ospf capability vrf lite

[Alexey Stytsenko]

 When the OSPF process is associated with the VRF, several checks are  performed when link-state advertisements (LSAs) are received. PE checks  are needed to prevent loops when the PE is performing a mutual  redistribution between OSPF and Border Gateway Protocol (BGP)  interfaces. 
 Table 1     PE Checks Performed 
 
 Type-3 LSA received 
 The DN bit is checked. If the DN bit is set, the Type-3 LSA is not considered during the shortest path first (SPF) calculation. 
 Type-5 or -7 LSA received 
 If the Tag in the LSA is equal to the VPN-tag, the Type-5 or-7 LSA is not considered during the SPF calculation. 

Can smb clarify how TYPE-3 LSA can be created due to "mutual  redistribution between OSPF and BGP"? Previously i have thought that only type 5 and type 7 LSAs can be created during redistribution, so i were a bit surprised when found these information.

[Rolf Fischer]

Hi,

this is something special when a MPLS VPN interconnects sites wich use OSPF as IGP.

You normally don't want OSPF routes from site A to be injected as external Routes in site B and vice versa, which would be normal behavior when redistributing into BGP or any other routing protocol. You would also loose the infomation about the LSA types.

That's why MPLS/MP-BGP acts as an OSPF "superbackbone" (google it!) and the other site's routes will be injected as LSA type 3 in such a case.

Hope that helps

Rolf

[Rolf Fischer]

Btw: There is an "ask the expert" for OSPF in the "other nw infra subjects" section, a great oportunity for everybody who's interested in OSPF!


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[Giuseppe Larosa · Accepted solution]

Hello Alexey,

>> Can smb clarify how TYPE-3 LSA can be created due to "mutual  redistribution between OSPF and BGP"? Previously i have thought that only type 5 and type 7 LSAs can be created during redistribution, so i were a bit surprised when found these information.

When using OSPF as PE-CE routing protocol some handy features are available. The SP network in its control plane uses MP BGP.

MP BGP has been extended in order to carry OSPF routes attributes within special extended BGP communities, by doing so the remote PE node can rebuild OSPF data structures and depending if some conditions are met OSPF LSA type 3 are built and sent to remote CE node ( co-located to remote PE node).

Because the extended communities used carry some information in order to rebuild OSPF data structures, one of this condition in order the routes seen as LSA type 3 is the use of the exact same OSPF process-id on the the two PE nodes.

Actually, when using sham-links intra area routes can be re-built at remote PE.

Hope to help

Giuseppe