From: Godswill Oletu (oletu@inbox.lv)
Date: Thu Jun 09 2005 - 22:25:52 GMT-3
Will this work for you?
Discard all Worries before Leaving Rome As the Original Mis-information
Sound's like a Neighbor's Idea.
Discard = DISCARD unreachable next hop.
Worries= highest WEIGHT
Leaving=highest LOCALpreference
Rome=Originated on this ROUTER
As=shortest AS_PATH
Original=ORIGIN code
Mis-information=lowest MED
Sound=SOURCE (external or internal)
Neighbor's=Closet IGP NEIGHBOR
Idea=lowest router ID
----- Original Message -----
From: "Daniel Kutchin" <daniel@kutchin.com>
To: "ccie2be" <ccie2be@nyc.rr.com>; <ccielab@groupstudy.com>
Sent: Thursday, June 09, 2005 4:21 PM
Subject: mnemonics - eg for BGP Route selection
> Tim and *,
>
> I'd like to know if you or anyone has any mnemonics he'd like to share
> with
> us.
>
> Especially, I'd like to know if you folks have any mnemonic for lerning
> the
> BGB Route selection algorithm below:
>
>
> Discard any route whose next hop is unreachable
> Prefer the route with the highest weight.
> Select the route with the highest local preference attribute.
> Prefer routes originated on this router.
> Prefer the route with the shortest AS_PATH.
> Prefer Interior to Exterior to Incomplete origin.
> Prefer those with the lowest MED value.
> Prefer routes with external rather than internal sources.
> Prefer the path through the closest IGP neighbor.
> Prefer the path with the lowest originating router ID.
>
> TIA,
>
>
> ---
>
> Dr. Daniel Kutchin
> Consultant Cisco(TM) Networks
> Heinrich-Pesch-Str. 55
> 41239 Moenchengladbach, Germany
> Tel. +49 173 5348189
> www.kutchin.com
>
>
>
> ----- Original Message -----
> From: "ccie2be" <ccie2be@nyc.rr.com>
> To: "Group Study" <ccielab@groupstudy.com>
> Sent: Tuesday, June 07, 2005 1:06 AM
> Subject: Re: STP- Port cost vs. Port-priority
>
>
> Bob,
>
> I also tend to give the benefit of a doubt to the double ccie if what I
> believe differs with what a double ccie.
>
> However, with the knowledge you now have regarding STP, I would think that
> you can verify any STP config for the scenario my comments applied to.
>
> Here's the mnemonic I used to keep this crap straight.
>
> D U D
>
> C P I
>
> Where D = downstream and U = Upstream (which includes root)
>
> And, C = cost; P = port priority; and I = interface #
>
> So, what the table says is this:
>
> If you want to influence which ports are blocking or not by using cost,
> make
> change on Downstream switch.
>
> If you want to influence which ports are blocking or not by using port
> priority, make change on Upstream switch.
>
> Of course, you can't change the interface # but it's in there because it's
> the last tiebreaker when all else is equal. But, just the same, you need
> to
> know that it's the Downstream which uses interface # as the tiebreaker.
>
> By knowing that fact, you can determine which port will go into forwarding
> mode next after a port failure.
>
> Bob, I think you're doing the right thing by experimenting with these
> variations. But, don't take my word for it or a double ccie or even a
> quad
> ccie.
>
> Only TRUST what the switch tells you empirically from your experiments.
>
> I read over your note quickly and didn't see any glaring mis-statements
> but
> at the moment I'm seeing double so don't go by that.
>
> I think that for you do this right, you need to do these experiments
> systematically.
>
> First, start by getting used to configuring STP to put ports in blocking
> and
> forwarding mode by changing these parameters, one at a time, for all
> vlans.
>
> IOW, for your first experiment, see which ports are blocking and
> forwarding,
> by default and determining which STP rules are causing the default result.
>
> Then, manually change which switch is the root and looking at the same
> show
> commands. What changed? Does this change make sense based on how you
> understand the STP rules? IF so, now, decide which ports you want to be
> in
> blocking and forwarding mode and the order of which ports change mode in
> case of failure. Write this down on paper.
