View source for Using HSRP On A /30 or /31

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It's common for business ISPs to use a /30 as a point-to-point to their customers, then route them a /29 or larger.  Normally, using a /30 precludes the possibility of having redundant routers terminating the connection.  The common belief is that you need a minimum of a /29 for HSRP: one IP for router A, one for router B, one for the HSRP High Availability address, and the rest for the clients.

In reality, the IPs on router A and B need to be in the same subnet, but it does not need to be the same subnet as the HA address. For example:

* ISP Gateway: 24.1.1.0/31
* Customer HA address: 24.1.1.1/31
* Customer Router A Outside Interface (Fa0): 192.168.0.1/30
* Customer Router B Outside Interface (Fa0): 192.168.0.2/30


==Interface Config==
====Router A====
<PRE>
interface FastEthernet0
 ip address 192.168.0.1 255.255.255.252
 standby 1 ip 24.1.1.1
</PRE>

====Router B====
<PRE>
interface FastEthernet0
 ip address 192.168.0.2 255.255.255.252
 standby 1 ip 24.1.1.1
</PRE>


==Route Config==
Routers A and B just need an interface route to be able to direct the traffic to the ISP's gateway, and their default route pointing to the ISP's gateway

====Router A====
<PRE>
ip route 24.1.1.0 255.255.255.254 fa0
ip route 0.0.0.0 0.0.0.0 24.1.1.0
</PRE>

====Router B==== 
<PRE>
ip route 24.1.1.0 255.255.255.254 fa0
ip route 0.0.0.0 0.0.0.0 24.1.1.0
</PRE>