topic Re: EIGRP delete lower metric route and install higher metric route in Routing and SD-WAN

EIGRP delete lower metric route and install higher metric route

chong.eric — Sat, 30 Jul 2022 05:00:57 GMT

Hi ,

Router is learning route 192.168.99.0/24 from 10.160.40.138 and 10.160.40.153. Router installed 192.168.99.0/24 which learned from 10.160.40.153 even metric from 10.160.40.138 is lower.

Can anyone able to explain why?

*Mar 1 01:04:44.875: RT: add 192.168.99.0/24 via 10.160.40.138, eigrp metric [170/28416]
*Mar 1 01:04:44.875: RT: NET-RED 192.168.99.0/24
*Mar 1 01:04:44.875: DUAL: RT installed 192.168.99.0/24 via 10.160.40.138
*Mar 1 01:04:44.875: DUAL: Send update about 192.168.99.0/24. Reason: metric chg
*Mar 1 01:04:44.875: DUAL: Send update about 192.168.99.0/24. Reason: new if
*Mar 1 01:04:44.963: DUAL: rcvquery: 192.168.99.0/24 via 10.160.40.138 metric 4294967295/4294967295, RD is 28416
*Mar 1 01:04:44.963: DUAL: Find FS for dest 192.168.99.0/24. FD is 28416, RD is 28416
*Mar 1 01:04:44.963: DUAL: 10.160.40.138 metric 4294967295/4294967295
*Mar 1 01:04:44.963: DUAL: 10.160.40.153 metric 30720/28160 found Dmin is 30720
*Mar 1 01:04:44.967: DUAL: send REPLY(r1/n1) about 192.168.99.0/24 to 10.160.40.138
*Mar 1 01:04:44.967: RT: delete route to 192.168.99.0 via 10.160.40.138, eigrp metric [170/28416]
*Mar 1 01:04:44.967: RT: SET_LAST_RDB for 192.168.99.0/24
OLD rdb: via 10.160.40.138, FastEthernet0/0
*Mar 1 01:04:44.967: RT: no routes to 192.168.99.0
*Mar 1 01:04:44.967: RT: NET-RED 192.168.99.0/24
*Mar 1 01:04:44.967: RT: delete network route to 192.168.99.0
*Mar 1 01:04:44.967: RT: NET-RED 192.168.99.0/24
*Mar 1 01:04:44.967: RT: SET_LAST_RDB for 192.168.99.0/24
NEW rdb: via 10.160.40.153
*Mar 1 01:04:44.967: RT: add 192.168.99.0/24 via 10.160.40.153, eigrp metric [170/30720]
*Mar 1 01:04:44.967: RT: NET-RED 192.168.99.0/24
*Mar 1 01:04:44.967: DUAL: RT installed 192.168.99.0/24 via 10.160.40.153
*Mar 1 01:04:44.967: DUAL: Send update about 192.168.99.0/24. Reason: metric chg
*Mar 1 01:04:44.967: DUAL: Send update about 192.168.99.0/24. Reason: new if
*Mar 1 01:04:45.003: DUAL: Removing dest 192.168.99.0/24, nexthop 10.160.40.138, infosource 10.160.40.138

Re: EIGRP delete lower metric route and install higher metric route

Peter Paluch — Sat, 30 Jul 2022 08:18:42 GMT

Hello Eric,

Looking through your debug output, very soon after your router received the update about 192.168.99.0/24 from 10.160.40.138, the same 10.160.40.138 sent out a Query indicating an infinite distance. That forced your router to move away from using 10.160.40.138 as the next hop and picking 10.160.40.153 instead. In fact, your router is not picking a worse route - it is picking the best route that is available at the time because the route through 10.160.40.138 is reported as unreachable.

The real question is: Why did 10.160.40.138 send out the Query indicating the infinite distance?

Can you share a diagram of the topology, and can you describe what experiment are you doing in it and how? (For example, tweaking interface metrics, or shutting them down competely... simply describe whether you are doing any action in the network during the time when this anomaly occurs.)