>
> IOW, as an experiment, let's say you decide you want the downstream
> switch's
> ports, fa0/x, fa0/y, and fa0/z to be forwarding blocking and blocking,
> respectively.
>
> And, if fa0/x fails, fa0/z should forward next and if both fa0/x and fa0/z
> fail, then fa0/y goes into forwarding mode.
>
> Now, config only the Upstream switch to achieve that result. Once that
> works, undo whatever config's you made and now config only the downstream
> to
> achieve that result.
>
> Once you're very comfortable with doing and verifying those types of
> config's, now config your various vlan load-balancing scenarios by
> changing
> those same parameters but on a per-vlan basis.
>
> Once you're good with those scenarios, you shouldn't have anything to be
> concerned about on the lab when it comes to STP.
>
> Remember, in the lab, there are only 2 3550's connected back to back. So,
> there are only so many potential STP scenarios they can throw at you.
>
> Of course, you also have to know how and when to use the various STP
> commands that lower STP convergence time. But, that's another topic.
>
> BTW, assuming you really want to understand STP, you should read the
> Kennedy
> and Clark classic book, LAN Switching especially the chapter on STP.
>
> Make sure you know how STP elects the root bridge and then determines
> which
> ports are put in forwarding and blocking states.
>
> HTH, Tim
>
> _____
>
> From: Bob Nelson [mailto:nelsnjr@cox.net]
> Sent: Monday, June 06, 2005 4:06 PM
> To: ccie2be
> Subject: Re: STP- Port cost vs. Port-priority
>
> Tim:
>
> I labbed up each combination of settings as per your last reply. I have a
> 5-6 page report on the combinations, but thought I would shorten it here
> with text.
> Sorry, this may be a little long. If you have input, it would be most
> appreciated.
>
> Scenario: Configuring STP load sharing, with two 3550 switches and one
> designated the root for all vlans.
> The switches are connected together with two trunks on ports 23 and 24.
> The testing was done using the cost and port-priority parameters to
> determine what effect each had on the
> forwarding and block of the specified vlans.
>
> Results:
> 1. On the primary switch, no matter what the configuration, both ports 23
> and 24 were always forwarding.
> The cost parameters and the port-priority parameters changed from their
> default STP values when using either
> cost command or port-priority configuration, however the ports remained in
> forwarding status.
>
> 2. Using the cost commands on the primary switch changed the costs
> parameters for STP, but did not
> cause a load balancing condition as the only port that was blocking was
> port 24 on the secondary switch for all vlans.
> Adding the cost commands to the secondary switch caused the load
> balancing conditions to become effective
> where vlans 3-6 were blocked on port 23 of the secondary switch, but
> were forwarded on port 24
> Vlans 8-10 were forwarded on port 23 but were blocked on port 24.
>
> 3. Using the port priority commands on the secondary switch changed the
> priority parameters for STP, but did not
> cause a load balancing condition as the only port that was blocking was
> port 24 on the secondary for all vlans.
> Adding the port priority commands on the primary switched caused the
> load balancing conditions to become effective
> where vlans 3-6 were blocked on port 23 of the secondary switch, but
> were forwarded on port 24
> Vlans 8-10 were forwarded on port 23 but were blocked on port 24.
>
> Conclusions:
> Configuring the switches for load balancing must be done using the
> port-priority commands on the primary (root) switch
> or the cost commands on the secondary (non-primary) switch to cause a load
> balancing condition between the switches.
>
> The commands need only be placed on a single switch, in the case of a two
> switch configuration lab scenario, to create the
> required load balancing condition. Putting cost commands on the primary
> or
> port-priority commands on the non-primary,
> would be an incorrect configuration, even though there was no effect. It
> is
> up to the test candidate to determine which method,
> cost or port-priority, to use depending on the language in the task. The
> answer may be given earlier in the switching scenario,
> where the root switch is to be designated for particular or all vlans.
> When
> in doubt ask the proctor.
