BGP: Aggregate address

    BGP: Aggregate address
    Technology: Routing

    Area: EGP

    Vendor: Cisco

    Title: BGP aggregation

    Software: 12.X , 15.X, IP Services

    Platform: Catalyst 3560, 3750, 3850, 4500, 6500, ISR/ASR Routers 

    Aggregation is a fundamental characteristic used by BGP to hide any number of prefixes from being advertised to neighbors. The key think is that have to be at least one prefix in a BGP table which is a part of aggregation scope. The newly created prefix has two additional attributes such as:

    • AGGREGATOR – consist of router information like Router-ID, AS_PATH which does actual job,
    • ATOMIC_AGGREGATE – inform other routers that some part of BGP attributes are missing like complete AS_PATH, NEXT_HOP.
      To prevent the routing loops from occurring, BGP process creates a new static route to Null0 for every aggregate statement. 

    To configure aggregation for prefixes encompasses by prefix 50.0.0.0/16 type:

    R1# configure terminal
    R1 (config)# router bgp 100
    R1 (config-router)# neighbor 80.50.0.2 remote-as 100
    R1 (config-router)# aggregate-address 50.0.0.0 255.255.0.0
    R1 (config-router)# exit

     Verification commands:

    R1# show bgp ipv4 unicast
    Network Next Hop Metric LocPrf Weight Path
    *> 50.0.0.0/24 0.0.0.0 0 32768 ?
    *> 50.0.0.0/16 0.0.0.0 32768 i
    *> 50.0.1.0/24 0.0.0.0 0 32768 ?
    *> 80.50.0.0/24 0.0.0.0 0 32768 ?

     

    R1# show ip route 50.0.0.0 255.255.0.0
    Routing entry for 50.0.0.0/16
    Known via "bgp 100", distance 200, metric 0, type locally generated
    Routing Descriptor Blocks:
    * directly connected, via Null0
    Route metric is 0, traffic share count is 1
    AS Hops 0 

    R2# show bgp ipv4 unicast neighbors 80.50.0.1 routes
    Network Next Hop Metric LocPrf Weight Path
    *>i50.0.0.0/24 80.50.0.1 0 100 0 ?
    *>i50.0.0.0/16 80.50.0.1 0 100 0 i
    *>i50.0.1.0/24 80.50.0.1 0 100 0 ?
    r>i80.50.0.0/24 80.50.0.1 0 100 0 ?

    R2# show bgp ipv4 unicast 50.0.0.0/16
    BGP routing table entry for 50.0.0.0/16, version 8
    Paths: (1 available, best #1, table Default-IP-Routing-Table)
    Not advertised to any peer
    Local, (aggregated by 100 1.1.1.1)
    80.50.0.1 from 80.50.0.1 (1.1.1.1)
    Origin IGP, metric 0, localpref 100, valid, internal, atomic-aggregate, best

     Extensions:

    Command offers a couple of keyword extensions to influence a final result:

    summary-only – to automatically suppress advertainment of more specific prefixes.

    R1# configure terminal
    R1 (config)# router bgp 100
    R1 (config-router)# aggregate-address 50.0.0.0 255.255.0.0 summary-only
    R1 (config-router)# exit

     Verification commands:

    R1# show bgp ipv4 unicast
    Network Next Hop Metric LocPrf Weight Path
    s> 50.0.0.0/24 0.0.0.0 0 32768 ?
    *> 50.0.0.0/16 0.0.0.0 32768 i
    s> 50.0.1.0/24 0.0.0.0 0 32768 ?
    *> 80.50.0.0/24 0.0.0.0 0 32768 ?

    R1# show bgp ipv4 unicast neighbors 80.50.0.2 advertised-routes
    Network Next Hop Metric LocPrf Weight Path
    *> 50.0.0.0/16 0.0.0.0 32768 i
    *> 80.50.0.0/24 0.0.0.0 0 32768 ?