Thank you!

Best regards,
Peter

Re: EIGRP delete lower metric route and install higher metric route

MHM Cisco World — Sat, 30 Jul 2022 11:06:57 GMT

can I see Network and your config, I think when the EIGRP is UP there is Routing Loop.

Re: EIGRP delete lower metric route and install higher metric route

Peter Paluch — Sat, 30 Jul 2022 11:32:19 GMT

Hello,

Very respectfully, I don't think that a routing loop is occurring - first of all, with EIGRP, a routing loop is not even possible. Second, the debugs show an event that in itself is clear - the next hop itself losing the path and querying for an alternate path. The real puzzle here is what made the next hop lose the path in the first place.

It will be interesting to see the configs and the topology diagrams that we have both requested 🙂

Best regards,
Peter

Re: EIGRP delete lower metric route and install higher metric route

MHM Cisco World — Sat, 30 Jul 2022 11:34:02 GMT

Reason: metric chg
Reason: new if

this make me think it routing Loop. and EIGRP design to detect and prevent it so the prefix next-hop is change.

Re: EIGRP delete lower metric route and install higher metric route

Peter Paluch — Sat, 30 Jul 2022 12:08:15 GMT

Hello,

Eric did not include the full debug output but I suspect that in the earlier lines that are not included, there would be information about the router learning a new better path about the 192.168.99.0/24, and therefore sending out an update. The two lines you have pointed out are not specifically related to routing loop prevention:

The "metric chg" indicates only that the metric of the 192.168.99.0/24 changed with respect to what it was previously.
The "new if" indicates only that the router changed the next hop interface for the network.

This sequence of events was caused by 10.160.40.138 first advertising the 192.168.99.0/24 network and then surprisingly querying for it using the infinite distance, as if it lost the path completely. We need to understand what happened on 10.160.40.138.

Best regards,
Peter

Re: EIGRP delete lower metric route and install higher metric route

MHM Cisco World — Sat, 30 Jul 2022 12:10:16 GMT

that why I ask him network and Config

Re: EIGRP delete lower metric route and install higher metric route

chong.eric — Sat, 30 Jul 2022 14:10:50 GMT

Hi Peter,

Here is network diagram

Network 192.168.99.0/24 is behind the R5. R3 has a static route for 192.168.99.0/24 point to R5 . R3 is redistributing the static route into EIGRP AS 100.

R3 config

router eigrp 100
redistribute static metric 100 100 255 1 1500 route-map STATIC-TO-EIGRP
network 10.43.0.0 0.0.255.255
no auto-summary
route-map STATIC-TO-EIGRP permit 10
match ip address prefix-list STATIC-TO-EIGRP
ip prefix-list STATIC-TO-EIGRP seq 10 permit 192.168.99.0/24

R7 then redistribute 192.168.99.0/24 from EIGRP into BGP (AS 65000).

R8 advertise route 192.168.99.0/24 to R6 via eBGP.

R6 redistribute 192.168.99.0/24 from BGP (AS 65001) into EIGRP AS 200.

R6 Config

router eigrp 200
redistribute bgp 65001 metric 100000 10 255 1 1500
network 10.160.40.152 0.0.0.3
no auto-summary

R4 then advertise 192.168.99.0/24 to its neighbour R9

R4 Config

router eigrp 200
network 10.160.40.136 0.0.0.3
network 10.160.40.152 0.0.0.3
no auto-summary

At the same time, R10 learned 192.168.99.0/24 from R3 and advertised to its neighbour R9.

R10 Config

router eigrp 100
network 10.43.13.0 0.0.0.255
network 10.160.40.144 0.0.0.3
no auto-summary

R9 is redistributing 192.168.99.0/24 from EIGRP AS 100 into EIGRP AS 200

R9 Config

router eigrp 100
redistribute eigrp 200 metric 1000000 10000 255 1 1500 route-map EIGRP200->EIGRP100
network 10.160.40.144 0.0.0.3
no auto-summary

router eigrp 200
redistribute eigrp 100 metric 1000000 10000 255 1 1500 route-map EIGRP100->EIGRP200
network 10.160.40.136 0.0.0.3
no auto-summary

route-map EIGRP100->EIGRP200 permit 30
match ip address prefix-list EIGRP100->EIGRP200
set metric 1000000 1 1 1 1
set tag 43

ip prefix-list EIGRP100->EIGRP200 seq 5 permit 192.168.99.0/24

For R4 point of view, the primary path to 192.168.99.0/24 is via R9. Backup path is via R6.

