From: ccie2be (ccie2be@nyc.rr.com)
Date: Sun Apr 17 2005 - 16:35:39 GMT-3
Alsontra,
I guess the only reason to use the verify-availability option with the set
ip default next-hop command is to save resource. But, I'm not sure how
much, if any, resource is actually saved when this option is used.
It appears the router will expend cpu resource with cdp when you use this
option instead of arping when you don't. And, you might save bandwidth but
again I don't know how much bandwidth would actually be saved.
Imagine you're using a low bandwidth f/r link. (On a high bandwidth
ethernet, you probably care if you're sending arp's continuously).
With the verify-avail option, u need to make sure cdp is enabled which it
might not be depending on the type of f/r int you're using. So, instead of
sending user traffic packets across the f/r link and having them dropped,
you're sending cdp frames across the link and dropping the packets locally.
So, I think the bottom line is this:
If the default next-hop is NOT available, packet to that next-hop will be
dropped no matter what.
The only issue left is where those packets get dropped on the router
checking availability or later.
What do you think? Does this sound right to you?
TIA, Tim
-----Original Message-----
From: alsontra [mailto:alsontra@hotmail.com]
Sent: Sunday, April 17, 2005 10:42 AM
To: 'ccie2be'; 'Group Study'
Subject: RE: What's the point of this?
Sorry, I'm not sure I understood your question.
>If the preferred next-hop isn't available, the router will check if the
>DEFAULT is available.
>If the DEFAULT isn't available, the packet is dropped whether you check if
>the DEFAULT is available or not. So, what's the point?
I think what this means is your not going to senselessly send traffic to a
nexthop that is unavailable. The " verify-availability " option doesn't
really seem to make a good amount of sense unless your using it in the "set
ip next-hop " snytax to make a more granular or discriminatory route-map.
In addition, this may be a feature that reclaims some of the routers
resources. Note the following:
"If the router is policy routing packets to the next hop and the next hop
happens to be down, the router will try unsuccessfully to use Address
Resolution Protocol (ARP) for the next hop (which is down). This behavior
will continue forever.
To prevent this situation from occurring, use this command to configure the
router to first verify that the next hops of the route map are the router's
CDP neighbors before routing to that next hop. "
http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123cgcr/iprr
p_r/ip2_s1g.htm#wp1038423
In a high traffic environment, this feature may save you some CPU. (if cdp
is an option)
HTH,
Al
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
ccie2be
Sent: Sunday, April 17, 2005 7:03 AM
To: 'alsontra'; 'Group Study'
Subject: RE: What's the point of this?
Alsontra,
Thanks for getting back to me.
Although I appreciate your response, it didn't really address my question.
My question was about the command:
set ip DEFAULT next-hop verify-availability
In this command, we're using DEFAULT and verify-availability in the same
command. In other words, we are telling the router to verify the default
before we use it. So, what happens when the DEFAULT is not available?
By definition, a DEFAULT is what's used as a last resort. In your example,
you had a preferred next-hop which would be used if it's available and a
default next-hop if the preferred next-hop isn't available. That makes
sense.
Now, what happens when you add this command to your config.
set ip next-hop 1.1.1.1
set ip next-hop verify-availability
set ip default next-hop 2.2.2.2
set ip DEFAULT next-hop verify-availability
If the preferred next-hop isn't available, the router will check if the
DEFAULT is available.
If the DEFAULT isn't available, the packet is dropped whether you check if
the DEFAULT is available or not. So, what's the point?
The only difference I can see by verifying if the DEFAULT is available when
it's not is where the packet is dropped. But, either way the packet is
dropped.
Maybe that's all there is to that but I would appreciate any comments.
TIA, Tim
-----Original Message-----
From: alsontra [mailto:alsontra@hotmail.com]
Sent: Saturday, April 16, 2005 1:44 PM
To: 'ccie2be'; 'Group Study'
Subject: RE: What's the point of this?
Tim,
There are at least two situations that demonstrate the point of this option.
(at least from my humble perspective.)
#1. Using only policy routing to create a DDR type solution.
set ip next-hop 1.1.1.1
set ip next-hop verify-availability
set ip default next-hop 2.2.2.2
In this form, the router will policy route all packets to 1.1.1.1 as long as
the adjacent router or 1.1.1.1 route destination is in the CDP table.
(set ip next-hop verify-availability uses cdp to verify adjacency)
When this route is not verifiable via CDP, all packets will be routed to
2.2.2.2... Thereby creating your backup solution.
#2. You can combine policy routing and normal routing to produce a hybrid
backup situation.
set ip next-hop 1.1.1.1
set ip next-hop verify-availability
ip route 0.0.0.0 0.0.0.0 2.2.2.2
Once again, if the route to 1.1.1.1 is not verifiable all packets get routed
to 2.2.2.2. (pls note policy routing happens before normal routing)
(shooting from the hip)
HTH,
Alsontra
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
ccie2be
Sent: Friday, April 15, 2005 11:53 AM
To: Group Study
Subject: What's the point of this?
Hi guys,
That prior question about conditional redist static got me thinking about
the verify-availability command.
In the Command Reference I found another command which also uses the
verify-availability.
set ip default next-hop x.x.x.x
set ip default next-hop verify-availability
What's the point of this?
Just think about this. If a certain next-hop is the default next-hop that
means this next-hop should be used if there's no other choice.
But, if there's no other choice, what happens if this default next-hop is
NOT available?
Doesn't this seem to be a contradiction?
Can someone provide an example where this command should be used and what
happens if the default next-hop is NOT available?
TIA, Tim
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