TWO INTERFACES SAME SUBNET(R)

[isaaco001]

Dear Community,

I would like to have my router 2911 to have two interfaces with the same ip for the LAN,however, i have limited L3 devices so its just one router and two L2 switches. vrrp,glbp or hsrp or subif's is not to be used.What can be done?Thank you all.

Regards,

Isaac.

[Julio E. Moisa · Accepted solution]

Hi

You cannot have 2 interface at the same subnet on the same router because they will overlap each other, but you can create a bridge group and associate your interfaces to it and you will have only 1 IP on the router. 

Example:

Router

interface Fa0/0
no ip address
bridge-group 2
!

interface Fa0/1
no ip address
bridge-group 2
!

!
interface BVI2
ip address 10.10.2.1 255.255.255.0
!

!

bridge irb
bridge 2 protocol ieee
bridge 2 route ip

Switch 1

vlan 2

interface Vlan2
ip address 10.10.2.2 255.255.255.0
end

Switch#ping 10.10.2.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.2.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms

Switch 2

vlan 2

interface Vlan2
ip address 10.10.2.3 255.255.255.0
end

Switch2#ping
*Feb 13 13:06:47.111: %SYS-5-CONFIG_I: Configured from console by console
Switch2#ping 10.10.2.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.2.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms
Switch2#ping 10.10.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/2 ms
Switch2#ping 10.10.2.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.2.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 5/5/5 ms
Switch2#

Please rate the comment if it is useful

:-)

[isaaco001]

Julio,

Thanks for your response. However, i have concerns that need to be settled.This bvi ip should act like any other ip i.e routable with no glitches(caveats),right?Its just odd..

Regards,

Isaac.

[Julio E. Moisa · Accepted solution]

Hi Isaac,

You are welcome, Yes this IP can be used on routing processes like any IP on a router.  

 :-) 

[Jubei]

I know this is an older post but, I was working on a lab using VRF; in that specific instance, you can have the same IP address on two separate interfaces within the same router. Without VRF running it won't work. 





boot-end-marker
!
!
vrf definition Client-A
!
address-family ipv4
exit-address-family
!
vrf definition Client-B
!
address-family ipv4
exit-address-family
!
!
no aaa new-model
mmi polling-interval 60
no mmi auto-configure
no mmi pvc
mmi snmp-timeout 180
!

!
!
!
!

!
!
!
!
ip cef
no ipv6 cef
!
multilink bundle-name authenticated
!
!
!
!
!
!
!
!
!
!
interface Ethernet0/0
vrf forwarding Client-A
ip address 10.10.2.3 255.255.255.0
!
interface Ethernet0/1
vrf forwarding Client-B
ip address 10.10.2.3 255.255.255.0

!
interface Ethernet0/2
no ip address
shutdown
!
interface Ethernet0/3
no ip address
shutdown

[Joseph W. Doherty]

True, but it may be unknown, to OP or some readers, such identical IPs cannot be within the same IP domain.

[Ricky Sandhu]

Currently working on my CCIE EI lab in GNS3 and I noticed on a given router, I can have two serial interfaces using the same IP configuration.  Is it because serial interfaces are point-to-point and theoretically can be connected to different networks using the same IP schema?

Serial1/0 10.1.10.10 YES NVRAM up up
Serial1/1 10.1.10.20 YES manual up up