Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
New Member

External route with Down Bit set by CE router

I have a question regarding the Down Bit. All the documentation I see concentrates on the redistribution of the MP-BGP into OSPF but I want to know more about the OSPF redistribution into MP-BGP. I think the rule is different for Type3 and Type5 LSAs.

Lets say the CE redistributes a RIP route into OSPF, creating an LSA Type 5 route sent to the PE router - with the Down Bit set.

Will this PE router redistribute this route into MP-BGP?

I know that LSA Type 3 summary routes will not be redistributed into MP-BGP if the Down Bit is set but I am wondering about external OSPF routes with the Down Bit set????

I am assuming that redistribution of RIP into OSPF at the CE does infact set this Down Bit before passing on to the PE router?

13 REPLIES
Silver

Re: External route with Down Bit set by CE router

hi walter

Down bit concept is used in MPLS when ospf route redistribute in MP-BGP and PE sets the downbit for stopping loop. I think CE annot set the downbit.

regards

shivlu jain

Cisco Employee

Re: External route with Down Bit set by CE router

Walter,

For type 5 LSAa, IOS uses the route tag to control routing loops between BGP and OSPF. Routes are tagged as they are redistributed from BGP to OSPF and the tag is checked when redistributing OSPF back into BGP.

This behavior is described by RFC 4577, section 4.2.5.2.

So in your case,the RIPs route should be redistributed into BGP.

BTW, redistribution of RIP into OSPF will not set the down bit.

Regards

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

Re: External route with Down Bit set by CE router

Thanks for the reply:

I think this link provided some wrong info...

check out under the heading OSPF Route Tag or VPN Route Tag

http://fengnet.com/book/IOS_MPLS/ch05lev1sec1.html

This is wrong isn't it???

Silver

Re: External route with Down Bit set by CE router

walter

In the link author is considering about another domain.

regards

shivlu jain

New Member

Re: External route with Down Bit set by CE router

How can step 2 in the sequence description say that PE2-AS1 receives an OSPF route with the Down Bit already set? This comes from the CE.

I find this all very confusing when I check out these sequences - I thought only the PE router assigns Down Bit to LSA Type 3 routes?

All the other references I see show how PE1-AS1 would normally set the Down Bit as it is redistributed back into OSPF?

Thanks for your help - I feel I am getting closer. It is just this second step that is throwing me!

Cheers

Walter

Cisco Employee

Re: External route with Down Bit set by CE router

Walter,

In IOS implementation, the down bit is never set on type 5 LSAs, only for type 3 LSAs.

Regards

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Silver

Re: External route with Down Bit set by CE router

As I have said only PE router add the downbit. But when PE1-AS1 forwards the route to CE1-A by removing downbit and CE1-A will forward the route to PE3-AS1. Now PE3-AS1 will advertise the route to PE1-AS1 & PE2-AS1. Both PE1-AS1 & PE2-AS1 should reject the route because they are already having the route bit down bit set.

regards

shivlu jain

Cisco Employee

Re: External route with Down Bit set by CE router

Walter,

I think this document is just stating that a routing loop would take place if the VPN route tag was not implemented. It is true though that the down bit is not being used for type 5 LSA.

Regards

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Silver

Re: External route with Down Bit set by CE router

hritter is saying right author tries to conveince the message that there would be routing loop.

regards

shivlu jain

Silver

Re: External route with Down Bit set by CE router

hritter

I have one query if CE is giving E2 routes then a loop can be occured becuase PE router cannot mark downbit to it. I think this is what the author wants to say.

regards

shivlu jain

Cisco Employee

Re: External route with Down Bit set by CE router

Shivlu,

The loop would be prevented using the VPN route tag. It doesn't matter if the route is E1 or E2.

Regards

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Silver

Re: External route with Down Bit set by CE router

hritter

If the same is there then there is no requirement of downbit. It would be helpful if you can put more light on to it or simply provide any link.

regards

shivlu jain

Cisco Employee

Re: External route with Down Bit set by CE router

Shivlu,

The down bit is used for the summary LSA (type 3) as route tag is only available for external LSAs.

Regards

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
445
Views
4
Helpful
13
Replies
CreatePlease to create content