>
> In addition to using the cost and port priority commands, I can accomplish
> the same result by setting one switch as
> the primary for vlans 3-6 and the other switch primary for vlans 8-10. In
> this instance, port 23 on both switches
> forwards vlans 3-6 and port 24 on both switches forwards vlans 8-10.
> Again
> the lab will dictate which of the three
> methods to use.
>
> Tim, if you want to see the interface configs and the show spanning-tree
> commands for this, let me know
> as I have them in a Word document. The reason I was so concerned about
> learning this is that I attended
> a CCIE lab prep class and the instructor gave the following result for
> this
> question of load balancing.
> I did not agree with him, but he told me I was wrong. Thanks to you, I
> believe I have the correct
> answer. He had 2 CCIEs already, so I gave him credit for the CCIEs.
>
> From my studies of this, I think this is incorrect.
> Configure VLANs 4, 12, and 57 to use F0/23 and VLANs 111,112, and 113 to
> use
> F0/24.
>
> SW1
> Interface F0/23
> Spanning-tree vlan 4 port-priority 240
> Spanning-tree vlan 12 port-priority 240
> Spanning-tree vlan 57 port-priority 240
> Interface F0/24
> Spanning-tree vlan 111 port-priority 0
> Spanning-tree vlan 112 port-priority 0
> Spanning-tree vlan 113 port-priority 0
>
> SW2
> Interface F0/23
> Spanning-tree vlan 4 port-priority 0
> Spanning-tree vlan 12 port-priority 0
> Spanning-tree vlan 57 port-priority 0
> Interface F0/24
> Spanning-tree vlan 111 port-priority 240
> Spanning-tree vlan 112 port-priority 240
> Spanning-tree vlan 113 port-priority 240
>
>
> Thanks again!!
>
> Bob
> ----- Original Message -----
> From: "ccie2be" < <mailto:ccie2be@nyc.rr.com> ccie2be@nyc.rr.com>
> To: "'Bob Nelson'" < <mailto:nelsnjr@cox.net> nelsnjr@cox.net>
> Sent: Friday, June 03, 2005 2:01 PM
> Subject: RE: STP- Port cost vs. Port-priority
>
>> Bob,
>>
>> What I was describing isn't load-sharing. In my description, the ports
> that
>> would be forwarding would be forwarding for all vlan's and the ports that
>> are blocking are blocking all vlans.
>>
>> However, what I described can very easily be adapted to vlan
> load-balancing
>> because the same rules apply.
>>
>> For example, instead of changing the port cost for all vlans, just change
> it
>> for one or more vlans. Likewise, for port priority.
>>
>> Just remember, for priority (port or per-vlan) to have any affect, it
>> must
>> be changed on the upstream switch and for cost to have any effect, it
>> must
>> be changed on downstream switch.
>>
>> Here's why.
>>
>> The root switch will always have all it's port in forwarding mode. But,
> the
>> downstream must use the STP decision tree to determine which ports are
>> designated and which are blocked. Also, the root or upstream switch
>> sends
>> priority to the downstream switch but doesn't send cost. Cost is always
>> determined based on the incoming port, not the outgoing port.
>>
>> Now, review my notes for the STP decision tree.
>>
>> -Cost is determined the same way for STP as it is for IGP's: Cost = cost
> of
>> local int (going towards root) plus the cost of all int's pointing
> upstream
>> towards destination.
>> -If there are multiple trunks connecting the 2 Cat's & each trunk is
> allowed
>> to carry traffic for all vlan's (the default), STP will block at least
>> one
>> interface. Assume Cat1 is the root bridge for vlan X. Once Cat1 is set
> (or
>> elected) Root Bridge, Cat2 must decide which port is its root port to
> Cat1.
>> 1. Cat2 will compare the cost of each trunk interface (by def, they're
>> all equal)
>> 2. Cat2 will compare BID's from each trunk - of course, they're equal.
>> 3. Cat2 will then compare the port priority for each link sent from
>> Cat1.