Re: EIGRP delete lower metric route and install higher metric route

MHM Cisco World — Sat, 30 Jul 2022 14:57:23 GMT

I think you hit the classic mutual redistribute between two routing.
make some routing Loop.
so check from your side,
later today I will do your lab and solve it.

ISSUE in redistribute BGP into EIGRP and EIGRP into BGP.

Re: EIGRP delete lower metric route and install higher metric route

Jon Marshall — Sat, 30 Jul 2022 14:42:25 GMT

As per Peter's request can you describe what you were doing at the time in your lab when you saw this in the debug ie. did you change metrics, did you shut an interface etc.

Also on R9 can you include the other route map details ie. the one for redistributing EIGRP AS 200 routes into AS 100.

Jon

Re: EIGRP delete lower metric route and install higher metric route

MHM Cisco World — Sat, 30 Jul 2022 18:21:39 GMT

I need Config for each router

Re: EIGRP delete lower metric route and install higher metric route

David Ruess — Sat, 30 Jul 2022 20:08:59 GMT

Hello,

As Peter mentioned R9 advertised an infinite metric so it chose the other route. And it looks like it had the route and then the metrics changed as Jon mentioned. I will say this:

1. You don't need to provide any metrics when redistributing between EIGRP AS's as EIGRP already knows how to propagate its metrics between other EIGRP routing domains.

2. You have metrics on top of metrics and its a lot to follow the logic. You have the metrics in the redistribute command as well as referencing a route map that also sets metrics.

I would remove all the route maps and metric redistribution commands from EIGRP related neighbors. You can still redistribute, but no need for the extra metrics yet.

The reason for that is you can see how EIGRP functions without any traffic manipulation and you will see EIGRP for what it does. Then you can add commands 1 by 1 to determine at which point it chooses the other route. Just a suggestions.

Hope that helps

-David

Re: EIGRP delete lower metric route and install higher metric route

MHM Cisco World — Sat, 30 Jul 2022 20:06:55 GMT

solution is config route-map add metric to prefix

Re: EIGRP delete lower metric route and install higher metric route

Peter Paluch — Sun, 31 Jul 2022 08:47:24 GMT

Hi Eric, MHM,

Speaking at least for myself, I do not yet have an explanation of what exactly is happening. I would like to dig deeper before suggesting any particular step (such as adding additional metric to a prefix).

@chong.eric , if you're still interested in finding out why this is happening, can you do me a favor please? When your network stabilizes into the situation that R4 picks R6 as the primary route even though it should be using R9, can you please provide me with the output of these commands from each router in your diagram?

show ip route 192.168.99.0 255.255.255.0 show ip eigrp topology 192.168.99.0/24 ! only on EIGRP speakers

Thank you!

Best regards,
Peter

Re: EIGRP delete lower metric route and install higher metric route

chong.eric — Sun, 31 Jul 2022 13:47:16 GMT

Hi Peter,

Here is the output

R4#sh ip route 192.168.99.0 255.255.255.0
Routing entry for 192.168.99.0/24
Known via "eigrp 200", distance 170, metric 30720
Tag 13879, type external
Redistributing via eigrp 200
Last update from 10.160.40.153 on FastEthernet1/1, 00:29:18 ago
Routing Descriptor Blocks:
* 10.160.40.153, from 10.160.40.153, 00:29:18 ago, via FastEthernet1/1
Route metric is 30720, traffic share count is 1
Total delay is 200 microseconds, minimum bandwidth is 100000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
Route tag 13879

