Why does EIGRP send routes with a metric of 4294967295 when i change the delay slightly?

[nsti_0682]

I'm currently labbing EIGRP in GNS3 and have hit an issue that i cant seem to figure out...While trying to get used to FS's and looking at the metrics and the FC's, i've been changing the delay on one of my EIGRP neighbors interfaces. Everytime i change the delay to, say 11, the router sends it's downstream neighbor an updated route with the metric of 4294967295 which is the 'inaccessible' route metric....any ideas?

[Akash Agrawal]

Hi,

Below extra information will help everyone to understand issue better

- A brief topology

- show ip eigrp topology output from router you are making change before delay change and after delay change

- show ip eigrp topology output from router, seeing metric 4294967295

Regards,

Akash

[nsti_0682]

Below is the topology...

R3 - Before delay 50 'Sh ip ei top' & 'Sh ip ei top all'

R3(config-if)#do sh ip ei top
IP-EIGRP Topology Table for AS(1)/ID(192.168.254.3)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 1.1.1.0/24, 1 successors, FD is 156160
        via 192.168.254.1 (156160/128256), FastEthernet0/0
P 172.16.253.0/24, 1 successors, FD is 30720
        via 172.16.254.4 (30720/28160), FastEthernet0/1
P 172.16.254.0/24, 1 successors, FD is 28160
        via Connected, FastEthernet0/1
P 111.111.111.0/24, 1 successors, FD is 30720
        via 192.168.254.1 (30720/28160), FastEthernet0/0
P 192.168.253.0/24, 1 successors, FD is 30720
        via 192.168.254.1 (30720/28160), FastEthernet0/0
P 192.168.254.0/24, 1 successors, FD is 28160
        via Connected, FastEthernet0/0
R3(config-if)#do sh ip ei top al
IP-EIGRP Topology Table for AS(1)/ID(192.168.254.3)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 1.1.1.0/24, 1 successors, FD is 156160, serno 29
        via 192.168.254.1 (156160/128256), FastEthernet0/0
P 172.16.253.0/24, 1 successors, FD is 30720, serno 3
        via 172.16.254.4 (30720/28160), FastEthernet0/1
        via 192.168.254.1 (33280/30720), FastEthernet0/0
P 172.16.254.0/24, 1 successors, FD is 28160, serno 1
        via Connected, FastEthernet0/1
P 111.111.111.0/24, 1 successors, FD is 30720, serno 30
        via 192.168.254.1 (30720/28160), FastEthernet0/0
P 192.168.253.0/24, 1 successors, FD is 30720, serno 31
        via 192.168.254.1 (30720/28160), FastEthernet0/0
        via 172.16.254.4 (33280/30720), FastEthernet0/1
P 192.168.254.0/24, 1 successors, FD is 28160, serno 28
        via Connected, FastEthernet0/0

R3 - After delay 50 'Sh ip ei top' & 'Sh ip ei top all'

R3(config-if)#do sh ip ei top
IP-EIGRP Topology Table for AS(1)/ID(192.168.254.3)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 1.1.1.0/24, 1 successors, FD is 161280
        via 172.16.254.4 (161280/158720), FastEthernet0/1
        via 192.168.254.1 (166400/128256), FastEthernet0/0
P 172.16.253.0/24, 1 successors, FD is 30720
        via 172.16.254.4 (30720/28160), FastEthernet0/1
P 172.16.254.0/24, 1 successors, FD is 28160
        via Connected, FastEthernet0/1
P 111.111.111.0/24, 1 successors, FD is 35840
        via 172.16.254.4 (35840/33280), FastEthernet0/1
        via 192.168.254.1 (40960/28160), FastEthernet0/0
P 192.168.253.0/24, 1 successors, FD is 33280
        via 172.16.254.4 (33280/30720), FastEthernet0/1
        via 192.168.254.1 (40960/28160), FastEthernet0/0
P 192.168.254.0/24, 1 successors, FD is 38400
        via Connected, FastEthernet0/0
        via 172.16.254.4 (35840/33280), FastEthernet0/1

