; Use of ‘/31 IP subnet’ on point to point links; and their effect on routing protocols
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Use of ‘/31 IP subnet’ on point to point links; and their effect on routing protocols

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									Use of ‘/31 IP subnet’ on point to point links; and their effect on routing
protocols


The use of 31bit subnet masks on point to point links is documented in RFC 3021:
http://www.ietf.org/rfc/rfc3021.txt

If you know subnetting, then you should know that networks always need a subnet address (all
zeroes) and a ‘broadcast’ address. This is why when you generally use a /30 for point-to-point
links. 2 addresses used for the routers on each end, then 1 subnet address and 1 broadcast
address.

Essentially using a /31 gives you no option to use directed broadcasts. How does this affect
routing protocols though? These days most use multicast, but some of the older ones use
broadcasts. Let’s run a lab to find out!

I’ve got a simple topology going here. 2 routers connected to each other, each with a loopback
subnet advertised in the protocol.




Let’s see what happens!
The first thing I do is to give /31 addresses to both interfaces. Immediately I got the following
warning:
R2(config-ip)ip address 10.1.1.4 255.255.255.254
% Warning: use /31 mask on point-to-point interface cautiously
RIPV1:
R1(config)#router rip
R1(config-router)#network 10.0.0.0
R1(config-router)#network 192.168.1.0
R1#sh ip route 172.16.1.1
Routing entry for 172.16.0.0/16
Known via "rip", distance 120, metric 1
Redistributing via rip
Last update from 10.1.1.4 on FastEthernet0, 00:00:13 ago
R1#ping 172.16.1.1
Success rate is 100 percent (5/5)
Yes it works fine!
RIPV2:
R1(config)#router rip
R1(config-router)#version 2
R1#sh ip route 172.16.1.1
Routing entry for 172.16.0.0/16
Known via "rip", distance 120, metric 1
Redistributing via rip
Last update from 10.1.1.4 on FastEthernet0, 00:00:08 ago
R1#ping 172.16.1.1
Success rate is 100 percent (5/5)
Bingo again, there is no problem!
EIGRP
R1(config)#router eigrp 1
R1(config-router)#no auto-summary
R1(config-router)#network 10.1.1.5
R1(config-router)#network 192.168.1.1
R1#sh ip route 172.16.1.1
Routing entry for 172.16.0.0/16
Known via "eigrp 1", distance 90, metric 156160, type internal
Redistributing via eigrp 1
Last update from 10.1.1.4 on FastEthernet0, 00:01:45 ago
R1#ping 172.16.1.1
Success rate is 100 percent (5/5)
EIGRP has no problems either! ;)
OSPF:
R1(config)#router ospf 1
R1(config-router)#network 10.1.1.5 0.0.0.0 area 0
R1(config-router)#network 192.168.1.1 0.0.0.255 area 0
R1#sh ip route 172.16.1.1
Routing entry for 172.16.1.1/32
Known via "ospf 1", distance 110, metric 2, type intra area
Last update from 10.1.1.4 on FastEthernet0, 00:00:45 ago
R1#ping 172.16.1.1
Success rate is 100 percent (5/5)
OSPF also works fine. I guess we can see a pattern emerging here…
BGP
R1(config)#router bgp 100
R1(config-router)#neighbor 10.1.1.4 remote-as 200
R1(config-router)#network 192.168.1.0 mask 255.255.255.0
R1#sh ip route 172.16.1.1
Routing entry for 172.16.1.0/24
Known via "bgp 100", distance 20, metric 0
Tag 200, type external
Last update from 10.1.1.4 00:00:08 ago
R1#ping 172.16.1.1
Success rate is 100 percent (5/5)
Whew, BGP works just fine
To be honest everything seems to work just fine. I suppose I could test IS-IS and IGRP as well,
but who really uses IGRP anymore? (I suppose the same could be said about RIPv1)
IS-IS I suppose would work just fine, but I forget to test it. I’ll update this doc when I do though
:)
I’m going to make all my future topologies using /31 on point-to-point links. Not only will it be
good practice, but if I ever run into something that ‘doesn’t work’ in the lab, it’s a better place
to not work than in the real world!
If you have any bad experiences with /31’s please let me know in the comments.

								
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