topic Question_As_Path_Prepend in Routing and SD-WAN

Question_As_Path_Prepend

Vinayaka Raman — Mon, 04 Mar 2019 22:02:42 GMT

All the branch end MPLS routers injects sites BGP prefix in to the service provider’s cloud by forming EBGP neighbhorship with PE.

In my scenario, I need to inject same prefix from two of the branches so that the actual MPLS route is always better than the other backup route.

This back route is static to BGP redistributed route. While redistributing I want to prepend AS so that it becomes the worst route. Right now, MED is configured and it is not working as expected because the service provider is hoping through Multiple AS for the actual MPLS route.

Router bgp 65000

Redistribute static route-map prepend As

Route-map permit 10

Set As path 65000 65000

This isn’t working . Do you have some suggestion ?

I will post the actual error message in a while.

Question_As_Path_Prepend

cadet alain — Tue, 25 Oct 2011 13:23:01 GMT

Hi,

you're missing the route-map name. It should be:

Route-map permit prepend As 10

Set As path 65000 65000

Regards.

Alain.

Question_As_Path_Prepend

Vinayaka Raman — Tue, 25 Oct 2011 13:51:20 GMT

Router bgp 65000

Ip route X.X.X.X Y.Y.Y.Y next hop 150

redistribute static route-map Prepend_AS

Route-map Prepend_AS 10

Set As path 65000 65000

sorry for the typo...

I forget the exact error message. While I issue this redistribute command It throws me an error like this is not possibl

I will post the error message soon.

Is this configuration correct ?

My intention is to prepend the AS after redistributing from static to bgp and then advertise to eBGP peer.

Re: Question_As_Path_Prepend

Vaibhava Varma — Tue, 25 Oct 2011 14:11:22 GMT

Hi Raman

If we need to prepend the AS-Path which is being advertised to other BGP Peers the best way is t first redistribute the Static ROutes into BGP Process and then apply an outbound route-map on per neighbiur basis under BGP config which prepends the AS. I wonder if the redistribution of Static ROutes into BGP and same time prepending will be allowed or not.

The solution for prepending would be as follows:

route bgp 65000

neighbour x.x.x.x route-map Prepend_AS out

redistribute static

Ip route X.X.X.X Y.Y.Y.Y next hop 150

route-map Prepend_AS permit 10

set as-path prepend 65000 65000

Hope this helps to answer your query.

Regards

Varma

Question_As_Path_Prepend

cadet alain — Tue, 25 Oct 2011 14:16:04 GMT

Hi,

the correct config should be set as-path prepend 65000 65000 but you should configure this like this:

neighbour x.x.x.x route-map Prepend_AS out

Because AS_PATH prepending is for advertising BGP prefixes not for redistributing static routes.

I still wonder how I didn't realize it at first look.

Alain.

Question_As_Path_Prepend

Vinayaka Raman — Tue, 25 Oct 2011 14:21:28 GMT

I wanted only the redistributed static routes to be prpended with AS..

So do you want me to create a prefix list and match under route-map..

Re: Question_As_Path_Prepend

Vinayaka Raman — Tue, 25 Oct 2011 14:23:21 GMT

Forgot to mention i have 2 more major prefixes advertisde from my CE to PE, i dont want to prpend that...

only i want to prepend the static to bgp redistributed routes...

Re: Question_As_Path_Prepend

Vaibhava Varma — Tue, 25 Oct 2011 14:29:57 GMT

Hi Raman

If we want to prepend only specific prefix then we need to have 2 terms in the outbound route-map:

1. Term 10 for prepending the required prefixes matched via a Prefix-List and prepend the AS-Path

2. Term 20 to allow everything else remaining un-prepended

route bgp 65000

neighbour x.x.x.x route-map Prepend_AS out

redistribute static

Ip route X.X.X.X Y.Y.Y.Y next hop 150

route-map Prepend_AS permit 10

match ip address prefix-list Prepend_AS

set as-path prepend 65000 65000

route-map Prepend_AS permit 20

ip prefix-list Prepend_AS seq 5 permit x.x.x.x/24

Hope this helps to answer your query.

Regards

Varma

Question_As_Path_Prepend

cadet alain — Tue, 25 Oct 2011 14:30:27 GMT

Hi,

yes you can match the prefixes with a prefix-list and prepend them.

Don't forget to add an empty route-map permit 20 for other prefixes to be advertised.

Alain.