R4#show ip eigrp topology 192.168.99.0/24
IP-EIGRP (AS 200): Topology entry for 192.168.99.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 30720
Routing Descriptor Blocks:
10.160.40.153 (FastEthernet1/1), from 10.160.40.153, Send flag is 0x0
Composite metric is (30720/28160), Route is External
Vector metric:
Minimum bandwidth is 100000 Kbit
Total delay is 200 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
External data:
Originating router is 57.213.9.110
AS number of route is 65001
External protocol is BGP, external metric is 0
Administrator tag is 65003 (0x00003637)
10.160.40.138 (FastEthernet0/0), from 10.160.40.138, Send flag is 0x0
Composite metric is (2588160/2562560), Route is External
Vector metric:
Minimum bandwidth is 100000 Kbit
Total delay is 100100 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
External data:
Originating router is 10.160.40.138
AS number of route is 100
External protocol is EIGRP, external metric is 25630720
Administrator tag is 43 (0x0000002B)

R9#show ip eigrp topology 192.168.99.0/24
IP-EIGRP (AS 200): Topology entry for 192.168.99.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 33280
Routing Descriptor Blocks:
10.160.40.137 (FastEthernet0/0), from 10.160.40.137, Send flag is 0x0
Composite metric is (33280/30720), Route is External
Vector metric:
Minimum bandwidth is 100000 Kbit
Total delay is 300 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
External data:
Originating router is 57.213.9.110
AS number of route is 65001
External protocol is BGP, external metric is 0
Administrator tag is 65003 (0x00003637)
IP-EIGRP (AS 100): Topology entry for 192.168.99.0/24
State is Passive, Query origin flag is 1, 0 Successor(s), FD is 4294967295
Routing Descriptor Blocks:
10.160.40.146 (FastEthernet0/1), from 10.160.40.146, Send flag is 0x0
Composite metric is (25630720/25628160), Route is External
Vector metric:
Minimum bandwidth is 100 Kbit
Total delay is 1200 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
External data:
Originating router is 10.43.13.233
AS number of route is 0
External protocol is Static, external metric is 0
Administrator tag is 0 (0x00000000)

R10#show ip route 192.168.99.0 255.255.255.0
Routing entry for 192.168.99.0/24
Known via "eigrp 100", distance 170, metric 25628160, type external
Redistributing via eigrp 100
Last update from 10.43.13.233 on FastEthernet0/0, 00:28:57 ago
Routing Descriptor Blocks:
* 10.43.13.233, from 10.43.13.233, 00:28:57 ago, via FastEthernet0/0
Route metric is 25628160, traffic share count is 1
Total delay is 1100 microseconds, minimum bandwidth is 100 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1

R10#show ip eigrp topology 192.168.99.0/24
IP-EIGRP (AS 100): Topology entry for 192.168.99.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 25628160
Routing Descriptor Blocks:
10.43.13.233 (FastEthernet0/0), from 10.43.13.233, Send flag is 0x0
Composite metric is (25628160/25625600), Route is External
Vector metric:
Minimum bandwidth is 100 Kbit
Total delay is 1100 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
External data:
Originating router is 10.43.13.233
AS number of route is 0
External protocol is Static, external metric is 0
Administrator tag is 0 (0x00000000)

R3#show ip route 192.168.99.0 255.255.255.0
Routing entry for 192.168.99.0/24
Known via "static", distance 1, metric 0
Redistributing via eigrp 100
Advertised by eigrp 100 metric 100 100 255 1 1500 route-map STATIC-TO-EIGRP
Routing Descriptor Blocks:
* 10.43.13.249
Route metric is 0, traffic share count is 1

R3#show ip eigrp topology 192.168.99.0/24
IP-EIGRP (AS 100): Topology entry for 192.168.99.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 25625600
Routing Descriptor Blocks:
10.43.13.249, from Rstatic, Send flag is 0x0
Composite metric is (25625600/0), Route is External
Vector metric:
Minimum bandwidth is 100 Kbit
Total delay is 1000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 0
External data:
Originating router is 10.43.13.233 (this system)
AS number of route is 0
External protocol is Static, external metric is 0
Administrator tag is 0 (0x00000000)

