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I have 2 equal cost path links between PeerA and PeerB and all its interfaces are participating in the same OSPF area.

When i redistribute a static route (172.16.2.0/24) from PeerB to A, does the adverting router generate only 1 x external LSA and send it down from just 1 of the 2 links to the receiving peer? or the same external LSA will be send via both links to the receiving peer?

Reason for asking this is because i am only seeing 1 entry in PeerA for the external LSA even though in the RIB , there are 2 x OE2 routes to the redistributed network (172.16.2.0)

PeerA#show ip ospf database external adv-router 0.0.0.2

            OSPF Router with ID (0.0.0.1) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 502
  Options: (No TOS-capability, No DC)
  LS Type: AS External Link
  Link State ID: 172.16.2.0 (External Network Number )
  Advertising Router: 0.0.0.2
  LS Seq Number: 80000003
  Checksum: 0x9B6A
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 10
        Forward Address: 0.0.0.0
        External Route Tag: 0

Only 1 x External LSA shown in the OSPF database as above, but 2 x OE2 routes in the RIB.

RouterA#show ip route ospf
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       + - replicated route, % - next hop override

Gateway of last resort is not set

      172.16.0.0/24 is subnetted, 3 subnets
O E2     172.16.2.0
           [110/10] via 200.200.200.201, 00:11:04, GigabitEthernet1/0/6
           [110/10] via 100.100.100.101, 00:01:32, GigabitEthernet1/0/5
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  • "When i redistribute a static route (172.16.2.0/24) from PeerB to A" Redistribution happens inside a single router, not between routers. Are you redistributing in Router B and advertising the route to Router A? The LSA is probably sent on both links, but it is not advertising those links, it is between the two routers, not the links. There are different LSA types, and the LSA (Type 5) for external routes is different than the LSA types used for internal routes.
    – Ron Maupin
    Nov 18, 2018 at 14:32
  • @RonMaupin - yeap what I meant was the external LSA (type5) advertisement of the redistributed network in RouterB to RouterA - does it send the same LSA via both or a single link ? cause I am only seeing 1 entry for that network using "show ip ospf database external" in RouterA; however in the RouterA's RIB, there are 2 x same cost OE2 routes to that redisributed network
    – Noob
    Nov 19, 2018 at 14:54
  • 2
    As I explained, that LSA is not advertising links the way a Type 1 LSA does. It says that Router B knows how to reach that external route. Router B sends the LSA to it neighbor. How the LSA gets from Router B to Router A is of no concern. It could be over one link, or both, but it is a single database entry because it is pointing to the router ID, not the link. Router A knows two ways to get to Router B, so it knows it has two paths to the external network.
    – Ron Maupin
    Nov 19, 2018 at 15:11
  • @RonMaupin thanks for confirming the behavior. i thought i would have setup something wrong somewhere.
    – Noob
    Nov 19, 2018 at 15:17

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