OSPF, 2547bis VPNs, Loops, and LSA Options

1
OSPF, 2547bis VPNs, Loops, and LSA Options

• Scenario
• Customer multiple OSPF sites
• Service Provider (SP) connects sites over shared
  backbone
• Not with tunnels, but by using BGP to distribute
  customer routes from site to site
• Use of BGP transparent to customer SP converts
  BGP routes back to OSPF (type 3 usually)

2
Topology
Customer site
PE1
SP Network
CE1
BGP
PE2
PE3
OSPF
CE2
CE3
PE Provider Edge
CE Customer Edge
3
Route Distribution

• PEs function like border routers
• PE3 receives LSAs from CE3
• PE3 determines prefixes reachable via CE3
• PE3 generates BGP updates for those prefixes,
  distributes to PE1 and PE2
• PE1 converts BGP updates to LSAs, type 3 or 5,
  and sends to CE1

4
The Problem

• LSA which PE1 sent to CE1 may be sent by CE2 to
  PE2
• This may cause PE2 to generate a BGP update,
  incdicating that a prefix at CE1s site is
  reachable at CE2s site.
• Can happen if LSA is type 5, or if LSA is type 3
  and PE-CE link is area 0 link
• May cause looping

5
Proposed Solution

• Allocate bit from LSA Options field
• PE sets bit when it sends LSA to CE
• When PE receives LSA from CE with bit set, do NOT
  generate BGP update
• Loop-freedom ensured.