EIGRP with the Same Router ID – part 1

What could possibly go wrong with routers with the same EIGRP router ID (RID)?

Figure 1 is the topology for this lab. See figure below.

eigrp-rid-topo

Figure 1

I am going to run two different scenarios. The first one is going to be a point-to-point between R1 and R2. The second scenario would be R1 and R3 will both have the same RID. We are going to examine what is going to happen if two routers in the topology have the same RID.

Let’s tackle scenario one. The Example 1 below is the routing table of R1. At this point, everything is configured correctly. Example 2 is the EIGRP and interfaces config of R1.

Example 1

R1(config-if)#do show 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 not set

     10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks
C       10.0.1.0/24 is directly connected, Loopback0
L       10.0.1.1/32 is directly connected, Loopback0
D       10.0.2.0/24 [90/409600] via 10.0.12.2, 00:02:11, Ethernet1/0
C       10.0.10.0/24 is directly connected, Loopback1
L       10.0.10.1/32 is directly connected, Loopback1
C       10.0.12.0/24 is directly connected, Ethernet1/0
L       10.0.12.1/32 is directly connected, Ethernet1/0
D       10.0.20.0/24 [90/409600] via 10.0.12.2, 00:02:02, Ethernet1/0
D       10.0.23.0/24 [90/307200] via 10.0.12.2, 00:02:17, Ethernet1/0
R1(config-if)#

Example 2

R1(config-if)#do show run | section router
router eigrp 1
 network 10.0.0.0
R1(config-if)#do show run int e1/0
!
interface Ethernet1/0
 ip address 10.0.12.1 255.255.255.0
 duplex full
end

R1(config-if)#do show run int lo0 
!
interface Loopback0
 ip address 10.0.1.1 255.255.255.0
end

R1(config-if)#do show run int lo1
!
interface Loopback1
 ip address 10.0.10.1 255.255.255.0
end

Example 3 is the routing table of R2, and Example 4 is the EIGRP config and interfaces config of R2.

Example 3

R2#show 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 not set

       10.0.0.0/8 is variably subnetted, 10 subnets, 2 masks
D        10.0.1.0/24 [90/409600] via 10.0.12.1, 00:11:38, Ethernet1/0
C        10.0.2.0/24 is directly connected, Loopback0
L        10.0.2.1/32 is directly connected, Loopback0
D        10.0.10.0/24 [90/409600] via 10.0.12.1, 00:11:30, Ethernet1/0
C        10.0.12.0/24 is directly connected, Ethernet1/0
L        10.0.12.2/32 is directly connected, Ethernet1/0
C        10.0.20.0/24 is directly connected, Loopback1
L        10.0.20.1/32 is directly connected, Loopback1
C        10.0.23.0/24 is directly connected, Ethernet1/1
L        10.0.23.2/32 is directly connected, Ethernet1/1
R2#

Example 4

R2#show run | section router
router eigrp 1
 network 0.0.0.0
R2#show run int eth1/0
!
interface Ethernet1/0
 ip address 10.0.12.2 255.255.255.0
 duplex full
end

R2#show run int lo0 
!
interface Loopback0
 ip address 10.0.2.1 255.255.255.0
end

R2#show run int lo1
!
interface Loopback1
 ip address 10.0.20.1 255.255.255.0
end

R2#

Right now, the EIGRP between R1 and R2 is working correctly and the eigrp neighbor relationship look fine as shown in Example 5 and Example 6.

Example 5

R1#show ip eigrp neighbors 
EIGRP-IPv4 Neighbors for AS(1)
H     Address     Interface     Hold Uptime      SRTT     RTO     Q     Seq
                                (sec)            (ms)             Cnt   Num
0    10.0.12.2    Et1/0         11 00:18:43      45       270     0     57
R1#

Example 6

R2#show ip eigrp neighbors 
EIGRP-IPv4 Neighbors for AS(1)
H     Address     Interface     Hold Uptime      SRTT     RTO     Q     Seq
                                (sec)            (ms)             Cnt   Num
0    10.0.12.1    Et1/0         10 00:19:05      54       324     0     25
R2#

Now, I am going to manually change the RID of both routers to 1.1.1.1.

Example 7

R1#show run | sec router
router eigrp 1
 network 10.0.0.0
 eigrp router-id 1.1.1.1
R1#

!!!!!!!!!!!!!!!!!!!!!!!!!!

R2#show run | sec router
router eigrp 1
 network 0.0.0.0
 eigrp router-id 1.1.1.1
R2#

Right after I changed both routers’ RID, R1’s route table drops all eigrp routes from R2. R2 did the same thing. Example 8 shows the route table for both routers.

Example 8

R1#show ip route
...
     10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
C      10.0.1.0/24 is directly connected, Loopback0
L      10.0.1.1/32 is directly connected, Loopback0
C      10.0.10.0/24 is directly connected, Loopback1
L      10.0.10.1/32 is directly connected, Loopback1
C      10.0.12.0/24 is directly connected, Ethernet1/0
L      10.0.12.1/32 is directly connected, Ethernet1/0
R1#

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

R2#show ip route
...
     10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks
C      10.0.2.0/24 is directly connected, Loopback0
L      10.0.2.1/32 is directly connected, Loopback0
C      10.0.12.0/24 is directly connected, Ethernet1/0
L      10.0.12.2/32 is directly connected, Ethernet1/0
C      10.0.20.0/24 is directly connected, Loopback1
L      10.0.20.1/32 is directly connected, Loopback1
C      10.0.23.0/24 is directly connected, Ethernet1/1
L      10.0.23.2/32 is directly connected, Ethernet1/1
R2#

Here are the show eigrp iprotocols for both routers. Here you can see their router IDs.

