OSPF Virtual link

EdTheLad

https://www.flickr.com/photos/117689526@N08/12594740014/


I'm interested in the subnet 10.108.1.0/24, which is the point-to-point link between R8 and R10. This prefix will be summarized from area 3 into the backbone as a type 3 lsa. R3 will receive the type 3 lsa in area 0 and advertise it as a type 3 lsa to area 4.

So both R2 and R4 in area 1 will have a type 3 lsa which was advertised by R3 for prefix 10.108.1.0/24.

Since R2 has a virtual-link to R3, R2 will also have a type 3 lsa in area 0 advertised by R8.

R2's preferred path is via the VL

R2# sh ip route 10.108.1.0
Routing entry for 10.108.1.0/24
Known via "ospf 1", distance 110, metric 40, type inter area
Last update from 10.24.1.4 on Ethernet0/1, 02:24:42 ago
Routing Descriptor Blocks:
* 10.24.1.4, from 8.8.8.8, 02:24:42 ago, via Ethernet0/1
Route metric is 40, traffic share count is 1


R2#sh ip ospf database summary 10.108.1.0

OSPF Router with ID (2.2.2.2) (Process ID 1)

Summary Net Link States (Area 0)

Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 2 (DoNotAge)
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.108.1.0 (summary Network Number)
Advertising Router: 8.8.8.8
LS Seq Number: 80000001
Checksum: 0x5A41
Length: 28
Network Mask: /24
MTID: 0 Metric: 10


Summary Net Link States (Area 1)

LS age: 484
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.108.1.0 (summary Network Number)
Advertising Router: 3.3.3.3
LS Seq Number: 80000001
Checksum: 0x5550
Length: 28
Network Mask: /24
MTID: 0 Metric: 20



Now since R2 is a backbone router, i expect it to advertise all summaries to any attached area's. R2 is an ABR for area's 0,2 and 1. So i expect the prefix to be advertised to routers R1 in area 2 and R3,R4 in area 1.




R1#sh ip ospf database summary 10.108.1.0

OSPF Router with ID (1.1.1.1) (Process ID 1)

Summary Net Link States (Area 2)

Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 1227
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.108.1.0 (summary Network Number)
Advertising Router: 2.2.2.2
LS Seq Number: 80000005
Checksum: 0x345D
Length: 28
Network Mask: /24
MTID: 0 Metric: 40




R4#sh ip ospf database summary 10.108.1.0

OSPF Router with ID (4.4.4.4) (Process ID 1)

Summary Net Link States (Area 1)

Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 900
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.108.1.0 (summary Network Number)
Advertising Router: 3.3.3.3
LS Seq Number: 80000001
Checksum: 0x5550
Length: 28
Network Mask: /24
MTID: 0 Metric: 20

R4#

The summary from R2 is not being received on R4, it is just seeing the summary from R3.


Is there a rule for virtual links to say "don't advertise backbone summaries into the virtual link transit area"? It makes sense not to, but i haven't seen this rule anywhere.

mistabrumley89

Would the actual physical link type (nbma, broadcast, p2p, p2mp, etc) have an effect on the outcome? Not sure of the answer to your question, but it just made me think of that.

EdTheLad

No nothing to do with ospf network type.

EdTheLad

So there is a split-horizon rule for summaries, i found it in the rfc.

To determine which routes to advertise into an attached Area
A, each routing table entry is processed as follows.

o If the area associated with this set of paths is
the Area A itself, do not generate a summary-LSA for the
route.[17]

o Else, if the next hops associated with this set of paths
belong to Area A itself, do not generate a summary-LSA
for the route.[18] This is the logical equivalent of a
Distance Vector protocol's split horizon logic.

Dieg0M

OSPF will never summarize backbone-area prefixes when injecting them into a TRANSIT area.
Understanding OSPF Transit Capability

EdTheLad

Dieg0M wrote: »

OSPF will never summarize backbone-area prefixes when injecting them into a TRANSIT area.
Understanding OSPF Transit Capability

You cant condense summarizes using the range command into a transit area, but the converting of type 1 to type 3 i.e. the summary prefixes can be advertised into a transit area. My discussion above was purely in regards to type 3 lsa's, i've been playing around with this alot, the area split-horizon behavior when using virtual links was my final obstacle in completely understanding lsa flooding.