Comments on: Secondary IP Addresses

By: 34

34 — Mon, 15 Aug 2011 11:55:41 +0000

Javed, I believe the reason for using secondary ip addressing in this example is because the connecting switch is unmanaged. This implies it is unable to be connected in “Trunk” mode and seperate the subnets via VLANs.

By: Javed

Javed — Wed, 22 Jun 2011 00:02:56 +0000

Hi There ,

One more silly question guy’s , Cant we use sub-interfaces instead of secondary ip address ?

Thanks in Advance

By: Jim

Jim — Wed, 15 Dec 2010 12:20:53 +0000

So if I wanted to add a second router (R2) into the picture for diversity (without EIGRP), how would I do this?

By: 27

27 — Sun, 17 Oct 2010 04:54:47 +0000

guys just a question
can both network primary and secondary get DHCP IPs from a Windows Server DHCP if the two pools are configured on the server

By: Colby

Colby — Wed, 28 Jul 2010 20:31:38 +0000

Haha. The post itself was trivial. Leave it to the comments to get interesting.

By: errtime

errtime — Wed, 28 Jul 2010 20:22:18 +0000

yes, it would be fair to put it to the semantics because secondary addresses are taking a huge role in the neighborship forming. “Network x.x.x.x” just triggers the router to start advertise Hello packets out of the corresponding interface, all the rest neighborship is formed using primary addresses.

p.s. And you said one small n easy post about the secondary addresses. Not so trivial actually

By: Colby

Colby — Wed, 28 Jul 2010 19:38:06 +0000

R1:
interface FastEthernet0/0
 ip address 192.168.1.1 255.255.255.0 secondary
 ip address 10.1.1.1 255.255.255.0
!
router eigrp 100
 network 192.168.1.0
 
R2:
interface FastEthernet0/0
 ip address 192.168.1.2 255.255.255.0 secondary
 ip address 10.1.1.2 255.255.255.0
!
router eigrp 100
 network 192.168.1.0
 
*Mar  1 00:05:31.295: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.1.1 (FastEthernet0/0) is up: new adjacency

The network statement is using secondary IPs. I suppose it comes down to semantics. The neighbor is seen by it’s primary IP, but we’re sort of using secondary IPs.

By: errtime

errtime — Wed, 28 Jul 2010 19:16:11 +0000

Colby, if i remember correctly EIGRP sources it’s packets only from primary addresses not secondary, so its impossible to establish neighborship over the secondary addresses. Technically you could “network x.x.x.x” only the secondary addresses and your neighborship would come up but only if both routers believe they are on the same subnet from primary address point of view.

By: Oliver Gorwits

Oliver Gorwits — Mon, 26 Jul 2010 20:15:53 +0000

A few tidbits for secondaries:

Traffic between two hosts on the connected LAN but in e two networks will go via the router. This often catches non network folks and those running firewalls – local traffic on the outside interface.

DHCP relay will come from the primary address (I think there’s an IOS command to alter this behaviour).

For IPv6 there’s no secondary keyword. All v6 addresses are created equal!

HTH,
Oliver.

By: Colby

Colby — Sun, 25 Jul 2010 21:27:58 +0000

You can use secondary IPs to peer with EIGRP, but not OSPF. You should be able to advertise secondary networks with both. Not sure about RIP, never tried.