Example 9

R1#show eigrp protocols 
EIGRP-IPv4 Protocol for AS(1)
  Metric weight K1=1, K2=0, K3=1, K4=0, K5=0
  NSF-aware route hold timer is 240
  Router-ID: 1.1.1.1
  Topology : 0 (base) 
    Active Timer: 3 min
    Distance: internal 90 external 170
    Maximum path: 4
    Maximum hopcount 100
    Maximum metric variance 1

R1#

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

R2#show eigrp protocols 
EIGRP-IPv4 Protocol for AS(1)
  Metric weight K1=1, K2=0, K3=1, K4=0, K5=0
  NSF-aware route hold timer is 240
  Router-ID: 1.1.1.1
  Topology : 0 (base) 
    Active Timer: 3 min
    Distance: internal 90 external 170
    Maximum path: 4
    Maximum hopcount 100
    Maximum metric variance 1

R2#

At this point, R1 has lost the routes that R2’s advertising and vice versa.

R1#show ip eigrp neighbors 
EIGRP-IPv4 Neighbors for AS(1)
H     Address     Interface     Hold Uptime      SRTT     RTO     Q     Seq
                                (sec)            (ms)             Cnt   Num
0    10.0.12.2    Et1/0         14 00:14:10      58       378     0     69
R1#

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

R2#show ip eigrp neighbors 
EIGRP-IPv4 Neighbors for AS(1)
H     Address     Interface     Hold Uptime      SRTT     RTO     Q     Seq
                                (sec)            (ms)             Cnt   Num
0    10.0.12.1    Et1/0         14 00:14:03      56       504     0     37
R2#

Despite both routers have the same RID, the neighbor relationship is still up. Let’s fix R1’s network statement to be more specific and let’s advertise R1’s loopback1 via redistribution. Let’s see what’s going to happen.

Example 10

R1#show run | sec router
router eigrp 1
 network 10.0.1.0 0.0.0.255
 network 10.0.12.0 0.0.0.255
 redistribute connected metric 10000 10 255 1 1500
 eigrp router-id 1.1.1.1
R1#

R2 is still not accepting any eigrp routes from R1. Example 11 is R2’s route table

Example 11

R2#show ip route
...
    10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks
C     10.0.2.0/24 is directly connected, Loopback0
L     10.0.2.1/32 is directly connected, Loopback0
C     10.0.12.0/24 is directly connected, Ethernet1/0
L     10.0.12.2/32 is directly connected, Ethernet1/0
C     10.0.20.0/24 is directly connected, Loopback1
L     10.0.20.1/32 is directly connected, Loopback1
C     10.0.23.0/24 is directly connected, Ethernet1/1
L     10.0.23.2/32 is directly connected, Ethernet1/1
R2#

What is happening now is the eigrp loop prevention. If an eigrp router receives an eigrp advertisement, it will check the RID of who advertised the route. Now, if it sees its RID, then the router will not install the routes because it thinks that there is a routing loop. If the topology is loop free, the router should not receive an advertised route with its RID in the eigrp update packet.

Now, let’s turn on R3 and see its routing table

Example 12

R3#show ip route
...
     10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks
D      10.0.2.0/24 [90/409600] via 10.0.23.2, 00:00:26, Ethernet1/1
C      10.0.3.0/24 is directly connected, Loopback0
L      10.0.3.1/32 is directly connected, Loopback0
D      10.0.12.0/24 [90/307200] via 10.0.23.2, 00:00:26, Ethernet1/1
D      10.0.20.0/24 [90/409600] via 10.0.23.2, 00:00:26, Ethernet1/1
C      10.0.23.0/24 is directly connected, Ethernet1/1
L      10.0.23.3/32 is directly connected, Ethernet1/1
C      10.0.30.0/24 is directly connected, Loopback1
L      10.0.30.1/32 is directly connected, Loopback1
R3#

As you can see, routes from R1 is not getting to R3 because R2 is not installing the routes into its topology table; therefore, R2 will not forward the route to R3.

We will tackle scenario two on my next post.

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About networkshinobi

This blog is about the things I learned about computers and networking to help me to remember them as I push further my studies. I created this blog to help myself to continue my education; and if you find this blog helpful for your studies, that is great. That is one of the reasons why I made this blog, to share my interest and knowledge. Also, all the entries/posts I made are based on my views, opinion and for educational purposes only. If you see some mistakes, feel free to drop some comments. I would appreciate all the helpful comments. Thanks BTC: 14wVPFBWNAKmfNsgUrPpw8EytkXFLjxYoU ETH: 0x8528793dF77a57186f5B15dA6DC1eaA3c5e92c4a LTC : LMpW2rGYnYdUwvnHA4huB6TGcPEEc1JzXw
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