From: Scott Morris (swm@emanon.com)
Date: Thu Nov 15 2007 - 16:44:01 ART
I think we're looking at some fundamental functionality questions...
2 ports in VLAN 1 will get all broadcasts, multicasts and unknown unicast
traffic for all VLAN 1. but each port would not see each others' direct
unicast traffic once a switch learned the appropriate port.
As for the idea with secondary addresses... From a switch's perspective of
forwarding multicast or broadcast traffic, then copies of the packet would
go to every device in that VLAN (regardless of subnet).
Your routers, however, may not pay any attention to subnet broadcasts (like
192.168.1.255) that aren't part of their configured subnet(s). The
255.255.255.255 packets will be decoded by everyone though.
We see this a lot of times with RIPv2 with devices in different address
ranges (because anything you source will, by default, be from your primary
IP address). Where multicast packets are received (because the switch
forwards them to all ports in VLAN 1 as it's supposed to) but end devices
ignore the information because it's sourced from someone they don't "know".
HTH,
Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, JNCIE-M
#153, JNCIS-ER, CISSP, et al.
CCSI/JNCI-M/JNCI-ER
VP - Technical Training - IPexpert, Inc.
IPexpert Sr. Technical Instructor
A Cisco Learning Partner - We Accept Learning Credits!
Telephone: +1.810.326.1444
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-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Vladimir Sousa
Sent: Thursday, November 15, 2007 1:32 PM
To: Sean C
Cc: ccielab@groupstudy.com
Subject: Re: Secondary IPs and broadcasts
Hello Sean,
Just one question, what makes the broadcast segmentation? the vlan or the
ip?
if you set 2 ports in vlan 1, will it receive traffic for vlan 1 in both
IPs?
If you take 2 routers and connect to these 2 ports, and you enable the same
pair of IP, Sec IP, will they receive the broadcast packets for each subnet?
will they respond in both cases?
I think the answer to your question is, yes, packets will be delivered to
both ports, but will be responded by the hosts configured on the same subnet
IP.
Vlad
On 11/15/07, Sean C <upp_and_upp@hotmail.com> wrote:
>
> Hi Group,
>
> If I have a 3550 with Vlan 1:
>
> int vlan 1
> ip address 10.1.1.254 255.255.255.0
> ip address 10.1.2.254 255.255.255.0 secondary
>
> Interface f0/1 is in Vlan1 and has a host connected (host1) with IP
> 10.1.1.1
> /24
> Interface f0/2 is also in Vlan1 and has a host connected (host2) with
> IP
> 10.1.2.1 /24
>
> If host1 sends a local broadcast (10.1.1.255), will host2 receive this
> since both IP subnets are in the same layer 2 segment?
>
> This leads me into other areas, like what if it's an IP broadcast
> (255.255.255.255) or layer 2 broadcast (f.f.f) or if host2 is across a
> couple of 3550s, with each switch's SVI in both IP subnets. I'm
> thinking that
> host2
> will receive the frame (what host2 does with the frame is a different
> story).
> I'm just curious to see how the packets are switched.
>
> I looked pretty hard on-line, but can't find a definitive answer.
> Here's some things I found for guidance (buyer beware):
> http://www.groupstudy.com/archives/cisco/199912/msg00535.html
>
> http://books.google.com/books?id=77h9SA94kasC&pg=PA53&lpg=PA53&dq=%22s
> econdar
>
> y+address%22+broadcast+interface&source=web&ots=ZK2MDq5iey&sig=niwz8-D
> y+79xoTWM
> Jt97wCCinJfcQ
> http://tcpmag.com/forums/forum_posts.asp?tid=1383&pn=10
>
> Even found a decent post from Howard Berkowitz (I miss his posts):
> http://www.groupstudy.com/archives/cisco/200306/msg01434.html
>
> I don't have immediate access to a LAN sniffer or I'd bang on this myself.
> Curious to hear the group's thoughts. Thanks in advance, Sean C.
> CCIE #17085
>
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