Same Extcommunity RT on different PEs

sourabhdsourabhd ✭
in CCIE Routing & Switching Technical

Hello Friends,

I have some PEs where same RT 1278 has been configured with ext community additive command in route-map and exported

so is this fine if we configure same RTs for different subnets in PEs.?

how actually PE will come to know and install all subnets with same RTs in respective vrf routing table?

thanks and regards,

sourabh

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  • Narayan.NeupaneNarayan.Neupane ✭

    how actually PE will come to know and install all subnets with same RTs in respective vrf routing table?

    Route-Target is the mechanism used to control the ip prefixes going from VRF table to BGP table and coming from BGP table to VRF table. We use route-target import <RT> and route-target export <RT> or route-target both <RT>. If you have to controll particular ip prefixes within route-target we use import map or export map using route-map.

    Can we use configure same RT for different subnets? It works even you configure same RT on different sites. see here I have two sites with same RT 1:100:

    R2#show run | section ip vrf
    ip vrf TEST
     rd 1:100
     route-target export 1:100
     route-target import 1:100

    Route 1.1.1.1 is coming from CE1 which is connected to R2 and 5.5.5.5 is coming from CE2 which is connected to R4 and VPNv4 between R2 and R4 plus redistribution.

    See this debug output what happens if we have same RT on both sites:

    I just turned on debug ip bgp vpnv4 unicast updates

    *Mar 19 15:52:22.191: BGP: TX VPNv4 Unicast Net TEST 1:100:1.1.1.1/32 Changed.
    *Mar 19 15:52:22.199: BGP: TX VPNv4 Unicast Net TEST 1:100:12.12.12.0/24 Changed.
    *Mar 19 15:52:22.371: BGP(4): nettable_walker 1:100:1.1.1.1/32 route sourced locally
    *Mar 19 15:52:22.375: BGP: topo TEST:VPNv4 Unicast:base Remove_fwdroute for 1:100:1.1.1.1/32
    *Mar 19 15:52:22.375: BGP: TX VPNv4 Unicast Net TEST 1:100:1.1.1.1/32 RIB done.
    *Mar 19 15:52:22.379: BGP(4): nettable_walker 1:100:12.12.12.0/24 route sourced locally
    *Mar 19 15:52:22.379: BGP: topo TEST:VPNv4 Unicast:base Remove_fwdroute for 1:100:12.12.12.0/24
    *Mar 19 15:52:22.379: BGP: TX VPNv4 Unicast Net TEST 1:100:12.12.12.0/24 RIB done.
    *Mar 19 15:52:22.379: BGP: TX VPNv4 Unicast Tab RIB walk done version 7, added 2 topologies.
    *Mar 19 15:52:22.379: BGP: TX VPNv4 Unicast Tab All topologies are EOR ready.
    *Mar 19 15:51:43.323: BGP: TX VPNv4 Unicast Net TEST 1:100:5.5.5.5/32 Changed.
    *Mar 19 15:51:43.323: BGP: TX VPNv4 Unicast Net TEST 1:100:45.45.45.0/24 Changed.
    *Mar 19 15:51:43.327: BGP(4): Revise route installing 1 of 1 routes for 5.5.5.5/32 -> 4.4.4.4(TEST) to TEST IP table
    *Mar 19 15:51:43.331: BGP: TX VPNv4 Unicast Net TEST 1:100:5.5.5.5/32 RIB done.
    *Mar 19 15:51:43.331: BGP(4): Revise route installing 1 of 1 routes for 45.45.45.0/24 -> 4.4.4.4(TEST) to TEST IP table
    *Mar 19 15:51:43.335: BGP:

    It looks like both PE routers sending all imported routes from VRF table to each other to BGP peer and BGP is checking the route origination and removing it.I hope this helps you to understand how RT works.

    Good Luck

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  • dcanceriandcancerian ✭✭✭

    so is this fine if we configure same RTs for different subnets in PEs.?

    RT is used for controlling routes i.e which routes are importing/exporting from/to the vrf so its not a property of subnet as such.

