[j-nsp] BGP "Holdtime", " Active Holdtime" and "Preference" values

[Stefan Fouant]

On 8/9/2011 7:55 AM, Martin T wrote:
> Hi,
> in case one has following settings active with it's BGP peer:
>
>    Holdtime: 90 Preference: 170
>    Active Holdtime: 90
>    Keepalive Interval: 30
>
> ..then what do they mean? As I understand, "Holdtime" is the maximum
> number of seconds allowed to elapse between the time that a BGP system
> receives successive keepalive or update messages from a peer. So if
> the holdtime is configured to 90s, the "Holdtime" value under "show
> bgp neighbor I.I.P.P" doesn't change, does it? How is "Active
> Holdtime" different from "Holdtime"? And what does this "Preference"
> mean?

Holdtime is the configured holdtime on the local device, whereas Active 
Holdtime is the negotiated holdtime between the two peers, which should 
be the minimum of the two peers holdtime configuration.

Preference is the value we assign to BGP routes learned from this 
neighbor, which in this case is 170, the default.  Preference is the 
equivalent of Administrative Distance in Cisco, and allows the router to 
determine which route to prefer should the identical route be learned 
from multiple sources (ex: RIP[100] vs. BGP[170]).

> Last but not least- in case of the configuration above, is it possible
> that the connection between peers is down for example 80 seconds and
> then comes back up, but as holdtime is set to 90s, the session stays
> up?

Yes.

Stefan Fouant
JNCIE-ER, JNCIE-M, JNCIE-SEC, JNCI
Technical Trainer, Juniper Networks
http://www.shortestpathfirst.net
http://www.twitter.com/sfouant