Friday, January 28, 2011

can you assign same ip address to two interfaces

cisco if the interface type is point to point we don't need to assign ip address related to RFC 1812 2.2.7 section :
Related to this if the interface type is P2P we can assign same ip address to two interfaces.

Serial1/0 192.168.1.1 YES manual up up
Serial1/1 192.168.1.1 YES manual up up


Ok if i ping 192.168.1.2 where it will go ? lets explore it ..

Basic diagram

R2 (s1/0)--<(s1/1) R1 (s1/0)>--(s1/0) R3 -- LO 192.168.6.1/32


R1#show ip route 192.168.1.2
Routing entry for 192.168.1.0/30
Known via "connected", distance 0, metric 0 (connected, via interface)
Routing Descriptor Blocks:
* directly connected, via Serial1/0
Route metric is 0, traffic share count is 1
directly connected, via Serial1/1
Route metric is 0, traffic share count is 1

so basically load sharing :)
further more ,

R1#show ip cef 192.168.6.1
192.168.6.1/32
nexthop 192.168.1.2 Serial1/0
nexthop 192.168.1.2 Serial1/1


if i enabled the debug ip icmp and interface changed the loadsharing mode to per packet :

R1#ping 192.168.6.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.6.1, timeout is 2 seconds:
U!
*Jan 28 22:24:25.455: ICMP: dst (192.168.1.1) host unreachable rcv from 192.168.1.2
*Jan 28 22:24:25.555: ICMP: echo reply rcvd, src 192.168.6.1, dst 192.168.1.1.!U
Success rate is 40 percent (2/5), round-trip min/avg/max = 76/88/100 ms
R1#
*Jan 28 22:24:27.635: ICMP: echo reply rcvd, src 192.168.6.1, dst 192.168.1.1
*Jan 28 22:24:27.743: ICMP: dst (192.168.1.1) host unreachable rcv from 192.168.1.2


So we can assign the ip address but if its connecting to same device no issue otherwise will be problem

1 comment:

Anonymous said...

Nice one. Thanks