R7#show ip route 192.168.99.0 255.255.255.0
Routing entry for 192.168.99.0/24
Known via "eigrp 100", distance 170, metric 25628160, type external
Redistributing via eigrp 100, bgp 65000
Advertised by bgp 65000
Last update from 10.43.13.14 on FastEthernet0/1, 01:13:45 ago
Routing Descriptor Blocks:
* 10.43.13.14, from 10.43.13.14, 01:13:45 ago, via FastEthernet0/1
Route metric is 25628160, traffic share count is 1
Total delay is 1100 microseconds, minimum bandwidth is 100 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1

R7#show ip eigrp topology 192.168.99.0/24
IP-EIGRP (AS 100): Topology entry for 192.168.99.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 25628160
Routing Descriptor Blocks:
10.43.13.14 (FastEthernet0/1), from 10.43.13.14, Send flag is 0x0
Composite metric is (25628160/25625600), Route is External
Vector metric:
Minimum bandwidth is 100 Kbit
Total delay is 1100 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
External data:
Originating router is 10.43.13.233
AS number of route is 0
External protocol is Static, external metric is 0
Administrator tag is 0 (0x00000000)

R6#show ip eigrp topology 192.168.99.0/24
IP-EIGRP (AS 200): Topology entry for 192.168.99.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 28160
Routing Descriptor Blocks:
57.213.9.109, from Redistributed, Send flag is 0x0
Composite metric is (28160/0), Route is External
Vector metric:
Minimum bandwidth is 100000 Kbit
Total delay is 100 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 0
External data:
Originating router is 57.213.9.110 (this system)
AS number of route is 65001
External protocol is BGP, external metric is 0
Administrator tag is 65003 (0x00003637)
10.160.40.154 (FastEthernet0/1), from 10.160.40.154, Send flag is 0x0
Composite metric is (2590720/2588160), Route is External
Vector metric:
Minimum bandwidth is 100000 Kbit
Total delay is 100200 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
External data:
Originating router is 10.160.40.138
AS number of route is 100
External protocol is EIGRP, external metric is 25630720
Administrator tag is 43 (0x0000002B)

R8#sh ip route 192.168.99.0 255.255.255.0
Routing entry for 192.168.99.0/24
Known via "bgp 65003", distance 20, metric 25628160
Tag 65000, type external
Last update from 57.213.223.194 01:13:08 ago
Routing Descriptor Blocks:
* 57.213.223.194, from 57.213.223.194, 01:13:08 ago
Route metric is 25628160, traffic share count is 1
AS Hops 1
Route tag 65000

Re: EIGRP delete lower metric route and install higher metric route

MHM Cisco World — Sun, 31 Jul 2022 14:57:18 GMT

R6#show ip eigrp topology 192.168.99.0/24
Hop count is 0 <<<- Hop count Zero !!! issue in BGP into EIGRP
External protocol is BGP, external metric is 0 <<<- as I note in my photo

ANYWAY GOOD LUCK in Your Issue.

Re: EIGRP delete lower metric route and install higher metric route

Peter Paluch — Sun, 31 Jul 2022 19:33:32 GMT

Hello Eric, MHM,

@MHM Cisco World - very respectfully, I don't believe that we are yet looking at the root cause. Hop count is a consequence, not the cause - to EIGRP, hop count is not and has never been a parameter influencing path selections; it does not influence the resulting metric and can never be configured to influence it. You can't even manipulate it manually. On R6, hop count 0 merely says that the network has been locally injected into EIGRP's database as opposed to learning it from an EIGRP neighbor; in this case, it is injected by redistributing. And the external metric (MED) in BGP has no influence to EIGRP anyway because EIGRP cannot redistribute a scalar metric from another protocol directly - rather, it always needs to have it broken down to the bandwidth, delay, reliability, load and MTU components, no exception. Arguing whether situation would be different if the metric in BGP was raised is not relevant because with dissimilar routing protocols, we would compare administrative distances - and eBGP's 20 will always beat EIGRP's 90 for internal and 170 for external routes. BGP would always win. There is something else going on here.

