From: Roche-Kelly, Edmund B. (Edmund.B.Roche-Kelly@xxxxxxx)
Date: Thu Jun 07 2001 - 11:04:02 GMT-3
This loop free design is the latest fad in Cisco design circles, mainly due
to 4 years of spanning tree loops breaking networks. The idea is to keep
each vlan
on only one access layer switch. This gives a V shape instead of the
traditional
triangle.
DISTRIBUTION A----vlan 10----ACCESS A----vlan 10----DISTRIBUTION B
If a vlan needs to be on two access layer switches, you use a U shape.
DISTRIBUTION A----vlan 10----ACCESS A===ether channel===ACCESS B----vlan
10----DISTRIBUTION B
You can trunk multiple vlans between the Access layer and the distribution,
but the
point is that vlan 10, for example, is on only one link between the
distribution and
the access layer. If that link goes down, the vlan interface on the RSM goes
down since
there are no active ports on the switch in the vlan. HSRP works fine.
If you did something like this:
DISTRIBUTION A----vlan 10----ACCESS A----vlan 10----DISTRIBUTION B
|
|
vlan 10
|
|
ACCESS B
Then the vlan interface would not go down on the first RSM if the link to
ACCESS A broke, because of the active port to ACCESS B and both RSMs would
be HSRP
active, with standby router unknown.
Spanning tree is very unfashionable since layer 3 switching means the old
routing latency
is gone, and layer 2 recovery times are so much longer than routing protocol
recovery times.
Ed
-----Original Message-----
From: Williams, Glenn [mailto:WILLIAMSG@PANASONIC.COM]
Sent: Thursday, June 07, 2001 8:51 AM
To: Ccielab (E-mail)
Subject: Spanning Tree & HSRP
Hi all. This is an actual design in progress that was sent to Cisco for a
response. As we await, I wanted to see your comments. From what I
understand (???) Cisco would like to see switch designs without any loops in
the initial design so as not to depend on spanning tree. (However even
though it may be awkward, don't loops provide for redundancy?) But this has
me wondering about a redundant distribution switch design involving HSRP
with two RSMs inside two 4006s. Below is what I sent to Cisco. What do ya
think? BTW I cannot test this yet and I'm not as learned as many of you. So
I appreciate your input.
Message to Cisco:
We will be implementing 2 distribution switches with RSMs that will connect
to all our access level switches. This will provide fault tolerance. The
RSMs will run HSRP. In many Cisco designs I noticed that a trunk is usually
run between the two switches. I imagined that this is a way for the two
HSRP routers to maintain communication and as a path in case a port that
connects to a particular access switch fails or the wire connecting the
access to distribution switches fail. However I heard that we may be not
putting in a trunk between the two distribution switches which has posed a
technical question that I have not been able to get my mind around. I
understand that by not putting in the trunk we eliminate a possible loop
that would be a problem if spanning tree were disabled. I also understand
that we may or may not put in a separate VLAN between the two distribution
switches for routing updates or some other purpose. (We were thinking of
turning off any routing protocols and putting in a default route to our WAN
7505 router.)
Here is what I do not understand.
The only path for the two distribution switches to communicate is either
between the VLAN (if we connect it) between the two switches or the VLAN
that would go from a distribution switch to an access switch, then back down
to the other distribution switch. If I break the wire to one of the access
switches, what happens, especially for HSRP?
I am assuming that the hello messages were flowing from one distribution
HSRP router to the other via the connection through the access switch. With
this connection broken the backup HSRP router becomes active. But wouldn't
the original active HSRP also stay active since it did not fail, it just
lost connectivity to one switch in that VLAN. This being the case, when it
receives a packet from another subnet, destined for that VLAN that the
access switch was on won't it drop the packet? This is why I thought we
need a trunk between the two so there would continue to be a path to the mac
address being sought and so the HSRP routers could continue to talk.
Straighten me out.
Thanks
Glenn
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