[c-nsp] 2851 and full BGP

[Paul Cosgrove]

Forgot to cc the list on this earlier email.

Paul Cosgrove wrote:
> Hi Chuck,
> 
> Indeed it is apparently more than that: Jay mentioned receiving 20,000 
> routes before he sees the issue, so I guess about 75%.  I had similar 
> thoughts about this but wasn't (and still am not) sure how frequently in 
> practice BGP with a full table is likely to have to send large updates.
> 
> My (admittedly basic) understanding is that individual update messages 
> contain details about prefixes which share the same attributes.  If the 
> attributes are different, different update messages will be used.
> 
> If I have this right, the number of update messages will vary according 
> to the number of distinct attribute sets, and the size of each update 
> varies according to the number of NLRI which have those particular 
> attributes.
> 
> This makes me think that MTU issues could indeed occur at any point 
> during the update process.  A software bug might indeed turn out to be 
> the cause, but I wouldn't rule MTU issues out at this stage.
> 
> Paul.
> 
> 
> Church, Charles wrote:
>> Oh, yeah.  Sorry, I didn't catch the 'WAN' part of it the first time.
>> That does make MTU a possibility.  But didn't he get like 20% of his
>> routes before the error message?  Since it was 12.4(20)T (pretty
>> bleeding edge), I'd lean towards that still.  I'd think that an MTU
>> problem would show up way before you got to 20%.  Does BGP set the DF
>> bit?
>>
>> Chuck
>>
>> -----Original Message-----
>> From: Paul Cosgrove [mailto:paul.cosgrove at heanet.ie] Sent: Monday, 
>> August 11, 2008 4:33 AM
>> To: Church, Charles
>> Cc: mtinka at globaltransit.net; cisco-nsp at puck.nether.net
>> Subject: Re: [c-nsp] 2851 and full BGP
>>
>>
>> Hi Chuck,
>>
>> Jay will be able to clarify, but I took the following to mean that the 
>> two are separated via third party infrastructure: "two 2851s connected 
>> to each other over gigabit Ethernet WAN".
>>
>> May well be a bug though.
>>
>> Paul.
>>
>> Church, Charles wrote:
>>> Wasn't the original problem the iBGP connection over his own network?
>> Sounds like a bug more than anything else.
>>> Chuck
>>>
>>> ----- Original Message -----
>>> From: cisco-nsp-bounces at puck.nether.net
>> <cisco-nsp-bounces at puck.nether.net>
>>> To: mtinka at globaltransit.net <mtinka at globaltransit.net>
>>> Cc: cisco-nsp at puck.nether.net <cisco-nsp at puck.nether.net>
>>> Sent: Sun Aug 10 15:52:03 2008
>>> Subject: Re: [c-nsp] 2851 and full BGP
>>>
>>>
>>> Keep in mind that if the peerings are not between directly connected
>> IP,
>>> disabling PMTUd for BGP will cause it to use an MSS of 536 bytes.
>>>
>>> You could check the achievable MTU using extended pings with the DF
>> bit
>>> set, and compare it with the segment size listed by BGP before you 
>>> decide whether to make that change.
>>>
>>> Paul.
>>>
>>> Mark Tinka wrote:
>>>> On Saturday 09 August 2008 10:28:40 Jay Nakamura wrote:
>>>>
>>>>  
>>>>> Any ideas on what could be causing this issue?  Is there
>>>>> a better IOS version to use?
>>>>>     
>>>> Sounds like an MTU issue.
>>>>
>>>> Try disabling TCP PMTUd for BGP and see if that helps:
>>>>
>>>> router bgp 1234
>>>>  no bgp transport path-mtu-discovery
>>>>
>>>> If that works, consider checking with your provider on the supported 
>>>> MTU, end-to-end, and adjust your interface MTU if it helps.
>>>>
>>>> Cheers,
>>>>
>>>> Mark.
>>>>  
>> ------------------------------------------------------------------------
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>>
>>
> 
> 


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