>> 4. Last, Cat2 will use its own port's ID as the last tiebreaker
>>
>> -As a result of above process, if you have to config a particular link to
> be
>> in forwarding mode & you can't config Cat2 to do this, then change Port
>> Priority on Cat1.
>> -If the change can only be made on Cat2, lower port cost on Cat2. Notice
>> that if you're restricted to modifying either port cost or port priority,
>> that restriction determines on which Cat you have to make the change.
>> -Verify config with show spanning-tree [options].
>>
>> Please let me know if you find any discrepancies between what I say and
> what
>> your experimenting shows.
>>
>> HTH, Tim
>>
>>
>>
>> -----Original Message-----
>> From: Bob Nelson [mailto:nelsnjr@cox.net]
>> Sent: Friday, June 03, 2005 4:44 PM
>> To: ccie2be
>> Subject: Re: STP- Port cost vs. Port-priority
>>
>> Tim:
>>
>> I have been researching this topic for some time, and I do not
>> necessarily
>> understand your answer.
>> The usual case for this question seems to be the load sharing of traffic
>> between switches, where
>> you are given the requirement that vlan abc uses port one of the trunk
>> and
>> vlan xyz uses the other
>> port of the trunk.
>>
>> I have labbed this up with my 3550s and found no difference in the
>> functioning using port-priority or cost
>> from either the root switch or the secondary. Using vlans 3-6 and 8-10,
>> I
>> can make 3-6 prefer trunk fa0/23 and
>> 8-10 prefer trunk fa0/24 using either method. The correct ports are
> blocked
>> in either case. This example is
>> directly out of the 3550 DOC CD.
>>
>> Cisco does make a distinction between the two methods:
>> You configure load sharing on trunk ports by using STP port
>> priorities or STP path costs.
>> For load sharing using STP port priorities, both load-sharing
> links
>> must be connected to the same switch.
>> For load sharing using STP path costs, each load-sharing link can
> be
>> connected to the same switch or to two different switches
>>
>> Tim, in taking your answer literally, I would believe it to mean that I
>> could use the priority command from the root to the secondary,
>> but I would have to use the cost to configure this from the secondary to
> the
>> primary. However, it does not work, at least for me,
>> to do this. Can you elaborate on your answer a little more?
>>
>> Thanks,
>>
>> Bob
>> ----- Original Message -----
>> From: "ccie2be" < <mailto:ccie2be@nyc.rr.com> ccie2be@nyc.rr.com>
>> To: "'Sumit'" < <mailto:sumit.kumar@comcast.net>
>> sumit.kumar@comcast.net>;
> < <mailto:ccielab@groupstudy.com> ccielab@groupstudy.com>
>> Sent: Thursday, June 02, 2005 1:08 PM
>> Subject: RE: STP- Port cost vs. Port-priority
>>
>>
>> > Sumit,
>> >
>> > Which parameter you change depends on which switch you're making the
>> change.
>> >
>> > The port priority is sent by the upstream switch (root) to the
> downstream
>> > switch so changing this value on the downstream switch wouldn't do any
>> good.
>> >
>> > Likewise, changing cost on the upstream switch wouldn't affect the
>> selection
>> > of which port the downstream switch decides is the root port.
>> >
>> > Make sure you know this stuff. It's very important.
>> >
>> > HTH, Tim
>> >
>> >
>> > -----Original Message-----
>> > From: <mailto:nobody@groupstudy.com> nobody@groupstudy.com
> [mailto:nobody@groupstudy.com] On Behalf Of
>> > Sumit
>> > Sent: Thursday, June 02, 2005 3:27 PM
>> > To: <mailto:ccielab@groupstudy.com> ccielab@groupstudy.com
>> > Subject: STP- Port cost vs. Port-priority
>> >
>> > Mates,
>> >
>> > As I understand the "Spanning tree port cost " manipulation should be
>> able
>> > to make vlan traffic prefer a trunk port over another one.
>> > Is there any case we need to change "port-priority" unless changing
> port
>> > cost is restricted?
>> >
>> > thanks
>> > Sumit
>> >
>> > _______________________________________________________________________
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