tags and communities in MPLS

[Ahmed Dockrat]

When to tags and BGP communiyu values get stripped off in a MPLS environment.

So for example on a PE router if a tag or BGP community value is set on incoming routes from a CE, would this be propogated throughout the MPLS network or would it be local to that device and get stripped of once its routed through the core network.

Can either of these (tags/communities) be propgated through multiple AS networks, so for example set a tag/community on the CE, honour on PE, route through MPLS to remote PE into another MPLS network. At what point do these get stripped off.

thanks

[Giuseppe Larosa]

Hello Ahmed,

BGP normal communities and BGP extended communities like route targets can both be propagated if PE devices have a configuration for address-family vpnv4 that allows for this

router bgp AS

address-family vpnv4

send-community both

with both normal and extended communities are propagated.

If at SP AS border the same is done BGP standard communities and  BGP extended communities can be extended also.

For route-targets in inter-AS VPN sometimes the ASBR performs route target rewrite at border.

Hope to help

Giuseppe

[Ahmed Dockrat]

Thanks,

What about route tags set via a route map, can they be propogated.

[milan.kulik]

Hi,

not supported, I'm afraid.

When you try to apply

neighbor x.x.x.x route-map map-with-tag out

you get an error message:

% "tag" used as BGP outbound route-map, set tag not supported

BR,

Milan

[Ahmed Dockrat]

Thanks.

So i suppose then if a tag was set on an incoming route, it would be stripped out when routed within MPLS or to another EBGP peer.

[milan.kulik]

Hi,

if you try to use set tag command within a route-map applied as incoming route-map on a BGP neighbor or a route-map applied when redistributing IGP into BGP, you always get the same error message:

"..., set tag not supported"

So IMHO route tagging is not supported at all in BGP, you need to use BGP communities instead.

BR,

Milan

[Giuseppe Larosa]

Hello Ahmed,

as Milan has noted when dealing with BGP we should or actually must use BGP communities instead of route tags.

If PE-CE protocol is not eBGP you may need to use route tags with IGP, but then the route tag is NOT propagated to MP BGP as it would be a BGP community with appropriate configuration.

The PE router might be configured to add BGP communities based on route tags but it is not straightforward.

By the way, BGP communities are more powerful then route tags as multiple BGP communities can be associated to a single NLRI prefix.

Hope to help

Giuseppe

[hank]

I am looking for an IOS code snippet that would show how a BGP community value can be "transferred" into the MPLS area so that it can checked.

Thanks,

Hank