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Static routes redistributed by two routers doesn't appear in OSPF database

ju.mahieu
Level 1
Level 1

Hello,

As show in the picture below.

OSPF - Problem.jpg

I redistribute the same static routes from my two ABR-ASBR routers (RTR1 & RTR2) inside OSPF area 0 and 1. The Next Hop for every static routes is FW1.

When I'm looking the Ospf database on every devices in Backbone Area or in Area 1, only the RTR2 LSA are present.

I should see also the same LSA from RTR1, isn't it ?

If anyone can help...

regards,

Julien

-------------------------------------------

Find below the Configurations:

-------------------------------------------

**** Same config For RTR1 and RTR2 ****

! Static Redistributed routes in Ospf

ip route xx.0.1.16 255.255.255.240 xx.18.225.1
ip route xx.0.2.16 255.255.255.240 xx.18.225.1

! Static-to-ospf matching

access-list 6 remark *** JUNIPER STATIC NETWORK
access-list 6 permit 55.0.0.0 0.0.255.255

! Static-to-ospf filtering

route-map STATIC-to-OSPF permit 10
match ip address 6

! Ospf config

router ospf 100
router-id xx.0.18.1
log-adjacency-changes
network xxxx area 0

network yyyy area 1
redistribute static metric 10 metric-type 1 subnets route-map STATIC-to-OSPF

-------------------------------------------

Find below the Ospf Database extract from RTR1:

-------------------------------------------

RTR1#sh ip ospf database external adv-router RTR1

            OSPF Router with ID ($RTR1) (Process ID 100)

RTR1#sh ip ospf database external adv-router RTR2

            OSPF Router with ID ($RTR1) (Process ID 100)

                Type-5 AS External Link States

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 16
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: xx.0.1.16 (External Network Number )
  Advertising Router: $RTR2
  LS Seq Number: 800000B6
  Checksum: 0x4C0
  Length: 36
  Network Mask: /28
        Metric Type: 1 (Comparable directly to link state metric)
        MTID: 0
        Metric: 10
        Forward Address: $FW1
        External Route Tag: 0

21 Replies 21

Thanks all for your replies,

I 'm currently in Christmas holidays and I can't access to my devices for five days.

I'll keep you posted when I come back to work.

Regards,

Ju

Hi all,

I think Giuseppe pointed to the correct direction. I think your case is described in RFC2328 (Section 12.4.4.1.  Examples of AS-external-LSAs):

"In figure 16, suppose instead that both RTA and RTB exchange BGP information with RTX.  In this case, RTA and RTB would originate the same set of AS-external-LSAs.  These LSAs, if they specify the same metric, would be functionally equivalent since they would specify the same destination and forwarding address (RTX).  This leads to a clear duplication of effort.  If only one of RTA or RTB originated the set of AS-external-LSAs, the routing would remain the same, and the size of the link state database would decrease.  However, it must be unambiguously defined as to which router originates the LSAs (otherwise neither may, or the identity of the originator may oscillate).  The following rule is thereby established: if two routers, both reachable from one another, originate functionally equivalent AS-external-LSAs (i.e., same destination, cost and non-zero forwarding address), then the LSA originated by the router having the highest OSPF Router ID is used.  The router having the lower OSPF Router ID can then flush its LSA. Flushing an LSA is discussed in Section 14.1."

Kind Regards,

Maria

Hello Maria,

Hats off! It seems that this could perfectly account for the behavior that Julien experienced. Thank you VERY much for highlighting it!

Best regards,

Peter

Hi Maria,

Thank you so much for your post. It perfectly explain my issue

Regards,

Ju

Hello Maria; Julien, Peter,

a lesson for all: we need to go back to RFCs to find explanations for standard based protocols.

Maria is very good on this.

Actually, the duplicated set of external LSAs is not needed thanks to recursion on the forwarding address

I think that in most cases when we see two sets of LSAs is because if FA = 0.0.0.0 this suppression is not triggered.

Hope to help

Giuseppe

Hi all,

Thank you for your kind remarks. Once upon a time I had to write code for specific IEEE 802.1q stuff. I don't know if you ever had the pleasure to read IEEE documents, but they look awful (I mean, what were those guys thinking when they wrote them? had they encrypted the documents, it would be easier to read, anyway). If anyone is interested in reading standards, but feels somewhat discouraged by their length, language or structure, I have a tip. A big problem with standards is that information about a specific feature is spread all over the document and you don't have the time to read it front to cover. So basically what I usually do is to search within the document for specific keywords. In this case, the keyword was 'forwarding'. In general, you may come across some irrelevant stuff or need to re-iterate with other keywords, but this works for me. And of course it is better to read some cisco documents first (if they exist) to familiarize with the basics, before going to standards. Cisco documents are good in this respect because they were written for humans and not some decryption engine.

Kind Regards,

Maria

RFC!!! It needs a long long time to read them.

Thanks for all replies!

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