BGP Route Refresh

[daudparvez]

hi all,

CISCO VOL 1 FOR BGP states that whenever we make changes in a BGP Inbound of Outbound filter, we must either use soft reset or hard reset to implement these changes.

However, i have noticed that once we make changes in a filter, these are automatically implemented after about 60 seconds.

Can some one please explain this??

BR

[Peter Paluch]

Hi Daud,

It seems that what you observed is valid only for outgoing filters. I've made an experiment using both inbound and outbound prefix-list. Only the application of an outbound prefix-list resulted in new updates being sent automatically after 60 seconds. For inbound prefix-list, I needed to use the clear ip bgp * in command. I did not use the Soft Reconfiguration Inbound feature - I have relied on the Route Refresh feature instead.

Why is this taking place - I am not sure myself. It seems to me that it has to do with the BGP walker process that runs every 60 seconds and obviously repopulates the Adj-RIB-Out for each neighbor according to the current filters and outbound policies. If there are any changes, they will obviously be sent to the appropriate neighbors.

Perhaps somebody else (Edison? Are you reading this thread?) could provide his/her insight.

Best regards,

Peter

[Amit Singh]

Hi Daud,


What you have observed is correct. With Route Refersh capabilities added from 12.0 version to the BGP, both the inbound and outbound filters are automatically updated. For this to work both the neghbours should be route refresh capable. If one of the neigbour is not capable, you have to implement the " soft reconfiguration inbound feature.Both the features are mutually exclusive and if you enable the " software inbound" configuration the dynamic route refresh gets disable.

The route refresh feature is a replacement for the soft reconfiguration feature. Route refresh is a capability that is negotiated at session initiation. The route refresh feature allows a BGP router to request that a remote peer resend its BGP Adj-RIB-Out.  This allows the BGP router to reapply the inbound policy.

With the route refresh feature available since the 12.0 release, inbound policy changes are updated dynamically to peers, so inbound soft reconfiguration is no longer required.
The route refresh feature is on automatically for supported releases. To verify whether  it is supported, execute the show ip bgp neighbor command.

Soft reconfiguration outbound does not require any additional resource. The BGP router can process the Adj-RIB-Loc through the outbound policy for the particular peer, creating
a new Adj-RIB-Out. The remote peer can be updated by any changes with BGP Update messages.

Here are the consequences:

Memory utilization will go down as each route that was rejected won't be stored in the memory

Bandwidth utilization could go up, as route-refresh would trigger the peers to send the routes again for processing

Soft-config inbound allows you to use the command "show ip bgp neighbor IP.ADDR received-routes" which is a key troubleshooting command in BGP. By forgoing the
soft-config inbound, you lose the ability to see what the peer sent.

Check this link for more info.


http://www.cisco.com/en/US/products/ps6599/products_data_sheet09186a0080087b3a.html
 
Hope this helps.


Cheers,
-amit singh

[Peter Paluch]

Hello Amit,

Thank you very much for joining this thread!

With Route Refersh capabilities  added from 12.0 version to the BGP, both the inbound and outbound  filters are automatically updated.

Hmm, this does not align to my observation. I have seen only the out filter to be applied automatically. The in filter required a manual usage of clear ip bgp in to come into effect. Are you sure that this automation applies to inbound filters?

I've tested this on C2691 12.4(15)T13 Advanced IP Services.

Best regards,

Peter

[Edison Ortiz]

Peter,

You are correct and the link Amit posted supports your findings.

The Route Refresh feature eliminates storing the entire BGP table in the router's memory but does not automatically update the BGP table. I believe the right term, also used in the document, is that it dynamically updates the BGP table.

The clear ip bgp in command is still needed. You can also trigger an update on the remote router with the clear ip bgp out command w/o the local router needing the clear ip bgp in.

[daudparvez]

Thank you all for your help,

My point to clarify is that, how the BGP table is actually automatically updated after 60 seconds when some changes to the Outbound filter against some neighbor is made? Does the IOS scans the filter after every 60 seconds due to some of it's other feature?

Also, when changes are made to an Inbound filter, the changes are not automatically implemented.

Best Regards,