Question_As_Path_Prepend

Vinayaka Raman — Tue, 25 Oct 2011 14:34:02 GMT

route bgp 65000

neighbour x.x.x.x route-map Prepend_AS out

redistribute static

Ip route X.X.X.X/24 next hop 150

Ip route Z.Z.Z.Z/24 next hop 150

route-map Prepend_AS permit 10

match prefix-list Static-routes

set as-path prepend 65000 65000

route-map Prepend_AS permit 20

Ip prefix-list Static-routes permit X.X.X.X/24

Ip prefix-list Static-routes permit Z.Z.Z.Z/24

Question_As_Path_Prepend

Vinayaka Raman — Tue, 25 Oct 2011 14:37:53 GMT

just review and let me know if the configuration looking good.

I had to use Clear ip bgp* for the configs to take effect immediately

Do you suggest an alternate way like

clear ip bgp * soft in

clear ip bgp * soft out

I dont think this worked when i tried for the first time

Question_As_Path_Prepend

Vaibhava Varma — Tue, 25 Oct 2011 14:43:02 GMT

Hi Raman

In my personal opinion we should never do a hard reset on BGP peers, the soft way which you have mentioned is the most optimal way withiut interrupting traffic flow.

Regards

Varma

Question_As_Path_Prepend

Vinayaka Raman — Tue, 25 Oct 2011 14:45:02 GMT

Ok...got it...

Do you suggest any changes in the config i posted. ?

Question_As_Path_Prepend

Vaibhava Varma — Tue, 25 Oct 2011 14:53:29 GMT

Hi Raman

Yes the config logic is correct. I did not look at the syntax typo though. You can refer my sample config snapshot which I shared in earlier post and look in the below excellent cisco docs for config syntax 🙂

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml

Regards

Varma

Question_As_Path_Prepend

Vinayaka Raman — Tue, 25 Oct 2011 14:58:04 GMT

Thank you Vaibhav and Alain..I will implement this and let you know how it goes

Re: Question_As_Path_Prepend

Vinayaka Raman — Fri, 28 Oct 2011 23:34:50 GMT

I have posted my configuration below.

In my config i wanted to prpend only 172.20.1.1/32 with as 65000 65000..all other perfixes i could see in the remote site..but for the prefix 172.20.1.1/32 i dont see the route at all...

If i just remove thr route-map, it is advertising zero routes.

I have one more route-map applied in bound, is it causing a problem?

Is this something service provider is restricting from prepending AS?

router bgp 65457
no synchronization
bgp log-neighbor-changes
network 10.1.181.80 mask 255.255.255.240
network 10.200.3.88 mask 255.255.255.255
network 100.179.40.0 mask 255.255.255.0
redistribute static
neighbor 152.161.42.85 remote-as 65000
neighbor 152.161.42.85 route-map blocked-sites in
neighbor 152.161.42.85 route-map Prepend_AS out
no auto-summary

route-map Prepend_AS, permit, sequence 10
Match clauses:
    ip address prefix-lists: Static-routes
Set clauses:
    as-path prepend 65000 65000
Policy routing matches: 0 packets, 0 bytes
route-map Prepend_AS, permit, sequence 20
Match clauses:
Set clauses:
Policy routing matches: 0 packets, 0 bytes
route-map blocked-sites, permit, sequence 10
Match clauses:
    ip address prefix-lists: blocked-sites
Set clauses:
Policy routing matches: 0 packets, 0 bytes

ip prefix-list Static-routes: 1 entries
seq 10 permit 172.20.1.1/32

APASACR1#show ip bgp nei 152.161.42.85 advertised-routes
BGP table version is 1905, local router ID is 10.200.3.88
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 10.1.181.80/28   0.0.0.0                  0         32768 i
*> 10.200.3.88/32   0.0.0.0                  0         32768 i
*> 100.179.40.0/24 0.0.0.0                  0         32768 i
*> 172.20.1.1/32    0.0.0.0                  0         32768 ?

Total number of prefixes 4

___________________________________________________

Re: Question_As_Path_Prepend

Vaibhava Varma — Sat, 29 Oct 2011 02:29:37 GMT

Hi Raman

I would guess this is an MPLS VPN connection between the two sites and using BGP as PE-CE routing protocol.

The route-map configuration seems to be fine but I would lile to see the Prefix-List Static-routes once and the output of "sh ip route".

Coming back to the second point of no routes being advertised on removing the outbound route-map should never happen as route-maps are used for some conditonal advertisement but if there is no route-map then all the BGP routes plus those defined under network statement and present as an exact route in RT will get advertised.

