[afnog] local pref issue with HE

[Patrick Okui]

On 23 Dec 2015, at 10:16, Tarig Yassin wrote:

> Hi Afnog
>
> We're trying to loadshare two uplinks from different upstream 
> providers using AS-PREPEND.
>
> the issue is how matter we prepend the 2nd link still the 1st link is 
> preferred.
>
> To clarify more, despite the 2nd route below has
> longer AS-PATH,
>
> but still HE prefers the 1st route, I try to
> understand the reason,and
>
> why the prepend won't work and why HE changes the
> value of Local-pref.


Local pref is set by the AS that receives the route and is not passed 
onto the next AS. I.e if you set a local pref of 200 for a route that 
setting only applies within your AS. Other ASes are free to do what they 
want.


>
>
>
>
>> 41.202.160.0/24 AS-PATH: 1299, 2914,33788                        
>> Local-pref:70
>
>> 41.202.160.0/24 AS-PATH:
> 15412,33788,33788,33788
> Local-pref:100


So I see that this block in whois is part of 41.202.160.0/19. One thing 
you could do is

1. announce the full 41.202.160.0/19 on both links.
2. Only announce the 41.202.160.0/24 on the link you’d prefer to 
attract traffic to.

The reason is irrespective of routing protocol, the more specific route 
(longer prefix in this case /24) will be preferred to the less specific 
one (in this case /19).

If the /24 disappears (because for example the link has gone down) then 
the internet will fall back to routing to that /24 address space using 
the remaining /19 announcement in the routing table.

If you do it this way consider load balancing with blocks which prefixes 
shorter than /24. say /20s or /21s for example. This avoids you having 
to list each and every /24 in your announcements and helps reduce the 
overall size of the routing table.

--
patrick