View source for Manual:BGP soft reconfiguration alternatives in RouterOS

==What is soft reconfiguration?==

When a route is received from a dynamic routing protocol, it is passed through routing filters. These filters may change some attributes of the route
or discard it altogether.

When the routing filters changes, they must be reapplied to routes from BGP (and other protocols, but we are focusing on BGP here). One way to do is reset BGP session, that is, tear down the connection with peer and re-establish it again. The disadvantages of this approach are obvious.

Soft reconfiguration means that filtering policy can be reapplied after a change without session reset. For RouterOS, both dynamic and static variants are possible.

==Static soft-reconfiguration==
What could be the effect of routing filters to a route? There are two possible cases.

''CASE 1: Filters only change some attributes of the route.''
The orginal received attributes always are stored with the route. They are use to calculate new routing table attributes if filters changes. This process is trigerred automatically.

''CASE 2: The route is discarded by filters.''
If the route is discarded, original attributes are not saved and information about it is lost. To avoid that, use '''action=reject''' in filters instead of '''action=discard'''. Now the route is saved, but is not eligible to become active (that is, it will not be installed in kernel routing table or redistributed to protocols). 

* + Router does not lose routing information, because session is not reset.
* - Memory overhead for storing rejected routes.

'''Example:'''

Original configuration (routes are rejected):
 [admin@A] > routing filter add chain=bgp-in action=reject prefix=4.0.0.0/8 prefix-length=8-32
 [admin@A] > routing bgp peer set peer1 in-filter=bgp-in
 [admin@A] > ip route print
 Flags: X - disabled, A - active, D - dynamic,
 C - connect, S - static, r - rip, b - bgp, o - ospf,
 B - blackhole, U - unreachable, P - prohibit
  #      DST-ADDRESS        PREF-SRC        G GATEWAY         DISTANCE INTERFACE
  0 A S  0.0.0.0/0                            10.0.0.1        1        ether1
  1 ADb  3.0.0.0/8                            192.65.184.3    200      ether1
  2  Db  4.0.0.0/8                            192.65.184.3    20       ether1
  3  Db  4.21.104.0/24                        192.65.184.3    20       ether1
  4  Db  4.21.112.0/23                        192.65.184.3    20       ether1
  5  Db  4.21.130.0/23                        192.65.184.3    20       ether1

Change filters to less restrictive:
 [admin@A] > routing filter disable 0
 [admin@A] > ip route pr
 Flags: X - disabled, A - active, D - dynamic,
 C - connect, S - static, r - rip, b - bgp, o - ospf,
 B - blackhole, U - unreachable, P - prohibit
  #      DST-ADDRESS        PREF-SRC        G GATEWAY         DISTANCE INTERFACE
  0 A S  0.0.0.0/0                            10.0.0.1        1        ether1
  1 ADb  3.0.0.0/8                            192.65.184.3    200      ether1
  2 ADb  4.0.0.0/8                            192.65.184.3    200      ether1
  3 ADb  4.21.104.0/24                        192.65.184.3    200      ether1
  4 ADb  4.21.112.0/23                        192.65.184.3    200      ether1
  5 ADb  4.21.130.0/23                        192.65.184.3    200      ether1

==Dynamic soft-reconfiguration==

In this case, your BGP routing peer must support route refresh capability. Enter '''/routing bgp peer print status''' in CLI to check this.
* + No additional memory is used
* - Peer must support this capability.
* - It's not done automatically. You must issue '''/routing bgp peer refresh''' command after changes in filters are finished.

'''Example:'''

Original configuration (routes are discarded):
 [admin@A] > routing filter add chain=bgp-in action=reject prefix=4.0.0.0/8 prefix-length=8-32
 [admin@A] > ip route pr
 Flags: X - disabled, A - active, D - dynamic,
 C - connect, S - static, r - rip, b - bgp, o - ospf,
 B - blackhole, U - unreachable, P - prohibit
  #      DST-ADDRESS        PREF-SRC        G GATEWAY         DISTANCE INTERFACE
  0 A S  0.0.0.0/0                            10.0.0.1        1        ether1
  1 ADb  3.0.0.0/8                            192.65.184.3    200      ether1

Change filters to less restrictive and send refresh request:
 [admin@A] > routing filter disable 0
 [admin@A] > routing bgp peer refresh peer1
 [admin@A] > ip route pr
 Flags: X - disabled, A - active, D - dynamic,
 C - connect, S - static, r - rip, b - bgp, o - ospf,
 B - blackhole, U - unreachable, P - prohibit
  #      DST-ADDRESS        PREF-SRC        G GATEWAY         DISTANCE INTERFACE
  0 A S  0.0.0.0/0                            10.0.0.1        1        ether1
  1 ADb  3.0.0.0/8                            192.65.184.3    200      ether1
  2 ADb  4.0.0.0/8                            192.65.184.3    200      ether1
  3 ADb  4.21.104.0/24                        192.65.184.3    200      ether1
  4 ADb  4.21.112.0/23                        192.65.184.3    200      ether1

==Summary==
* Do nothing unless the filter change changes discard status for some prefixes.
* Use '''routing bgp peer refresh''' comand after filter change if peer supports this capability.
* Use '''action=reject''' in filters in other cases.

[[Category: Routing]]