R3(config-if)#do sh ip ei top all
IP-EIGRP Topology Table for AS(1)/ID(192.168.254.3)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 1.1.1.0/24, 1 successors, FD is 161280, serno 40
        via 172.16.254.4 (161280/158720), FastEthernet0/1
        via 192.168.254.1 (166400/128256), FastEthernet0/0
P 172.16.253.0/24, 1 successors, FD is 30720, serno 3
        via 172.16.254.4 (30720/28160), FastEthernet0/1
        via 192.168.254.1 (43520/30720), FastEthernet0/0
P 172.16.254.0/24, 1 successors, FD is 28160, serno 1
        via Connected, FastEthernet0/1
P 111.111.111.0/24, 1 successors, FD is 35840, serno 41
        via 172.16.254.4 (35840/33280), FastEthernet0/1
        via 192.168.254.1 (40960/28160), FastEthernet0/0
P 192.168.253.0/24, 1 successors, FD is 33280, serno 39
        via 172.16.254.4 (33280/30720), FastEthernet0/1
        via 192.168.254.1 (40960/28160), FastEthernet0/0
P 192.168.254.0/24, 1 successors, FD is 38400, serno 32
        via Connected, FastEthernet0/0
        via 172.16.254.4 (35840/33280), FastEthernet0/1

R4 - Before delay 50 'Sh ip ei top' & 'Sh ip ei top all'

R4#sh ip ei top
IP-EIGRP Topology Table for AS(1)/ID(172.16.254.4)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 1.1.1.0/24, 2 successors, FD is 158720
        via 172.16.254.3 (158720/156160), FastEthernet0/0
        via 172.16.253.2 (158720/156160), FastEthernet0/1
P 172.16.253.0/24, 1 successors, FD is 28160
        via Connected, FastEthernet0/1
P 172.16.254.0/24, 1 successors, FD is 28160
        via Connected, FastEthernet0/0
P 111.111.111.0/24, 2 successors, FD is 33280
        via 172.16.254.3 (33280/30720), FastEthernet0/0
        via 172.16.253.2 (33280/30720), FastEthernet0/1
P 192.168.253.0/24, 1 successors, FD is 30720
        via 172.16.253.2 (30720/28160), FastEthernet0/1
P 192.168.254.0/24, 1 successors, FD is 30720
        via 172.16.254.3 (30720/28160), FastEthernet0/0

R4#sh ip ei top all
IP-EIGRP Topology Table for AS(1)/ID(172.16.254.4)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 1.1.1.0/24, 2 successors, FD is 158720, serno 38
        via 172.16.254.3 (158720/156160), FastEthernet0/0
        via 172.16.253.2 (158720/156160), FastEthernet0/1
P 172.16.253.0/24, 1 successors, FD is 28160, serno 1
        via Connected, FastEthernet0/1
P 172.16.254.0/24, 1 successors, FD is 28160, serno 2
        via Connected, FastEthernet0/0
P 111.111.111.0/24, 2 successors, FD is 33280, serno 39
        via 172.16.254.3 (33280/30720), FastEthernet0/0
        via 172.16.253.2 (33280/30720), FastEthernet0/1
P 192.168.253.0/24, 1 successors, FD is 30720, serno 7
        via 172.16.253.2 (30720/28160), FastEthernet0/1
        via 172.16.254.3 (33280/30720), FastEthernet0/0
P 192.168.254.0/24, 1 successors, FD is 30720, serno 37
        via 172.16.254.3 (30720/28160), FastEthernet0/0
        via 172.16.253.2 (33280/30720), FastEthernet0/1

R4 - After delay 50 'Sh ip ei top' & 'Sh ip ei top all'

R4#sh ip ei top
IP-EIGRP Topology Table for AS(1)/ID(172.16.254.4)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 1.1.1.0/24, 1 successors, FD is 158720
        via 172.16.253.2 (158720/156160), FastEthernet0/1
P 172.16.253.0/24, 1 successors, FD is 28160
        via Connected, FastEthernet0/1
P 172.16.254.0/24, 1 successors, FD is 28160
        via Connected, FastEthernet0/0
P 111.111.111.0/24, 1 successors, FD is 33280
        via 172.16.253.2 (33280/30720), FastEthernet0/1
P 192.168.253.0/24, 1 successors, FD is 30720
        via 172.16.253.2 (30720/28160), FastEthernet0/1
P 192.168.254.0/24, 1 successors, FD is 33280
        via 172.16.253.2 (33280/30720), FastEthernet0/1

