[110073] in North American Network Operators' Group
Re: What is the most standard subnet length on internet
daemon@ATHENA.MIT.EDU (Alex H. Ryu)
Tue Dec 23 09:26:15 2008
Date: Tue, 23 Dec 2008 08:25:40 -0600
From: "Alex H. Ryu" <r.hyunseog@ieee.org>
To: Grzegorz Janoszka <Grzegorz@Janoszka.pl>
In-Reply-To: <4950EC4C.5080504@Janoszka.pl>
Cc: nanog list <nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
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Also one of the reason why not putting default route may be because of
recursive lookup from routing table.
If you have multi-homed site within your network with static route, and
if you use next-hop IP address instead of named interface, you will see
the problem when you have default route in routing table.
For an example, if you have "ip route 1.0.0.0 255.0.0.0 2.2.2.2".
If the interface for 2.2.2.2 is down, 1.0.0.0/8 will be still be in the
routing table because 2.2.2.2 can be reached via default route
(0.0.0.0/0) from routing table recursive lookup.
Therefore the traffic for 1.0.0.0/8 will be forwarded to "0.0.0.0/0"
next-hop ip address, and customer fail-over scenario will not be working
at all.
Only way to resolve this problem is... Actually three...
1) Use named interface such as "serial 1/0" instead of "x.x.x.x" IP
next-hop address.
But sometimes this is not an option if you use ethernet circuit or
something like Broadcast or NBMA network.
2) Use BGP with private ASN...
3) Do not install default route in your routing table
Grzegorz Janoszka wrote:
> Nathan Ward wrote:
>>>> Let me rephrase; Are there people who are filtering /24s received from
>>>> eBGP peers who do not have a default route?
>>>
>>> of course.
>>
>> Curiously, it was really meant as a rhetorical question where the
>> answer was "no".
>>
>> Why are people doing this? Are they lacking clue, or, is there some
>> reasonable purpose?
>
> Memory mostly I think. /24 prefixes are ~ the half of all prefixes,
> but they cover only a small percent of the address space.
> If your router has > 6 full BGP sessions, you can filter /24 on half
> of them, your memory usage will drop significantly.
>
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