OSPF Down-bit and SOO

[sparkofwisdom]

Hi,

If I understand correctly, SOO is used to prevent routing information loop in the case of a site is multi-homed to MPLS/VPN backbone.

OSPF Down-bit also serves the similiar purose. But it is only restricted to Summary LSA, but not External LSA.

So can I make the conclusion that, to fully avoid routing information loop, SOO is already sufficient enough? SOO should be equal to Down-bit for Summary LSA and Tagging for External LSA.

If not, what is the pros and cons for both methods?

Thanks!

SShang

[mmolina2]

It's been awhile but I am pretty sure that the OSPF domain-tag might apply to External LSA's in the same way the down bit applies to the Summary LSA'a. I could be wrong.

[oettls]

yes you're perfectly right. For External LSAs the PE redistributing the MP-BGP route into OSPF computes an auto tag value. Other PE's receiving the external LSA check for the domain tag and don't add the route to the RIB if the values are identical. This way the routes aren't redistributed back to MP-BGP.

SoO doesn't apply for PE-CE OSPF, it's used e.g. for PE-CE eBGP and EIGRP.

cheers,

Stefan

[sparkofwisdom]

Hi, all:

Thank you very much replies. One further question, how does PE router calculate the default tag for external routes? Does it have any relation with domain-id? Is it the same as domain-tag?

If I remember correctly, the default domain-id is equal to the OSPF process number. Initially I have OSPF process 400 on one PE router R1. I saw this router attached a tag of 3489725928 with all external routes originated from its site. Then, I changed the OSPF process number from 400 to 300 on the same router R1. However, the tag for external routes still stayed the same when displaying it on the receiving PE router R2:

R2#show ip ospf 400 database external 10.10.9.0

OSPF Router with ID (172.16.2.1) (Process ID 400)

Type-5 AS External Link States

............

Options: (No TOS-capability, DC)

LS Type: AS External Link

Link State ID: 10.10.9.0 (External Network Number )

Advertising Router: 172.16.2.1

.............

Network Mask: /24

Metric Type: 2 (Larger than any link state path)

TOS: 0

Metric: 10

Forward Address: 0.0.0.0

External Route Tag: 3489725928

So it seems like ospf process number has nothing to do with it.

I know we can change the domain-tag by cisco command. But just for curiosity, how does PE router generate the tag?

Thanks!

SShang

[oettls · Accepted solution]

The tag value is computed acccording to RFC 1745 "BGP4/IDRP for IP---OSPF Interaction".

See section 4.3.5. for details.

In your case the binary value is

"1101 0000 0000 0000 1111 1101 1110 1000"

The first 4bits are something like 'tag type', 12bits arbitrary tag set to '0', the last 16bits is your BGP AS 65000.

cheers,

Stefan

[sparkofwisdom]

Kewl, I will download the RFC and read it. Thanks!

SShang