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RFC 5243 Database Exchange Summary Optimization OSPF

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meddane
Frequent Contributor

OSPF RFC.png

To optimize the database description (DBD) packet exchange between two OSPF neighbors, use the compatible rfc5243 in router configuration mode or address family configuration mode for OSPFv3 AF. To disable RFC5243 optimization, use the no form of this command.

 

This command was integrated into Cisco IOS Release 15.5(2)T as shown below.

 

R1(config)#router osp 1

R1(config-router)#compatible ?

  rfc1583  compatible with RFC 1583

  rfc1587  compatible with RFC 1587

  rfc5243  supports DBD exchange optimization

 

R1(config-router)#

 

The compatible rfc5243 command optimizes the DBD packet exchange between two OSPF neighbors forming adjacency. This optimization helps to avoid announcing the DBD packets to neighbor link state advertisements (LSA) that have already received the announcements. This ensures that the neighbor's link state database (LSDB) receives only the newer instance of the LSA.

 

The compatible rfc5243 command can be used only in the router configuration mode for OSPFv2. For OSPFv3, this command can be used in both the router configuration and address-family configuration modes.

 

By default the RFC 5243 is enabled for OSPFv2 and OSPFv3 as shown below.

 

R1#sh ip os | s RFC

 Supports NSSA (compatible with RFC 3101)

 Supports Database Exchange Summary List Optimization (RFC 5243)

R1#

 

R1#sh ipv os | s RFC

 Supports NSSA (compatible with RFC 3101)

 Supports Database Exchange Summary List Optimization (RFC 5243)

 RFC1583 compatibility enabled

    Area BACKBONE(0)

 

R1#

 

The following example shows how to disable the default RFC 5243 optimization for OSPFv2:

 

R1(config)#router osp 1

R1(config-router)#no compatible rfc5243

 

The following example shows how to disable the default RFC 5243 optimization for OSPFv3:

 

R1(config)#ipv router osp 1

R1(config-rtr)#no compatible rfc5243

 

Verify that the RFC 5243 is disabled.

 

R1#sh ip os | s RFC

 Supports NSSA (compatible with RFC 3101)

 Does not support Database Exchange Summary List Optimization (RFC 5243)

R1#

 

R1#sh ipv os | s RFC

 Supports NSSA (compatible with RFC 3101)

 Does not support Database Exchange Summary List Optimization (RFC 5243)

 RFC1583 compatibility enabled

    Area BACKBONE(0)

 

R1#

 

The RFC 5243 OSPF Database Exchange Summary List Optimization is described in the following section that explains how this RFC optimizes the DBD exchange between two OSPF routers.

 

  1.   Specification of Optimization

 

   The Database Exchange summary list optimization is defined by

   modifying Section 10.6 "Receiving Database Description Packets" of

   RFC 2328 as follows.  The second-to-last paragraph of Section 10.6 is

   replaced with the following augmented paragraph:

 

   When the router accepts a received Database Description Packet as the

   next in sequence, the packet contents are processed as follows.  For

   each LSA listed, the LSA's LS type is checked for validity.  If the

   LS type is unknown (e.g., not one of the LS types 1-5 defined by this

   specification), or if this is an AS-external-LSA (LS type = 5) and

   the neighbor is associated with a stub area, generate the neighbor

   event SeqNumberMismatch and stop processing the packet.  Otherwise,

   the router looks up the LSA in its database to see whether it also

   has an instance of the LSA.  If it does not, or if the database copy

   is less recent, the LSA is put on the Link state request list so that

   it can be requested (immediately or at some later time) in Link State

   Request Packets.  In addition, if the Database summary list contains

   an instance of the LSA that is the same as or less recent than the

   listed LSA, the LSA is removed from the Database summary list.

 

   The above additional step (which updates the Database summary list)

   may be performed either before or after the router looks up the

   listed LSA in its database and possibly adds the LSA to the Link

   state request list.  However, to implement the optimization, the

   additional step must be performed for each LSA listed in the received

   DD packet (to fully update the Database summary list) before the next

   DD packet is sent in response.