Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

HSRP and Subnet Masking (basic Concept)

On my two MSFC2's I can configure HSRP without setting an IP address on the vlan interface:


Interface VLAN1

standby IP

So how is the subnet mask determined? I have several variably subnetted networks. Do I need to put in a line:

ip adress ?

I would be using EIGRP routing protocol with the statement


no auto-summary

Would not defining a subnet mask be a problem?

  • Other Network Infrastructure Subjects

Re: HSRP and Subnet Masking (basic Concept)

If I had to guess it would probably just put it at the classfull boundary unless you specify a specific mask , which in your case would be . Just do a show int vlan 1 and it will show you what mask it assigned .

New Member

Re: HSRP and Subnet Masking (basic Concept)

Doing a show int vlan1 didn't show the ip information. When I added the IP address line on the vlan interface, the Ip netmask information showed correctly in show int vlan1.

Many of the examples in Cisco documentation concerning HSRP did not show entering the IP address, on the vlan interface, only the standby address. However, I could not do inter-vlan routing without the IP address specified on the vlan interface. The network did not show up in the routing table.


Re: HSRP and Subnet Masking (basic Concept)

HSRP won't work on an interface without IP configured (?)--or at least it shouldn't. HSRP also doesn't care about the subnet mask in any way, it just assumes you are using the address you've listed as the default gateway on any hosts attached to the segment.

EIGRP won't pay any attention to the HSRP information, since EIGRP won't route to an HSRP address, etc. If you have two possible next hops across the same link, EIGRP will just build the right neighbor relationships, and install two routes in topology table, which will either result in load sharing, or having a backup route, normally.