@chong.eric , I think am starting to see what is going on but I have to ask you one thing before I explain what I think is happening: Did you toy around with the split horizon setting in EIGRP? Did you by any chance disable the split horizon on R4 f0/0?

Best regards,
Peter

Re: EIGRP delete lower metric route and install higher metric route

MHM Cisco World — Sun, 31 Jul 2022 19:45:22 GMT

@Peter Paluch
I know that BGP win, BUT the issue is
R4 learn two path
one via R6 other R9
the EIGRP need to calculate the Path to the Origin router.
The Issue is
path through R9 is complete EIGRP and there is redistribute (between process) and hence the metric value can calculate.
path through R6 is BGP+EIGRP, and hence the metric must be EIGRP + BGP "Metric"<<<
here the issue the BGP not keep the metric when redistribute the prefix from EIGRP.
this make R4 have path short via BGP.
we need to make
1-redistribute the BGP into EIGRP with value Large than the Value of other second path
2-config the R4-R6 delay or BW to make this path less prefer than path through R9.

Re: EIGRP delete lower metric route and install higher metric route

Peter Paluch — Sun, 31 Jul 2022 21:03:56 GMT

Hi MHM,

The situation is more complex.

R4 is placed in a position where it is looking at the same destination network through two different advertising routers, and those two advertising routers - R6 and R9 - also happen to be redistribution points for that network so they have a complete power over the metric of the network and can set it to an arbitrary value. The way the redistribution is set up, the path through R9 should be with a total cost of 28416, the path through R6 should be with a total cost of 30720. The redistribution configuration never changes, so neither R6 nor R9 should change their mind about the cost of the redistributed route. Yet, apparently, exactly that happens on R9.

Now, the path through R9 is a combination of two EIGRP processes, and in the process 200, the metric of 192.168.99.0/24 is significantly lower (thanks to an overriding metric configured in the redistribution) than in the process 100 which is the original proces that carries the information about this network. On R9, in process 100, the composite metric of the route to 192.168.99.0/24 through R10 is 25630720 while in process 200, the composite metric through R4 is merely 33280 (that is the result of R9 deciding to use the path through R4 instead of staying with the path through R10). You could say - sure, the path is shorter! - but that would be a wrong understanding.

If R9 first redistributed the route from process 100 to process 200, the network in process 100 was first in its routing table with the high metric of 25630720, and it just got reinjected into process 200 with an overriden metric of 2816 = (10+1)*256. However, this reinjected route in process 200 would itself never override the original route in RIB because that is the basic rule of redistribution: Even if the redistributed route ends in a local routing process that has a lower AD and/or a lower metric, the original route must stay in the routing table untouched because it is the source for the redistribution.

The output from R9, however, shows something quite different:

R9# show ip eigrp topology 192.168.99.0/24
IP-EIGRP (AS 200😞 Topology entry for 192.168.99.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 33280
Routing Descriptor Blocks:
10.160.40.137 (FastEthernet0/0), from 10.160.40.137, Send flag is 0x0
 Composite metric is (33280/30720), Route is External
 Vector metric:
  Minimum bandwidth is 100000 Kbit
  Total delay is 300 microseconds
  Reliability is 255/255
  Load is 1/255
  Minimum MTU is 1500
  Hop count is 2
 External data:
  Originating router is 57.213.9.110
  AS number of route is 65001
  External protocol is BGP, external metric is 0
  Administrator tag is 65003 (0x00003637)

IP-EIGRP (AS 100😞 Topology entry for 192.168.99.0/24
State is Passive, Query origin flag is 1, 0 Successor(s), FD is 4294967295
Routing Descriptor Blocks:
10.160.40.146 (FastEthernet0/1), from 10.160.40.146, Send flag is 0x0
 Composite metric is (25630720/25628160), Route is External
 Vector metric:
  Minimum bandwidth is 100 Kbit
  Total delay is 1200 microseconds
  Reliability is 255/255
  Load is 1/255
  Minimum MTU is 1500
  Hop count is 2
 External data:
  Originating router is 10.43.13.233
  AS number of route is 0
  External protocol is Static, external metric is 0
  Administrator tag is 0 (0x00000000)