If this is an MPLS VPN I would not assume that ISP would be doing any kind of inbound route-filtering on their side and you can send a X/32 also. Had it been some Internet Link then definitly ISP won't alllow a X/32 proprogation.

Inbound route-map has no relation to outbound advertisement in BGP

About the case of AS-Path prepending not working I would first like to see the Static-ROutes Prefix List. Please share the same.

Hope this helps to provide some insight in the issue.

Regards

Varma

Re: Question_As_Path_Prepend

Vinayaka Raman — Sat, 29 Oct 2011 10:07:29 GMT

Thanks for your continued suuport.

I re-applied the route-map and it started working..

One thing little wierd is

I am prepnding my local as two times 65457 ..but in the remote sites I see it is prepended with the my remote as 65000 65000...I am not sure why is this behaviour..Do you have any idea..?

If i prepend with 65000 two times in my route-map, then this route is not getting advertised..

____________________________________________________________________

router bgp 65457

no synchronization

bgp log-neighbor-changes

network 10.1.181.80 mask 255.255.255.240

network 10.200.3.88 mask 255.255.255.255

network 100.179.40.0 mask 255.255.255.0

redistribute static

neighbor 152.161.42.85 remote-as 65000

neighbor 152.161.42.85 route-map blocked-sites in

neighbor 152.161.42.85 route-map Prepend_AS out

no auto-summary

________________________________________________________________

route-map Prepend_AS, permit, sequence 10

Match clauses:

ip address prefix-lists: Static-routes

Set clauses:

as-path prepend 65457 65457

Policy routing matches: 0 packets, 0 bytes

route-map Prepend_AS, permit, sequence 20

Match clauses:

Set clauses:

Policy routing matches: 0 packets, 0 bytes

__________________________________________________________________

ip prefix-list Static-routes: 1 entries

seq 10 permit 172.20.1.1/32

_____________________________________________________________________

ip route 172.20.1.1 255.255.255.255 10.13.0.1

______________________________________________________________________

APASACR1#show ip bgp nei 152.161.42.85 advertised-routes

BGP table version is 1905, local router ID is 10.200.3.88

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

*> 10.1.181.80/28 0.0.0.0 0 32768 i

*> 10.200.3.88/32 0.0.0.0 0 32768 i

*> 100.179.40.0/24 0.0.0.0 0 32768 i

*> 172.20.1.1/32 0.0.0.0 0 32768 ?

Total number of prefixes 4

___________________________________________________

Re: Question_As_Path_Prepend

Kishore Chennupati — Sat, 29 Oct 2011 10:19:13 GMT

Hi Vinayaka,

Can you please paste the output of " sh ip bgp neigh 152.161.42.85 advertised-routes" This command will show you what prefixes you are advertising to your ISP bgp peer..

If i just remove thr route-map, it is advertising zero routes.

I have one more route-map applied in bound, is it causing a problem?

___________________________________________________

I dont see "neighbor 152.161.42.85 soft-configuration outbound" . Because the command clear ip bgp * soft out

won't work if you dont have the above command.. Actually it will complain when you try to do it so maybe you have an IOS that allows you to do it even when you don't have it configured. I always use "clear ip bgp * out" . For this command to work you dont need the "neighbor 152.161.42.85 soft-configuration outbound".

You can created on route-map per direction so your inbound route-map shouldn't be causing any issues.

Aslo please post the prefix list config as well

HTH

Kishore

Re: Question_As_Path_Prepend

Kishore Chennupati — Sat, 29 Oct 2011 10:31:13 GMT

Hi Raman

If i prepend with 65000 two times in my route-map, then this route is not getting advertised..

This is BGP's loop avoidance mechanism. when you prepend 65000 and send it to the neighbouring AS which is

65000, BGP will detect that in the AS_PATH and will drop the prefix because it's seeing a route that should be generated within its AS. So, essentially its a loop prevention mechanism.

It's like this if someone is trying to sell your car to you. Will you buy it? You will just say "hey this is my car. how come you are selling it to me? hehe

I am prepnding my local as two times 65457 ..but in the remote sites I see it is prepended with the my remote as 65000 65000...I am not sure why is this behaviour..Do you have any idea..?


This behaviour is called "as path override" , If your router on the remote end is also in the same 
AS which is 65457 then if you prepend using 65457 then your remote router will see this again 
as a loop prevention mechanism and drop it. Hence ISP's normally use something called "AS-override" option to counter this.

You can also something "allowas-in" to do the same on your CE Router but let the ISP do the job for you 


HTH

Kishore