I have two data centers interconnected through a MPLS backbone running MP-BGP (typical L3 VPN setup with two sites).
If rz1n1-dc redistributes its locally configured static default route into OSPF, the following happens:
- one type 5 LSA for 0.0.0.0 in the left OSPF domain with Advertising Router: 10.40.32.246 (=rz1n1-dc)
- two type 5 LSAs for 0.0.0.0 in the right OSPF domain with Advertising Router: 10.40.31.251 (=rz2n1-c) and 10.40.31.252 (=rz2n2-c)
That's pretty much what I would expect to happen. The right side learns the default route through BGP and the two PE routers rz2n1-c and rz2n2-c redistribute it into OSPF.
Now comes the interesting part:
If rz2n1-dc also starts to redistribute its locally configured static default route into OSPF, the following happens (so we now have rz1n1-dc and rz2n1-dc redistributing a default route into OSPF):
- one type 5 LSA for 0.0.0.0 in the left OSPF domain with Advertising Router: 10.40.32.246 (=rz1n1-dc)
- two type 5 LSAs for 0.0.0.0 in the right OSPF domain with...
> Advertising router: 10.40.31.246 (rz2n1-dc) (the router which redistributes its static default route into OSPF) (expected)
> Advertising router: 10.40.31.252 (rz2n1-c) (one of the two PE routers which redistributes the default route from the left side from BGP to OSPF) (not expected)
I don't understand why I see the second type 5 LSA on the right side. I would expect to see only one type 5 LSA from rz2n1-dc (just like I see only one type 5 LSA on the left side).
By the way: If I look at the OSPF database for a different route (not the default route) which is configured identically (also redistributed from static into OSPF on rz1n1-dc and rz2n1-dc), I only see one type 5 LSA on both sides, just like I would expect.
Any ideas what I am missing? Is this some expected OSPF feature related to default route I am not aware of?
By the way: I see the same behavior in production environment (running NX-OS) and also in GNS3 simulation (running IOS).
rz2n1-dc#sh ip ospf database external 0.0.0.0
OSPF Router with ID (10.40.31.246) (Process ID 1)
Type-5 AS External Link States
LS age: 11
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 10.40.31.246
LS Seq Number: 80000001
Checksum: 0x93C9
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 1
Forward Address: 10.0.1.2
External Route Tag: 1
Routing Bit Set on this LSA
LS age: 215
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 10.40.31.252
LS Seq Number: 80000022
Checksum: 0xE181
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 3489725969
