Down bits for OSPF LSA type 5

Unanswered Question
Feb 20th, 2007

Dear All,

I have read from 2 diff sources regarding OSPF down bits for LSA Type 5

source 1

The down bit helps prevent routing loops between MP-BGP and OSPF, but not when external routes are announced, such as when redistribution between multiple OSPF domains or when external routes are injected in an area that is dual-homed to the provider network. The PE router redistributes an OSPF route from a different OSPF domain into an OSPF domain as an external route. "The down bit is not set because LSA Type 5 does not support the down bit". The redistributed route is propagated across the OSPF domain.

source 2

process steps for routing loops across OSPF domains

1. The PE router redistributes a non-ospf route into an OSPF domain as an external route. The down bit is set because the route should not be redistributed back into MP-BGP

2. A CE router redistribute the OSPF route into another OSPF domain. The down bit is lost if the CE router does not

understand this OSPF extension

3.The OSPF route is propagated through the other OSPF domain with the down bit cleared

4. A PE router receives the OSPF route;the down bit is not set,so the route is redistributed back into MP-BGP backbone resulting in a routing loop.

Question : it seems that source 1 is saying the down bits is not supported in OSPF type 5 LSA while source 2 states that it is supported. Please advise which is the correct one.

Thanks , birdy

I have this problem too.
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mheusinger Tue, 02/20/2007 - 23:29


for LSA5 the tag field will be used to prevent routing loops. An excerpt fronm an OSPF database:

OSPF Router with ID ( (Process ID 10)

Type-5 AS External Link States

Routing Bit Set on this LSA

LS age: 4

Options: (No TOS-capability, DC)

LS Type: AS External Link

Link State ID: (External Network Number )

Advertising Router:

LS Seq Number: 80000001

Checksum: 0x38C9

Length: 36

Network Mask: /30

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

TOS: 0

Metric: 1

Forward Address:

External Route Tag: 3489661028

! External Route tag 3489661028 = 0xD0000064 = 0xD0000000 + 0x64

! 0x64 = 100 = AS Nummer of the MPLS BB!


! So: External Route tag = 0xD0000000 + (AS-number of PE)


CE82#show ip ospf database summary

OSPF Router with ID ( (Process ID 10)

Summary Net Link States (Area 2)

Routing Bit Set on this LSA

LS age: 118

Options: (No TOS-capability, DC, Downward) ! Downward means Down Bit = 1

LS Type: Summary Links(Network)

Link State ID: (summary Network Number)

Advertising Router:

LS Seq Number: 80000001

Checksum: 0x1601

Length: 28

Network Mask: /30

TOS: 0 Metric: 1

As you can see there is no down bit visible for LSA5, only for LSA3.

Hope this helps! Please use the rating system.

Regards, Martin

birdy Wed, 02/21/2007 - 00:43


Thanks for yr response.

So are u saying that source 1 is correct ?

Thanks, birdy

mheusinger Wed, 02/21/2007 - 05:20

Helo birdy,

yes, source 1 is correct. Instead of the down bit a route tag is used to avoid routing loops with LSA5. The PE sets the route tag according to the rule described in my first posting and does not redistribute any external route with a route tag containing its AS number as described above.

Hope this helps!

regards, Martin

ganpatspatil Sat, 02/24/2007 - 10:15

Thanks for all descriptions.

I have one more doubt .

If i have PE-CE as Eigrp or RIP.And my CE site is multihomed to PE then which bit will used to avoid Loop ?


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