multi-subnet broadcast domains?

If I configure multiple IP addresses on one router interface, are they all then part of the same broadcast domain?

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milliamp wrote:

If I configure multiple IP addresses on one router interface, are they all then part of the same broadcast domain?

I am sure that any traffic send to 255.255.255.255 or FF:FF:FF:FF:FF:FF would hit all devices on the interface (ie arp traffic).

In my opinion I would call that ONE broadcast domain, but I am talking to someone who does not agree with me on this point.

devices on the same ip domain/subnet would talk nativily with devices in the ip domain. broadcasts do not cross the router to other devices on other subnets. if ip helper is added to the interface dhcp related broadcasts would pass only to the address specified as the helper

milliamp wrote:

I guess you could argue that traffic for 10.10.10.4/24 sent to 10.10.10.255 still is not addressed to your 192.168.x.x addresses on the interface...

I don't know, what do you think?

milliamp

Maybe the questions is confusing, let me ask a different way.

example interface:

router#show run
Interface FastE1/1
ip address 10.10.10.1/24
ip address 10.10.20.1/24 secondary

Would hosts 10.10.10.5 and 10.10.20.5 be in the same broadcast domain?

thedons

I think the answer is No. Even though the two subnets are on the same interface, the router still won't pass broadcast packets from one subnet to the other.

darkuser

yes, it's one (vlan) broadcast domain , but two seperate logical subnets.

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800ca75b.html#wp1001012

EricO

Would hosts 10.10.10.5 and 10.10.20.5 be in the same broadcast domain?

The short answer is : YES. The long answer is that you most likely would not want to do this on a production network. You would want to configure sub interfaces on the router and vlan trunking on the switch.

milliamp

EricO wrote:

The short answer is : YES. The long answer is that you most likely would not want to do this on a production network. You would want to configure sub interfaces on the router and vlan trunking on the switch.

That is what I was after, thank you.

Hi folks,

I went through the link :
http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800ca75b.html#wp1001012

- But had some difficulty understanding the scenarios

Could one please list few of the scenarios when we will be assigning a secondary address to an interface?

Thanks...

he-man

Hey guys,

Not sure why the secondary address would be on a different subnet but i vaguely remember frame relay environments using them becuase of the lack of ability to send route updates on the interface that an update was recieved on...... or something like that.

milliamp

SV wrote:

Could one please list few of the scenarios when we will be assigning a secondary address to an interface?

Say for instance you have a CMTS (for cable modems) operating at layer 2. You have 2 /19's on the same interface, one on 10.x.x.x for cable modems and the other is a block of publics for PC's.

Lets say you need another ~250 public IP addresses, do you reclaim the /19 and assign a whole /18 and let everyone start working again after they pull new IP addresses?

Not lilkely, you can just assign a /24 or /23 of publics as a secondary address on the interface and add the range to DHCP.

The same can be done with static users who may have /30's ouside of the dynamic range.

To illustrate your interface would look like this.

Int GigE 0
ip address 10.10.0.1/19 (for cable modems)
ip address 62.62.128.1/19 secondary (for PC's)
ip address 70.70.0.1/23 secondary (for more PC's)
ip address 70.72.10.25/30 secondary (user with static IP address)

The reason for my asking the question was an argument with someone else who wanted to pretty much break down all the /19's etc. into basically a pile of /24's.

I am really sorry. Still a bit confused. Is the above diagram an example for the type of usage where 10.0.0.5 can communicate with 20.0.0.5 without any additional routing entries?

Shiju

EdTheLad

Yes you understood corectly,view the 2 ip addresses on the interface as 2 seperate logical ports.Its exactly the same concept of routing between 2 physical ports.

Thanks a lot folks