     

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  • Brian McGahanBrian McGahan mod

    You can use the same or different route targets for different sites, it just depends on how you want to setup the topology.  The route can also have more than one RT, which allows you to build complex topologies like Central Services MPLS L3VPN.

     

    Brian McGahan, CCIE #8593 (R&S/SP/Security)
    [email protected]
     
    Internetwork Expert, Inc.
    http://www.INE.com
    Online Community: http://www.IEOC.com
    CCIE Blog: http://blog.INE.com

     

    From: [email protected] [mailto:[email protected]] On Behalf Of Narayan.Neupane
    Sent: Monday, March 19, 2012 5:26 AM
    To: Brian McGahan
    Subject: Re: [CCIE R&S] Same Extcommunity RT on different PEs

     

    imagesourabhd:

    how actually PE will come to know and install all subnets with same RTs in respective vrf routing table?

    Route-Target is the mechanism used to control the ip prefixes going from VRF table to BGP table and coming from BGP table to VRF table. We use route-target import <RT> and route-target export <RT> or route-target both <RT>. If you have to controll particular ip prefixes within route-target we use import map or export map using route-map.

    Can we use configure same RT for different subnets? It works even you configure same RT on different sites. see here I have two sites with same RT 1:100:

    R2#show run | section ip vrf
    ip vrf TEST
     rd 1:100
     route-target export 1:100
     route-target import 1:100

    Route 1.1.1.1 is coming from CE1 which is connected to R2 and 5.5.5.5 is coming from CE2 which is connected to R4 and VPNv4 between R2 and R4 plus redistribution.

    See this debug output what happens if we have same RT on both sites:

    I just turned on debug ip bgp vpnv4 unicast updates

    *Mar 19 15:52:22.191: BGP: TX VPNv4 Unicast Net TEST 1:100:1.1.1.1/32 Changed.
    *Mar 19 15:52:22.199: BGP: TX VPNv4 Unicast Net TEST 1:100:12.12.12.0/24 Changed.
    *Mar 19 15:52:22.371: BGP(4): nettable_walker 1:100:1.1.1.1/32 route sourced locally
    *Mar 19 15:52:22.375: BGP: topo TEST:VPNv4 Unicast:base Remove_fwdroute for 1:100:1.1.1.1/32
    *Mar 19 15:52:22.375: BGP: TX VPNv4 Unicast Net TEST 1:100:1.1.1.1/32 RIB done.
    *Mar 19 15:52:22.379: BGP(4): nettable_walker 1:100:12.12.12.0/24 route sourced locally
    *Mar 19 15:52:22.379: BGP: topo TEST:VPNv4 Unicast:base Remove_fwdroute for 1:100:12.12.12.0/24
    *Mar 19 15:52:22.379: BGP: TX VPNv4 Unicast Net TEST 1:100:12.12.12.0/24 RIB done.
    *Mar 19 15:52:22.379: BGP: TX VPNv4 Unicast Tab RIB walk done version 7, added 2 topologies.
    *Mar 19 15:52:22.379: BGP: TX VPNv4 Unicast Tab All topologies are EOR ready.
    *Mar 19 15:51:43.323: BGP: TX VPNv4 Unicast Net TEST 1:100:5.5.5.5/32 Changed.
    *Mar 19 15:51:43.323: BGP: TX VPNv4 Unicast Net TEST 1:100:45.45.45.0/24 Changed.
    *Mar 19 15:51:43.327: BGP(4): Revise route installing 1 of 1 routes for 5.5.5.5/32 -> 4.4.4.4(TEST) to TEST IP table
    *Mar 19 15:51:43.331: BGP: TX VPNv4 Unicast Net TEST 1:100:5.5.5.5/32 RIB done.
    *Mar 19 15:51:43.331: BGP(4): Revise route installing 1 of 1 routes for 45.45.45.0/24 -> 4.4.4.4(TEST) to TEST IP table
    *Mar 19 15:51:43.335: BGP:

    It looks like both PE routers sending all imported routes from VRF table to each other to BGP peer and BGP is checking the route origination and removing it.I hope this helps you to understand how RT works.

    Good Luck




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