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Route not advertised due to EIGRP zero successor

Posted by ltlnetworker on December 4, 2013


This article’s topic really fits in this blog’s genre. It focuses on a case of classic routing protocol behaviour inspection.

I was looking at the subnets in the routing tables when I noticed that a network I picked up was absent in one of the devices. It is a VLAN routed on a pair of core switches (SW1,SW2). The switches advertise the VLANs by EIGRP and all other routers learn it as external EIGRP routes (due to redistribute connected). However, there’s a router R2 connected to SW2 by a L3 link that doesn’t have the network in the routing table.

Let’s see R2’s EIGRP config :

router eigrp 100
 redistribute ospf 1 metric 24000 1 255 1 1500 match internal external 1 external 2
 network 192.168.1.1 0.0.0.0
 network 192.168.1.22 0.0.0.0
 network 192.168.253.1 0.0.0.0
 distribute-list route-map OSPFtoEIGRP out GigabitEthernet0/1
 distribute-list route-map FROM-EIGRP in GigabitEthernet0/1
 distribute-list route-map TO-EIGRP out Tunnel0
 distance eigrp 90 91
 no auto-summary

OK, clearly that distribute-list must be blocking the incoming updates. But no. I looked into it and it is just a tag-check to prevent OSPF-EIGRP-OSPF routing loops.

Neither the routing table not the topology table contains the route:

R2#sh ip route 10.1.4.0
% Subnet not in table
R2#sh ip eigrp topology 10.1.4.0 255.255.255.0
% IP-EIGRP (AS 100): Route not in topology table
R2#sh ip eigrp topology all-links | i 10.1.4\.
R2#

What about the switches?

SW1#sh ip route 10.1.4.0
Routing entry for 10.1.4.0/24
Known via "connected", distance 0, metric 0 (connected, via interface)
Redistributing via eigrp 100
Advertised by eigrp 100
Routing Descriptor Blocks:
* directly connected, via Vlan4
Route metric is 0, traffic share count is 1

interface Vlan4
 ip address 10.1.4.252 255.255.255.0
 ip helper-address 10.2.0.131
 standby 4 ip 10.1.4.1
 standby 4 authentication 4secret

I have to check and compare the EIGRP configs:

SW1#sh run | s r ei
router eigrp 100
 network 172.25.5.32 0.0.0.7
 network 172.25.5.62 0.0.0.0
 network 192.168.75.0
 network 192.168.1.32 0.0.0.3
 network 192.168.1.36 0.0.0.3
 network 192.168.1.0
 network 192.168.2.0
 no auto-summary
 default-metric 1000000 1 255 20 1500
 redistribute connected
 !
 address-family ipv4 vrf GREEN
 autonomous-system 56
 network 172.25.5.17 0.0.0.0
 no auto-summary
 default-metric 1000000 10 255 1 1500
 distance eigrp 90 210
 redistribute static
 redistribute bgp 65001
 exit-address-family
!
 address-family ipv4 vrf BROWN
 autonomous-system 6
 network 172.25.5.24 0.0.0.7
 no auto-summary
 default-metric 1000000 10 255 1 1500
 redistribute static
 redistribute bgp 65001
 exit-address-family

interface Vlan4
 ip address 10.1.4.253 255.255.255.0
 ip helper-address 10.2.0.131
 standby 4 ip 10.1.4.1
 standby 4 authentication 4secret

SW2# sh run | s r ei
router eigrp 100
!
 address-family ipv4 vrf BROWN autonomous-system 6
 default-metric 1000000 10 255 1 1500
 redistribute bgp 65001
 redistribute static
 network 172.25.5.24 0.0.0.7
 exit-address-family
!
 address-family ipv4 vrf GREEN
 default-metric 1000000 10 255 1 1500
 redistribute bgp 65001
 redistribute static
 network 172.25.5.18 0.0.0.0
 distance eigrp 90 210
 autonomous-system 56
 exit-address-family
 network 172.25.5.32 0.0.0.7
 network 172.25.5.4 0.0.0.15
 network 192.168.1.4 0.0.0.0
 network 192.168.1.17 0.0.0.0
 network 192.168.1.20 0.0.0.3
 network 192.168.1.32 0.0.0.3
 network 192.168.1.40 0.0.0.3

The difference in global part is these missing lines in SW2:
no auto-summary
default-metric 1000000 1 255 20 1500
redistribute connected

SW2 does not redistribute the connected VLAN into EIGRP but why is this a reason for not advertising it at all? After all, it learns the network from SW1 so it could advertise it further.

SW1#sh ip eigrp topology 10.1.4.0 255.255.255.0
EIGRP-IPv4 (AS 100): Topology Default-IP-Routing-Table(0) entry for 10.1.4.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2816
  Routing Descriptor Blocks:
  0.0.0.0, from Rconnected, Send flag is 0x0
      Composite metric is (2816/0), Route is External
      Vector metric:
        Minimum bandwidth is 1000000 Kbit
        Total delay is 10 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 0
      External data:
        Originating router is 192.168.1.3 (this system)
        AS number of route is 0
        External protocol is Connected, external metric is 0
        Administrator tag is 0 (0x00000000)
% EIGRP-IPv4 (AS 255): Route not in topology table TID(0)

