cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
Announcements

Cisco Community Designated VIP Class of 2020

575
Views
10
Helpful
3
Replies
Highlighted
Beginner

MPLS Route Target Queries and MPLS topology query

I have following queries in MPLS VPN

 

1. Route targets are configured in VRFs to form VPN route, if I am not wrong. Will the Route targets for export and import for a particular VRF will be same in all PE routers in case of any-any type VRF ? In hub spoke topology we know all the spoke will have Import and Export route targets which will then be configured reversely in HUB router. How about in any-any type VRFs ?

 

2. For configuring Route Targets should if follow any format as like ASN:XXX or it can be any value like XXX:XXX ? I guess in RD we follow ASN:XXXX format. Please confirm for Route Targets.

 

3. In MPLS network, Just by having Route reflectors how all the VPN routes are populated in all PEs correctly ?? I guess Route reflector will have only have IBGP peering with all PEs but no VRFs configured in it. Still how does Route Reflector populate all VPN routes for different VRFs correctly to the PEs ? What is the logic behind it ?

 

4. Can we have CE routers Connected to PE routers with eBGP routing protocol in MPLS environment ? If yes, How will it work ? CE<<eBGP>>PE<<MPBGP>>PE<<eBGP>>CE

Consider the above topology for typical MPLS topology in which both end CE routers are in AS 100. However PE routers in AS 200. Will CE router routes populated to Far end CE router inspite of same AS100 ?? Ideally it should not. How to make it work ?

Everyone's tags (1)
1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Hi Yogesh,1. Route targets

Hi Yogesh,

1. Route targets are configured in VRFs to form VPN route, if I am not wrong. Will the Route targets for export and import for a particular VRF will be same in all PE routers in case of any-any type VRF ? In hub spoke topology we know all the spoke will have Import and Export route targets which will then be configured reversely in HUB router. How about in any-any type VRFs ?

<Nagendra> On any VRF, when you configure RT import, it imports all the prefixes that matches the configued RT value as one of the atrribute in the BGP update. So in any-any VRF, it is common to see all PE configured with same RT value (You may see more RT depending on inter-VRF scenarios).

2. For configuring Route Targets should if follow any format as like ASN:XXX or it can be any value like XXX:XXX ? I guess in RD we follow ASN:XXXX format. Please confirm for Route Targets.

<Nagendra> It is not mandatory to use ASN as part of RD or RT. You can use XXX:YYY format. The use of ASN:XXX is useful from operational point of view.

3. In MPLS network, Just by having Route reflectors how all the VPN routes are populated in all PEs correctly ?? I guess Route reflector will have only have IBGP peering with all PEs but no VRFs configured in it. Still how does Route Reflector populate all VPN routes for different VRFs correctly to the PEs ? What is the logic behind it ?

<Nagendra> By default, when a PE (non-RR) receives a BGP udpate and if none of the RT attached with this update is imported in any local VRF, the update will be ignored. Normally, the role of RR is to simply reflect the updates from 1 PE to all other PEs with all the attributes (like RT). So when a router is configured as RR for VPNv4 address family, the above RT check will be disabled and it will import all the prefixes to its BGP table.

 

4. Can we have CE routers Connected to PE routers with eBGP routing protocol in MPLS environment ? If yes, How will it work ? CE<<eBGP>>PE<<MPBGP>>PE<<eBGP>>CE

Consider the above topology for typical MPLS topology in which both end CE routers are in AS 100. However PE routers in AS 200. Will CE router routes populated to Far end CE router inspite of same AS100 ?? Ideally it should not. How to make it work ?

<Nagendra> You are right that it will not work under normal condition. When remote CE receives update with its local AS number in AS-PATH attribute, it ignores it. But there are 2 ways to make it accept:

 

1. Configure "bgp allows-as in" on CE routers. - This will make the CE Accept the prefix even if the update carries its own AS number in AS-PATH. It should be carefully used in scenarios where there is no backdoor neighbors to your AS and the only neighbor is over you SP (to avoid loops).

 

2. Configure as-override" on PE devices. - This makes the PE router to check if the last number in AS-PATH is same as the neighbor to whcih it will update the prefix and if so, will rewrite the last AS number with its own AS number.

 

HTH,

Nagendra

View solution in original post

3 REPLIES 3
Cisco Employee

Hi Yogesh,1. Route targets

Hi Yogesh,

1. Route targets are configured in VRFs to form VPN route, if I am not wrong. Will the Route targets for export and import for a particular VRF will be same in all PE routers in case of any-any type VRF ? In hub spoke topology we know all the spoke will have Import and Export route targets which will then be configured reversely in HUB router. How about in any-any type VRFs ?

<Nagendra> On any VRF, when you configure RT import, it imports all the prefixes that matches the configued RT value as one of the atrribute in the BGP update. So in any-any VRF, it is common to see all PE configured with same RT value (You may see more RT depending on inter-VRF scenarios).

2. For configuring Route Targets should if follow any format as like ASN:XXX or it can be any value like XXX:XXX ? I guess in RD we follow ASN:XXXX format. Please confirm for Route Targets.

<Nagendra> It is not mandatory to use ASN as part of RD or RT. You can use XXX:YYY format. The use of ASN:XXX is useful from operational point of view.

3. In MPLS network, Just by having Route reflectors how all the VPN routes are populated in all PEs correctly ?? I guess Route reflector will have only have IBGP peering with all PEs but no VRFs configured in it. Still how does Route Reflector populate all VPN routes for different VRFs correctly to the PEs ? What is the logic behind it ?

<Nagendra> By default, when a PE (non-RR) receives a BGP udpate and if none of the RT attached with this update is imported in any local VRF, the update will be ignored. Normally, the role of RR is to simply reflect the updates from 1 PE to all other PEs with all the attributes (like RT). So when a router is configured as RR for VPNv4 address family, the above RT check will be disabled and it will import all the prefixes to its BGP table.

 

4. Can we have CE routers Connected to PE routers with eBGP routing protocol in MPLS environment ? If yes, How will it work ? CE<<eBGP>>PE<<MPBGP>>PE<<eBGP>>CE

Consider the above topology for typical MPLS topology in which both end CE routers are in AS 100. However PE routers in AS 200. Will CE router routes populated to Far end CE router inspite of same AS100 ?? Ideally it should not. How to make it work ?

<Nagendra> You are right that it will not work under normal condition. When remote CE receives update with its local AS number in AS-PATH attribute, it ignores it. But there are 2 ways to make it accept:

 

1. Configure "bgp allows-as in" on CE routers. - This will make the CE Accept the prefix even if the update carries its own AS number in AS-PATH. It should be carefully used in scenarios where there is no backdoor neighbors to your AS and the only neighbor is over you SP (to avoid loops).

 

2. Configure as-override" on PE devices. - This makes the PE router to check if the last number in AS-PATH is same as the neighbor to whcih it will update the prefix and if so, will rewrite the last AS number with its own AS number.

 

HTH,

Nagendra

View solution in original post

Beginner

Thanks for explaining all the

Thanks for explaining all the queries and making me understand.

 

Only for the first query re-phrasing again as my query was little different, whether the RT values for both export and import actions will be same in any-any VRF ? 

Cisco Employee

Hi Yogesh,Yes. By any-any VRF

Hi Yogesh,

Yes. By any-any VRF, I hope you are mentioning about VPN sites that belongs to same customer connected over MPLS provider. If yes, it is common to see same RT for both import and export.

 -Nagendra

CreatePlease to create content
Content for Community-Ad
FusionCharts will render here