Why BGP Route Refresh + Soft Reconfiguration Inbound are impossible at the same ?

[Tommy LE BALCH]

Hi,

Considering 2 BGP peers connected and capable of BGP route refresh.

As we know while we clear "in" a BGP peering on one side should send a BGP Route Refresh messages to the other peer neighbor.

But if the peering has been configured in "soft-reconfiguration inbound" the router won't send route refresh messages.

Route Refresh feature is usefull to get a confirmed advertisement from the peer.

Soft Inbound feature is also highly usefull to have the "recieved-route" list of advertisement even for what we don't learn.

These 2 usefull features conceptualy do not seem to be incompatible !!

So why this choice from Cisco to disable "BGP Route-refresh messages" when we enable "soft-reconfiguration inbound" ?

Can you explain the idea ?

Thank you.

[paul driver]

Hello

BGP RR is a better way to request bgp updates on the bgp router then using soft- reconfiguration, if RR isn't supported then you can use soft- reconfiguration but using the latter to obtain the same information the bgp router has to store unedited routing information to poll against and if you are receiving very large bgp prefixes then it can be very resource intensive for the router.

To check is you bgp peers support Route Refresh  - (sh ip bgp neighbors x.x.x.x | s neigh cap )

[Tommy LE BALCH]

Thank you Paul for your response.

But still my question is :

Why Cisco decided these 2 features to be exclusive ?

Why couldn't we have both feature at the same time ?

• Having a local database to store all unedited routing informations (so that the show  "received-route" works)

and

• Having a "Route Refresh" sent when we launch a clear bgp in ?

And because it is impossible, the next question is :

If I disable the soft-reconfiguration how could I know le global list of "recieved-route" advertised by the neighbor peer ?

This "received-route" information is very essential in our daily job because we have to know what a neighbor do advertise before we may accept and learn it.

Thanks.

[paul driver]

Hello

---

@Tommy LE BALCH wrote:

Thank you Paul for your response.

But still my question is :

Why Cisco decided these 2 features to be exclusive ?

Why couldn't we have both feature at the same time ?

• Having a local database to store all unedited routing informations (so that the show  "received-route" works)

and

• Having a "Route Refresh" sent when we launch a clear bgp in ?.

---

Not so sure why you would want to if BGP RR capability is being advertised and is applicable to be used between peers

As stated soft-reconfiguration is resource intensive to the router, and RR is a much better alternative for requesting the  readvertisement of any prefix changes advertised from it peer in a non disruptive way.

And because it is impossible, the next question is :

If I disable the soft-reconfiguration how could I know le global list of "recieved-route" advertised by the neighbor peer ?

This "received-route" information is very essential in our daily job because we have to know what a neighbor do advertise before we may accept and learn it.

---

sh ip bgp neighbors x.x.x.x routes    <-----show received routes
sh ip bgp neighbors x.x.x.x advertised-routes  <-----show advertised routes
clear ip bgp x.x.x.x soft in  <initiates a  RR request

[Tommy LE BALCH]

Thank you Paul

| Not so sure why you would want to if BGP RR capability is being advertised and is applicable to be used between peers

Ok I will elaborate.

Reminder : 

sh ip bgp neighbors x.x.x.x routes <-- Routes received and learnt in BGP table.

sh ip bgp neighbors x.x.x.x received-routes <-- All routes received even those filtered (not learnt in BGP table)

So why do we need to know ALL routes received (learnt AND not learnt) ? :

We have a network AS, and around our network AS we have many other AS clients that are connecting to us via eBGP (we are transit between all of them)

Then we need to know all the routes each AS client is trying to send to us.

Based on this information we can then decide which route we going to accept or not in our BGP table.

This kind of information is mandatory for us, because without the "received-routes" command we would be blind about our clients' AS routing tables.

So of course we could ask our client to launch an "advertised-routes" on their side every time we need it, but you know process....reply time : 2-5 days, if you manage to get the client guy who understand what you need and have access to routers. So, possible but, no time, no way...

-> With "received-routes" we get the same information instantly !!!

| soft-reconfiguration is resource intensive to the router

Not a problem in our network, it is private, this is not internet, in our corporation the number of route is around 10.000, no threat for our ASR1013...

Why we need route refresh ? :

We want to be sure that our "received-routes" soft-database is up to date by "asking" our clients neighbor BGP router to resend their full advertisment.

Tommy

[paul driver]

Hello

Are your routers RR capable?, if so by default then RR is active already, You also have soft-reconfiguration applied so if you perform either of these two commands then you should obtain the same output.

sh ip bgp neighbors x.x..x routes 
sh ip bgp neighbors x.x..x received-routes

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@Tommy LE BALCH wrote:

Why we need route refresh ? :

We want to be sure that our "received-routes" soft-database is up to date by "asking" our clients neighbor BGP router to resend their full advertisment.

---

As I have said my understanding RR is a much much better alternative to soft-reconfiguration and it will provide you with the same output bit it at less expense to the routers cpu/memory resources , but if you happy to keep soft-reconfiguration  active when your bgp rtrs are RR capable so be it.