From: Joe (joe.clyde@utah.edu)
Date: Fri Nov 14 2008 - 18:03:07 ARST
Thank you for all the responses. A real world scenario would be this....we
have several class B addresses in our network. We generate summary
addresses in BGP which obviously create supernets pointing to null 0 (/139s
and some /149s). We then have our customers connected on the edge (lets say
a small customer, /24 network like my example). Well the interface they are
connected to goes down, the route to them is still a valid route due to the
summary. So neighbor router sends traffic to this router due to the more
specific route, only to have the route send it back based on the summary
address( routing loop). Now we have seen this issue on some routes (those
where the summary route is a /14, but where the summary address is a /13 the
static route is removed from the table and thus no routing loop, saturated
link, et....) *On a side note, with the way MPLS TE is configured on our
backbone, the MPLS taf TTL is always new (doesn9t inherit the router TTL)
and thus creates a storm. I know those setting can be changed...but it9s a
little more complicated. Again a static with a next hop and interface fixes
the problem as well. The solution isn9t so much the concern, rather the
behavior and understanding it.
So while I understand the answers you guys are giving, I have seen the
route disappear on a /13 route but not a /14 route....To me it would make
sense that ANY valid route, minus a default, could be used for
recursion...but doesn9t appear to be the case. Didn9t know if there was any
rhyme or reason to it. Again, thank you everyone for your input.
On 11/14/08 12:44 PM, "Pavel Bykov" <slidersv@gmail.com> wrote:
> Yes, correct.
>
> On Fri, Nov 14, 2008 at 8:37 PM, Hyunseog Ryu <r.hyunseog@ieee.org> wrote:
>> As long as it can find any network containing next-hop IP address from
>> routing table.
>> So if you have default route (0.0.0.0/0 <http://0.0.0.0/0> ), it will be
hit
>> as very last
>> resort.
>> Therefore it will pick up the next-hop ip address from default route to
>> replace "down"ed interface ip address next-hop ip address.
>>
>>
>>
>> Joe wrote:
>>> > Ya, this is what I have seen. Again, at what point is the route NOT
>>> specific
>>> > enough for the static to disappear and is that something you can
>>> configure?
>>> >
>>> >
>>> >
>>> > On 11/14/08 11:02 AM, "Hyunseog Ryu" <r.hyunseog@ieee.org> wrote:
>>> >
>>> >
>>>> >> We had this issue in the past.
>>>> >> If int X fails, 11.11.11.0/24 <http://11.11.11.0/24> will still
exist
>>>> in routing table because
>>>> >> of recursive lookup.
>>>> >>
>>>> >> That's why you have to use interface name instead of next-hop IP
address
>>>> >> if possible.
>>>> >>
>>>> >>
>>>> >>
>>>> >> Joe wrote:
>>>> >>
>>>>> >>> I have a question on recursive lookups, hopefully I can phrase it in
a
way
>>>>> >>> that makes sense. Thanks
>>>>> >>>
>>>>> >>> If I have a static route like:
>>>>> >>> Ip route 11.11.11.0 <http://11.11.11.0> 255.255.255.0
>>>>> <http://255.255.255.0> 10.10.10.1 <http://10.10.10.1>
>>>>> >>>
>>>>> >>> And I have a route table that looks something like this:
>>>>> >>> C 10.10.10.0/24 <http://10.10.10.0/24> via int X
>>>>> >>> O 10.10.0.0/16 <http://10.10.0.0/16> via x.x.x.x
>>>>> >>> E 10.0.0.0/8 <http://10.0.0.0/8> via x.x.x.x
>>>>> >>> B 0.0.0.0 <http://0.0.0.0>
>>>>> >>>
>>>>> >>> Hopefully you get the idea of the route table...doesn't matter how
the
>>>>> >>> routes are learned... just the idea of multiple routes (each less
>>>>> specific).
>>>>> >>>
>>>>> >>> What happens when I lose my directly connected interface? How
>>>>> "un-specific"
>>>>> >>> of a route will the router use for a recursive lookup? *I know in
this
>>>>> >>> example if I lost the /24 then the next most specific /16 in this
case
is
>>>>> >>> next in line but at what point will the router say it won't use a
>>>>> valid
>>>>> >>> route (as far as the route table) for a recursive lookup?
>>>>> >>>
>>>>> >>> I don't believe it will ever use a default route but it seems like
>>>>> I've seen
>>>>> >>> it try to recurse off a /14 route. In my opinion that is an
>>>>> undesirable
>>>>> >>> behavior (let's say you have a summary address to null 0, you
wouldn't
>>>>> want
>>>>> >>> you statics still showing up as accessible because the next hop is
>>>>> reachable
>>>>> >>> via a /16 net lets say). I know, I can avoid that issue by adding a
>>>>> specific
>>>>> >>> interface to the static route, but would still like to know at what
>>>>> point do
>>>>> >>> I not do a recursive lookup. Thanks for the help
>>>>> >>>
>>>>> >>>
>>>>> >>> Blogs and organic groups at http://www.ccie.net
>>>>> >>>
>>>>> >>>
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-- Joe Clyde Network Engineer Utah Education Network jclyde@uen.org 801-883-4868 (Desk)Blogs and organic groups at http://www.ccie.net
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