Friday, March 4, 2011

/31 bit Point to Point ip address configuration .

Can we assign /31 address on the point to point link & save 50 % of ip address?
 Answer is yes.

R2(config)#int fa0/0
R2(config-if)#ip address 192.168.1.0 255.255.255.254
% Warning: use /31 mask on non point-to-point interface cautiously


R4(config)#int fa0/0
R4(config-if)#ip address 192.168.1.1 255.255.255.254
% Warning: use /31 mask on non point-to-point interface cautiously

Let’s try to ping :)

R4#ping 192.168.1.0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.0, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

Ok do we need privilege mode to ping ? no .

R4> ping 192.168.1.0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.0, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

Let’s check the ip route:

R4#show ip route
Codes: 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

Gateway of last resort is not set

192.168.1.0/31 is subnetted, 2 subnets
C 192.168.1.0 is directly connected, FastEthernet0/0
C 192.168.1.2 is directly connected, FastEthernet0/1

I tried to assign the same way on switches 3550,3560 there was no issues.

" im just wondering.... So wht is a the use of Network n Broadcast addrs anyway ?":
Fist if its real point to point interface ( eg: serial, ppp) we don’t need to assign the ip address at all , unnumbered or we could use /32 at both side 
But when we use Ethernet as Point to Point we need to consider the broadcast addresses:

RFC 1812 says in section 4.2.3.1 IP Broadcast Addresses :
“ (2) SHOULD silently discard on receipt (i.e., do not even deliver to
applications in the router) any packet addressed to 0.0.0.0 or {
, 0 }. If these packets are not silently
discarded, they MUST be treated as IP broadcasts (see Section
[5.3.5]). There MAY be a configuration option to allow receipt
of these packets. This option SHOULD default to discarding
them.

Broadcast fall in to two categories:

  • Limited broadcast (Local)
  1. 0.0.0.0
  2.   All one -255.255.255.255
  •   Directed broadcast
  1.   Network < all zero > - this is obsolete ( Reference RFC 3021 section 2.2 )
  2.   Network , < all one>
But our concern here is the directed broadcast , therefore if there is any broadcast it can uses limited broadcast . we have to understand the directed broadcast only affects the local router and  the segment which is the point to point link . Further routing protocols uses the multicast, limited broadcast or unicast addresses that is not going to affected by the /31 addressing scheme.
Anyway directed broadcasts are always filtered as the best practice.
 

Further Reading:
http://www.faqs.org/rfcs/rfc3021.html

http://www.faqs.org/rfcs/rfc1812.html
http://www.cisco.com/en/US/docs/ios/12_2t/12_2t2/feature/guide/ft31addr.html

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