SW2#sh ip eigrp topology  10.1.4.0 255.255.255.0
EIGRP-IPv4 Topology Entry for AS(100)/ID(192.168.1.4) for 10.1.4.0/24
  State is Passive, Query origin flag is 1, 0 Successor(s), FD is 4294967295
  Descriptor Blocks:
  192.168.1.18 (GigabitEthernet1/20), from 192.168.1.18, Send flag is 0x0
      Composite metric is (103424/100864), route is External
      Vector metric:
        Minimum bandwidth is 34013 Kbit
        Total delay is 1100 microseconds
        Reliability is 255/255
        Load is 50/255
        Minimum MTU is 1500
        Hop count is 12
      External data:
        Originating router is 192.168.1.3
        AS number of route is 0
        External protocol is Connected, external metric is 0
        Administrator tag is 0 (0x00000000)
  172.25.5.35 (Vlan80), from 172.25.5.35, Send flag is 0x0
      Composite metric is (3328/2816), route is External
      Vector metric:
        Minimum bandwidth is 1000000 Kbit
        Total delay is 30 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
      External data:
        Originating router is 192.168.1.3
        AS number of route is 0
        External protocol is Connected, external metric is 0
        Administrator tag is 0 (0x00000000)
  172.25.5.62 (Vlan81), from 172.25.5.62, Send flag is 0x0
      Composite metric is (3328/2816), route is External
      Vector metric:
        Minimum bandwidth is 1000000 Kbit
        Total delay is 30 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
      External data:
        Originating router is 192.168.1.3
        AS number of route is 0
        External protocol is Connected, external metric is 0
        Administrator tag is 0 (0x00000000)
  192.168.1.33 (GigabitEthernet1/22), from 192.168.1.33, Send flag is 0x0
      Composite metric is (3072/2816), route is External
      Vector metric:
        Minimum bandwidth is 1000000 Kbit
        Total delay is 20 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
      External data:
        Originating router is 192.168.1.3
        AS number of route is 0
        External protocol is Connected, external metric is 0
        Administrator tag is 0 (0x00000000)
Hey, the output says 0 successors!

SW2#sh ip eigrp topology zero-successors
EIGRP-IPv4 Topology Table for AS(100)/ID(192.168.1.4)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 10.1.4.0/24, 0 successors, FD is Inaccessible
        via 192.168.1.18 (103424/100864), GigabitEthernet1/20, serno 1907987
        via 172.25.5.35 (3328/2816), Vlan80
        via 172.25.5.62 (3328/2816), Vlan81
        via 192.168.1.33 (3072/2816), GigabitEthernet1/22

This SW2 table does not contain the Rconnected entry. Zero successor usually implies that the entry cannot be placed into the routing table because a lower administrative distance route is already in place. And that is the explanation as VLAN4 is connected and it means a very low admin distance. (The principle is similar to BGP RIB failure when a BGP best path is not placed in the routing table for the same reason.)

I added all 3 missing lines to SW2 and that fixed the problem (actually the redistribute connected did the trick). However, no auto-summary proved not to be a difference as the IOS versions slightly differ and SW2 does not display that line in the config. (Note the other differences in EIGRP syntax flavour too.)

R2#
Dec  3 14:44:49.938: IP-EIGRP(Default-IP-Routing-Table:100): Processing incoming UPDATE packet
Dec  3 14:44:49.938: IP-EIGRP(Default-IP-Routing-Table:100): Ext 10.1.4.0/24 M 3072 - 2560 512 SM 2816 - 2560 256
Dec  3 14:44:49.938: RT: add 10.1.4.0/24 via 192.168.1.21, eigrp metric [91/3072]
Dec  3 14:44:49.938: RT: NET-RED 10.1.4.0/24
Dec  3 14:44:49.938: IP-EIGRP(Default-IP-Routing-Table:100): route installed for 10.1.4.0  ()
Dec  3 14:44:49.954: IP-EIGRP(Default-IP-Routing-Table:100): 10.1.4.0/24 - denied by route-map
Dec  3 14:44:49.954: IP-EIGRP(Default-IP-Routing-Table:100): Ext 10.1.4.0/24 metric 3072 - 2560 512

R2#sh ip route 10.1.4.0
Routing entry for 10.1.4.0/24
  Known via "eigrp 100", distance 91, metric 3072, type external
  Redistributing via eigrp 100, ospf 1
  Advertised by ospf 1 metric-type 1 subnets route-map TO-OSPF
  Last update from 192.168.1.21 on GigabitEthernet0/1, 00:00:31 ago
  Routing Descriptor Blocks:
  * 192.168.1.21, from 192.168.1.21, 00:00:31 ago, via GigabitEthernet0/1
      Route metric is 3072, traffic share count is 1
      Total delay is 20 microseconds, minimum bandwidth is 1000000 Kbit
      Reliability 253/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1
R2#sh ip eigrp topology 10.1.4.0 255.255.255.0
IP-EIGRP (AS 100): Topology entry for 10.1.4.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 3072
  Routing Descriptor Blocks:
  192.168.1.21 (GigabitEthernet0/1), from 192.168.1.21, Send flag is 0x0
      Composite metric is (3072/2816), Route is External
      Vector metric:
        Minimum bandwidth is 1000000 Kbit
        Total delay is 20 microseconds
        Reliability is 253/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
      External data:
        Originating router is 192.168.1.4 
        AS number of route is 0
        External protocol is Connected, external metric is 0
        Administrator tag is 0 (0x00000000)

This config error showed that some routes may be missing in some parts of a common EIGRP domain even if no summarization or distribute-lists are applied. The domain-wide consistency (generally implicitly assumed as every router’s task is to advertise what it learns) is only guaranteed if all EIGRP routes can get into routing tables on all devices. Potential obstacles may be connected but unadvertised networks or other routing protocols that insert the same learned networks into the routing table.

Software versions:
SW1 s6523-advipservicesk9-mz.122-33.SXI2a.bin
SW2 s6523-advipservicesk9-mz.122-33.SXI4a.bin

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