscp
service-policy input AutoQos-4.0-Trust-Dscp-Input-Policy
service-policy output AutoQos-4.0-Output-Policy

router eigrp 1
network 172.19.1.1 0.0.0.0
network 172.19.1.9 0.0.0.0

passive-interface default

no passive-interface GigabitEthernet1/0/1

no passive-interface GigabitEthernet1/0/3

Router2:

interface GigabitEthernet1/0/1
description Uplink Bridge1_Werk2
no switchport
ip address 172.19.1.2 255.255.255.248
no ip proxy-arp
no ip route-cache
auto qos trust dscp
service-policy input AutoQos-4.0-Trust-Dscp-Input-Policy
service-policy output AutoQos-4.0-Output-Policy

interface Vlan666
ip address 172.19.1.10 255.255.255.248
delay 10

router eigrp 1
network 172.19.1.2 0.0.0.0
network 172.19.1.10 0.0.0.0
redistribute connected
passive-interface default
no passive-interface GigabitEthernet1/0/1
no passive-interface Vlan666
eigrp stub connected summary

sh ip eigrp topo output of Router1:

Router_Werk1(config-router)#do sh ip ei top
EIGRP-IPv4 Topology Table for AS(1)/ID(192.168.200.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status

P 192.168.3.0/24, 1 successors, FD is 3072
via 172.19.1.2 (3072/2816), GigabitEthernet1/0/1
via 172.19.1.10 (28416/2816), GigabitEthernet1/0/3
P 172.19.1.8/29, 1 successors, FD is 28160
via Connected, GigabitEthernet1/0/3
via 172.19.1.2 (5376/5120), GigabitEthernet1/0/1
P 192.168.1.0/24, 1 successors, FD is 3072
via 172.19.1.2 (3072/2816), GigabitEthernet1/0/1
via 172.19.1.10 (28416/2816), GigabitEthernet1/0/3
P 192.168.201.0/24, 1 successors, FD is 3072
via 172.19.1.2 (3072/2816), GigabitEthernet1/0/1
via 172.19.1.10 (28416/2816), GigabitEthernet1/0/3
P 0.0.0.0/0, 1 successors, FD is 2816
via Summary (2816/0), Null0
P 172.19.1.0/29, 1 successors, FD is 2816
via Connected, GigabitEthernet1/0/1

sh ip route output Router1:

Router_Werk1#sh ip route
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
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
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
+ - replicated route, % - next hop override

Gateway of last resort is 172.19.2.2 to network 0.0.0.0

S* 0.0.0.0/0 [1/0] via 172.19.2.2
172.19.0.0/16 is variably subnetted, 6 subnets, 3 masks
C 172.19.1.0/29 is directly connected, GigabitEthernet1/0/1
L 172.19.1.1/32 is directly connected, GigabitEthernet1/0/1
C 172.19.1.8/29 is directly connected, GigabitEthernet1/0/3
L 172.19.1.9/32 is directly connected, GigabitEthernet1/0/3
C 172.19.2.0/30 is directly connected, Vlan300
L 172.19.2.1/32 is directly connected, Vlan300
192.168.0.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.0.0/24 is directly connected, Vlan1
L 192.168.0.10/32 is directly connected, Vlan1
D EX 192.168.1.0/24 [170/3072] via 172.19.1.2, 01:37:58, GigabitEthernet1/0/1
D EX 192.168.3.0/24 [170/3072] via 172.19.1.2, 01:37:58, GigabitEthernet1/0/1
192.168.5.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.5.0/24 is directly connected, Vlan5
L 192.168.5.10/32 is directly connected, Vlan5
192.168.123.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.123.0/24 is directly connected, Vlan123
L 192.168.123.1/32 is directly connected, Vlan123
192.168.200.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.200.0/24 is directly connected, Vlan200
L 192.168.200.1/32 is directly connected, Vlan200
D EX 192.168.201.0/24
[170/3072] via 172.19.1.2, 01:37:58, GigabitEthernet1/0/1

Any Ideas or help is much appreciated!

Cheers

Thorsten

Seb Rupik · ‎09-07-2015

Hi Thorsten,

This is expected behaviour.

The EIGRP topology table will show all learned routes, but only sucessor routes will be installed in the routing table.

Hence you only see the EIGRP next-hop router 172.192.1.2 in your routing table.

Note that the second route via 172.19.1.10 is being held as a feasible sucessor, as its RD of 2816 is lower that the successor routes FD of 3072. Once the route to 172.19.1.2 is detected as down the feasible sucessor route is installed in the routing table.

If you want both routes to be installed in the routing table despite unequal metrics, use something called EIGRP variance. The sum of the sucessor FD multiplied by the variance must be greater or equal to the calculated metric of the other route. So...

28416 / 3072 = 9.25 Round this up to 10, this will be the variance required...

3072 * 10 = 30720

30720 > 28416 The route advertised by 172.19.1.10 now has an RD which is lower than the sucessor routes FD* variance product, so both route will appear in the routing table.

cheers,

Seb.

Richard Burts · ‎09-07-2015

Thorsten

As Seb notes the route advertisements have different metrics and so only the one with the best metric shows up in the routing table. It appears that the difference in metrics is due to the configuration of delay 10 for vlan 666. If you remove the delay command then I would expect the metrics to be equal and for both routes to show up in the routing table.

HTH

Rick

HTH

Rick