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[b]OSPF neighbors stuck in ExStart - case 1 - interface type mismatch[/b]


So we know that on a broadcast segment, two neighborship between two OSPF routers will allways be FULL given that all conditions are met. When there are more than two neighbors, the Designated Router and Backup Designated Router form a FULL adjacency with all routers on the segment and the DROthers will remain in 2-way state with each other. This is to avoid LSA flood storms on network segment with multiple OSPF speakers.


One of possible reasons why a neighbor would see FULL state neighborship with two other OSPF speakers on the broadcast segment and remain stuck in ExStart state with all the others is the interface type mismatch.


Let's see below:
[code]
admin@vMX-TEST> show ospf neighbor logical-system R3
Address Interface State ID Pri Dead
10.0.10.5 ge-0/0/3.10 Full 5.5.5.5 128 37
10.0.10.4 ge-0/0/3.10 ExStart 4.4.4.4 128 36
10.0.10.2 ge-0/0/3.10 ExStart 2.2.2.2 128 33
10.0.10.1 ge-0/0/3.10 ExStart 1.1.1.1 128 39
10.0.10.6 ge-0/0/3.10 Full 6.6.6.6 128 31
[/code]

What we see is that R3 has full ospf neighborship with RID 5.5.5.5 and 6.6.6.6. This is not a coincidence, these are routers with highest router ID and they are designated and backup designated routers.

RFC 2328 tells us that specific network types do not require election of designated and backup designated routers:

[quote]
https://www.ietf.org/rfc/rfc2328.txt

C.6 Point-to-MultiPoint network parameters

On Point-to-MultiPoint networks, it may be necessary to
configure the set of neighbors that are directly reachable over
the Point-to-MultiPoint network. Each neighbor is identified by
its IP address on the Point-to-MultiPoint network. Designated
Routers are not elected on Point-to-MultiPoint networks, so the
Designated Router eligibility of configured neighbors is
undefined.

[/quote]


[code]
admin@vMX-TEST> show ospf interface ge-0/0/3.10 extensive logical-system R3
Interface State Area DR ID BDR ID Nbrs
ge-0/0/3.10 PtToPt 0.0.0.0 0.0.0.0 0.0.0.0 5
Type: P2MP, Address: 10.0.10.3, Mask: 255.255.255.0, MTU: 1500, Cost: 1
Adj count: 2
Hello: 10, Dead: 40, ReXmit: 5, Not Stub
Auth type: None
Protection type: None
Topology default (ID 0) -> Cost: 1
[/code]

R3's interface is set to point-to-multipoint type. This means that, on ethernet segments, it will accept both unicast and multicast OSPF messages from all routers in the segment, but it will only send unicast. This explains why neighbourship with designated and backup designated routers is "FULL". The neighbourship with DROther routers is stuck in ExStart because the P2MP node tries to elevate the neighbourship state from 2-way to Exstart/Exchange/Loading/FUll, but DROther routers refuse to do this because they already have FULL neighborship with the DR and the BDR.

This is seen in a traffic capture taken on R3's interface:
[code]
09:35:05.079191 Out IP truncated-ip - 46 bytes missing! 10.0.10.3 > 10.0.10.6: OSPFv2, Hello, length 76
09:35:05.761154 In IP 10.0.10.1 > 224.0.0.5: OSPFv2, Hello, length 76
09:35:06.019392 Out IP truncated-ip - 14 bytes missing! 10.0.10.3 > 10.0.10.1: OSPFv2, Database Description, length 44
09:35:06.019469 Out IP truncated-ip - 14 bytes missing! 10.0.10.3 > 10.0.10.2: OSPFv2, Database Description, length 44
09:35:06.919188 Out IP truncated-ip - 46 bytes missing! 10.0.10.3 > 10.0.10.4: OSPFv2, Hello, length 76
09:35:07.210970 In IP 10.0.10.2 > 224.0.0.5: OSPFv2, Hello, length 76
09:35:07.399361 Out IP truncated-ip - 14 bytes missing! 10.0.10.3 > 10.0.10.4: OSPFv2, Database Description, length 44
[/code]

R3 tries to send OSPF Database Description messages in unicast form to DRother routers, but they do not reply, so the neighborship remains stuck in ExStart.

Question #2: If R3 has FULL neighborship with designated routers, in theory, R3 routes should be seen by all other routes, but that is not correct.