Note that in process 200, there is no mention of the locally redistributed route, only the route learned back from R4. In process 100, FD is set to infinity due to the fact that the process 100 was unable to install its own variant of the route into the routing table (a so-called zero-successor route) and so is marked as unusable even if learned. In fact, exactly because the process 100 does not own the route to 192.168.99.0/24 in the RIB on R9 anymore, there is no redistribution from process 100 to process 200 happening, and so there is no locally redistributed route in the process 200.

But clearly, at the beginning, the process 100 on R9 did have the route to 192.168.99.0/24 installed in the RIB and redistributed into process 200. So what happened that this has changed? The redistribution alone would not have caused process 200 overturn process 100.

A possible answer is that R4 advertised the route to 192.168.99.0/24 to R9. But let's break this up into details: Just what could have R4 advertised? The relevant debugs are:

*Mar 1 01:04:44.875: RT: add 192.168.99.0/24 via 10.160.40.138, eigrp metric [170/28416]
*Mar 1 01:04:44.875: RT: NET-RED 192.168.99.0/24
*Mar 1 01:04:44.875: DUAL: RT installed 192.168.99.0/24 via 10.160.40.138
*Mar 1 01:04:44.875: DUAL: Send update about 192.168.99.0/24. Reason: metric chg
*Mar 1 01:04:44.875: DUAL: Send update about 192.168.99.0/24. Reason: new if

Here, R4 picked the route through R9 and sent updates about it. But with EIGRP, we have split horizon with poisoned reverse. Unless it was deactivated on R4 - and that's why I asked Eric if he changed its setting - R9 would have received a poisoned reverse update from R4 which it should not have cared about at all because it wasn't using the path through R4 anyway.

So either R9 unexpectedly reacted even to the poisoned reverse from R4, or the split horizon on R4 was in fact disabled and then R4 advertised the network with the metric of 28416 to R9. R9 would then realize that it had a better path to 192.168.99.0/24 with a total metric of 33280 instead of 25630720... and that would explain some thing.

But unless we have this nailed down and clarified beyond doubt, we do not really know what is happening. There are quite a few things that should not have happened here and that don't make sense, at least not intuitively. Hence, I am not accepting an arbitrary manipulation with metrics as a solution - without understanding exactly what is going on and why the manipulation would solve it, it is a blind shot which, if it works, it works only by chance and coincidence.

Best regards,
Peter

Re: EIGRP delete lower metric route and install higher metric route

chong.eric — Mon, 01 Aug 2022 07:06:19 GMT

Hi Peter,

No, I didn't change any split horizon setting in EIGRP

R4#sh ip int fa0/0
FastEthernet0/0 is up, line protocol is up
Internet address is 10.160.40.137/30
Broadcast address is 255.255.255.255
Address determined by non-volatile memory
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Multicast reserved groups joined: 224.0.0.10
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Local Proxy ARP is disabled
Security level is default
Split horizon is enabled
ICMP redirects are always sent
ICMP unreachables are always sent
ICMP mask replies are never sent
IP fast switching is enabled
IP fast switching on the same interface is disabled
IP Flow switching is disabled
IP CEF switching is enabled
IP CEF Fast switching turbo vector
IP multicast fast switching is enabled
IP multicast distributed fast switching is disabled
IP route-cache flags are Fast, CEF
Router Discovery is disabled
IP output packet accounting is disabled
IP access violation accounting is disabled
TCP/IP header compression is disabled
RTP/IP header compression is disabled
Policy routing is disabled
Network address translation is disabled
BGP Policy Mapping is disabled
WCCP Redirect outbound is disabled
WCCP Redirect inbound is disabled
WCCP Redirect exclude is disabled