R4#sh ip ei top all
IP-EIGRP Topology Table for AS(1)/ID(172.16.254.4)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 1.1.1.0/24, 1 successors, FD is 158720, serno 42
        via 172.16.253.2 (158720/156160), FastEthernet0/1
P 172.16.253.0/24, 1 successors, FD is 28160, serno 1
        via Connected, FastEthernet0/1
P 172.16.254.0/24, 1 successors, FD is 28160, serno 2
        via Connected, FastEthernet0/0
P 111.111.111.0/24, 1 successors, FD is 33280, serno 43
        via 172.16.253.2 (33280/30720), FastEthernet0/1
P 192.168.253.0/24, 1 successors, FD is 30720, serno 7
        via 172.16.253.2 (30720/28160), FastEthernet0/1
P 192.168.254.0/24, 1 successors, FD is 33280, serno 41
        via 172.16.253.2 (33280/30720), FastEthernet0/1
        via 172.16.254.3 (40960/38400), FastEthernet0/0

Output of 'deb eigrp fsm' on R4 after delay 50 change on R3

*Mar  1 00:31:06.607: DUAL: rcvupdate: 111.111.111.0/24 via 172.16.254.3 metric 4294967295/4294967295
*Mar  1 00:31:06.607: DUAL: Find FS for dest 111.111.111.0/24. FD is 33280, RD is 33280
*Mar  1 00:31:06.607: DUAL:     172.16.253.2 metric 33280/30720
*Mar  1 00:31:06.607: DUAL:     172.16.254.3 metric 4294967295/4294967295 found Dmin is 33280
*Mar  1 00:31:06.611: DUAL: Removing dest 111.111.111.0/24, nexthop 172.16.254.3
*Mar  1 00:31:06.611: DUAL: RT installed 111.111.111.0/24 via 172.16.253.2

[Akash Agrawal]

Hi,

Thanks for the detail output. As per output link metric with normal delay is 2560 and metric to R1 from R2 and R3 is 30720 and from R4 it is 33280 (30720 + 2560). When you change delay to 50 on Fa0/0 of R3, metric gets increased and R3 starts preferring R4 to reach R1 with metric 35840 (30720+2560+2560). 

Now when R3 advertises route with metric 35840 to R4, FD is higher than its total distance (33280 via R2) so it can not be a feasible successor and can not be installed in the routing table.

-Akash

[nsti_0682]

Akash,

I understand why it's not an FS, but that route should still be in the EIGRP topology table and visible via the 'sh ip eigrp top all-links' show command regardless. The route is not in the EIGRP topology table due to the fact that R4 receives the route to/from R3 with a metric of 4294967295. Since 4294967295 is the inaccessible metric, EIGRP removes the routes from the table. My question is why is R4 receiving the route to R3 with a metric of 4294967295 to begin with?

[Akash Agrawal · Accepted solution]

Hi,

As per my understanding below could be explanation for this

After change in metric, R3 is preferring R4 to reach R6 and in distance vector protocol router advertises only preferred route. So it has to now advertise route with new metric of preferred route to all neighbors, but it can not advertise route on the link from where it has learned the route. So if it can not advertise the new route, it will have to withdraw the  previously advertised route. To withdraw that route it has to advertise route with maximum metric. 

-Akash

[Peter Paluch · Accepted solution]

Akash,

You are almost correct. What we see here is simply the Split Horizon with Poisoned Reverse in action. After the delay on R3 is changed, R3 needs to reevaluate the choice of next hops and advertise the route with the updated distance to all its neighbors. However, because R4 becomes the next hop itself, EIGRP on R3 follows the Split Horizon with Poisoned Reverse rule and advertises the route out the interface toward R4 with an infinite distance.

Best regards,
Peter

[nsti_0682]

Akash/Peter,

Thank you for setting me straight...What I was expecting to happen was that R3 would advertise it's FS route (via R1) to R4. I was confused because I didn't realize that EIGRP only advertises Successor routes and not FS routes as well. Knowing that now makes sense that R3 wouldn't advertise it's Successor route to 111.111.111.0/24 via R4, to R4 since that would indeed be the Split Horizon. Thanks again!