[code]
admin@vMX-TEST> show interfaces lo0.3 terse
Interface Admin Link Proto Local Remote
lo0.3 up up inet 3.3.3.3 --> 0/0
33.33.33.33 --> 0/0

admin@vMX-TEST> show ospf interface logical-system R3
Interface State Area DR ID BDR ID Nbrs
ge-0/0/3.10 PtToPt 0.0.0.0 0.0.0.0 0.0.0.0 5
lo0.3 DR 0.0.0.0 3.3.3.3 0.0.0.0 0
lo0.3 DR 0.0.0.0 3.3.3.3 0.0.0.0 0

admin@vMX-TEST> show ospf neighbor logical-system R3
Address Interface State ID Pri Dead
10.0.10.5 ge-0/0/3.10 Full 5.5.5.5 128 35
10.0.10.4 ge-0/0/3.10 ExStart 4.4.4.4 128 35
10.0.10.2 ge-0/0/3.10 ExStart 2.2.2.2 128 37
10.0.10.1 ge-0/0/3.10 ExStart 1.1.1.1 128 33
10.0.10.6 ge-0/0/3.10 Full 6.6.6.6 128 35[/code]


Here is R6's view of R3's router LSA:
[code]
admin@vMX-TEST> show ospf database lsa-id 3.3.3.3 extensive logical-system R6

OSPF database, Area 0.0.0.0
Type ID Adv Rtr Seq Age Opt Cksum Len
Router 3.3.3.3 3.3.3.3 0x8000002b 1063 0x22 0x9a16 84
bits 0x0, link count 5
id 10.0.10.3, data 255.255.255.255, Type Stub (3)
Topology count: 0, Default metric: 0
id 5.5.5.5, data 10.0.10.3, Type PointToPoint (1)
Topology count: 0, Default metric: 1
id 6.6.6.6, data 10.0.10.3, Type PointToPoint (1)
Topology count: 0, Default metric: 1
id 33.33.33.33, data 255.255.255.255, Type Stub (3)
Topology count: 0, Default metric: 0
id 3.3.3.3, data 255.255.255.255, Type Stub (3)
Topology count: 0, Default metric: 0
Topology default (ID 0)
Type: PointToPoint, Node ID: 6.6.6.6
Metric: 1, Unidirectional
Type: PointToPoint, Node ID: 5.5.5.5
Metric: 1, Unidirectional
Aging timer 00:42:16
Installed 00:17:42 ago, expires in 00:42:17, sent 00:17:42 ago
Last changed 00:17:42 ago, Change count: 10
admin@vMX-TEST> show ospf database lsa-id 6.6.6.6 extensive logical-system R6

OSPF database, Area 0.0.0.0
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *6.6.6.6 6.6.6.6 0x80000023 1158 0x22 0x5332 48
bits 0x0, link count 2
id 10.0.10.5, data 10.0.10.6, Type Transit (2)
Topology count: 0, Default metric: 1
id 6.6.6.6, data 255.255.255.255, Type Stub (3)
Topology count: 0, Default metric: 0
Topology default (ID 0)
Type: Transit, Node ID: 10.0.10.5
Metric: 1, Bidirectional
Gen timer 00:30:41
Aging timer 00:40:41
Installed 00:19:18 ago, expires in 00:40:42, sent 00:19:16 ago
Last changed 20:03:36 ago, Change count: 2, Ours
[/code]

R3 reports individual "PointToPoint" segments with R5 and R6 while R6 sees the segment as "Transit" and this causes R6 to ignore all IDs provided by R3 in it's type 1 LSA, and it will not install routes 3.3.3.3/32 and 33.33.33.33/32 presented by R3:
[code]
admin@vMX-TEST> show route protocol ospf logical-system R6

inet.0: 8 destinations, 8 routes (8 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

1.1.1.1/32 *[OSPF/10] 20:04:54, metric 1
> to 10.0.10.1 via ge-0/0/6.10
2.2.2.2/32 *[OSPF/10] 19:40:19, metric 1
> to 10.0.10.2 via ge-0/0/6.10
4.4.4.4/32 *[OSPF/10] 18:17:55, metric 1
> to 10.0.10.4 via ge-0/0/6.10
5.5.5.5/32 *[OSPF/10] 20:04:54, metric 1
> to 10.0.10.5 via ge-0/0/6.10
224.0.0.5/32 *[OSPF/10] 20:05:09, metric 1
MultiRecv[/code]