[c-nsp] aggregate-address with 2 neighbours

[Adam Vitkovsky]

> CiscoNSP List
> Sent: Wednesday, September 28, 2016 11:01 AM
>
> Hi Everyone,
>
>
> If we have a customer with 2 sites, that has there own /24 that they have
> split into 2 x /25's - Each site advertises its /25 to us (In a single VRF)...we then
> want to advertise the aggregate /24 to our upstreams....so, to achieve this
> we would need to use (under address-family ipv4 vrf foo) aggregate-address
> xxx.xxx.xxx.xxx 255.255.255.0 as-set summary only  (So customers AS is
> maintained, and the smaller prefixes are filtered from being re-advertised to
> upstream)....I haven't had a chance to lab this up, so Im not 100% sure of the
> behaviour of aggregate-address....when our PE receives one/both of the
> /25s via bgp, it aggregates them into the /24 - Im assuming(hoping), it also
> keeps the more specific prefix in bgp also (And in fib), so that routing for the
> /25s to the correct sites is maintained?
>
Yes as long as the PE will have at least one contributing /25s it will generate and advertise the summary route.

Yes after the summarization the more specific routes will remain in your PE's BGP table and FIB and as a matter of fact only the more specific /25 routes will be used to route traffic towards the customer.

This however can cause problems if one of the customer's links goes down -as one of the /25 routes disappears.
It can be solved by asking the customer to advertise both /25 routes via both links (with different local-pref)
Or by advertising the /24 in addition to a given /25 from each link.
So that in case of one link failure the other link can take over traffic for the affected /25.

adam




        Adam Vitkovsky
        IP Engineer

T:      0333 006 5936
E:      Adam.Vitkovsky at gamma.co.uk
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