    R2# show bgp ipv4 unicast neighbors 80.50.0.1 routes
    Network Next Hop Metric LocPrf Weight Path
    *>i50.0.0.0/16 80.50.0.1 0 100 0 i
    r>i80.50.0.0/24 80.50.0.1 0 100 0 ?

     

    suppress-map – to selectively suppress advertainment of some prefixes.

    R1# configure terminal
    R1 (config)# ip prefix-list SUPPRESS permit 50.0.1.0/24
    R1 (config)# route-map SUPPRESS_MAP permit 10
    R1 (config-route-map)# match ip address prefix-list SUPPRESS
    R1 (config)# router bgp 100
    R1 (config-router)# aggregate-address 50.0.0.0 255.255.0.0 suppress-map SUPPRESS_MAP
    R1 (config-router)# exit

     Verification commands:

    R1# show bgp ipv4 unicast
    Network Next Hop Metric LocPrf Weight Path
    *> 50.0.0.0/24 0.0.0.0 0 32768 ?
    *> 50.0.0.0/16 0.0.0.0 32768 i
    s> 50.0.1.0/24 0.0.0.0 0 32768 ?
    *> 80.50.0.0/24 0.0.0.0 0 32768 ?

    R1# show bgp ipv4 unicast neighbors 80.50.0.2 advertised-routes
    Network Next Hop Metric LocPrf Weight Path
    *> 50.0.0.0/24 0.0.0.0 0 32768 ?
    *> 50.0.0.0/16 0.0.0.0 32768 i
    *> 80.50.0.0/24 0.0.0.0 0 32768 ?


    R2# show bgp ipv4 unicast neighbors 80.50.0.1 routes
    Network Next Hop Metric LocPrf Weight Path
    *>i50.0.0.0/24 80.50.0.1 0 100 0 ?
    *>i50.0.0.0/16 80.50.0.1 0 100 0 i
    r>i80.50.0.0/24 80.50.0.1 0 100 0 ?

     

    as-set – to restore the whole set of AS_PATH information and include it in aggregated prefix, the ATOMIC_AGGREGATE attribute is not needed any more.


    R1# configure terminal
    R1 (config)# router bgp 100
    R1 (config-router)# aggregate-address 50.0.0.0 255.255.0.0 summary-only as-set
    R1 (config-router)# exit

     Verification commands:

     R1# show bgp ipv4 unicast
    Network Next Hop Metric LocPrf Weight Path
    s> 50.0.0.0/24 0.0.0.0 0 32768 ?
    *> 50.0.0.0/16 0.0.0.0 100 32768 ?
    s> 50.0.1.0/24 0.0.0.0 0 32768 ?
    *> 80.50.0.0/24 0.0.0.0 0 32768 ?

    R1# show bgp ipv4 unicast neighbors 80.50.0.2 advertised-routes
    Network Next Hop Metric LocPrf Weight Path
    *> 50.0.0.0/16 0.0.0.0 100 32768 ?
    *> 80.50.0.0/24 0.0.0.0 0 32768 ?

    R2# show bgp ipv4 unicast neighbors 80.50.0.1 routes
    Network Next Hop Metric LocPrf Weight Path
    *>i50.0.0.0/16 80.50.0.1 0 100 0 ?
    r>i80.50.0.0/24 80.50.0.1 0 100 0 ?

    R2# show bgp ipv4 unicast 50.0.0.0/16
    BGP routing table entry for 50.0.0.0/16, version 21
    Paths: (1 available, best #1, table Default-IP-Routing-Table)
    Flag: 0x820
    Not advertised to any peer
    Local, (aggregated by 100 1.1.1.1)
    80.50.0.1 from 80.50.0.1 (1.1.1.1)

    Origin incomplete, metric 0, localpref 100, valid, internal, best

    attribute-map – to assign specific attributes for new aggregate prefix and override the current ones.


    R1# configure terminal
    R1 (config)# route-map ATT_MAP permit 10
    R1 (config-route-map)# set community 100:100
    R1 (config-route-map)# set origin incomplete
    R1 (config)# router bgp 100
    R1 (config-router)# neighbor 80.50.0.2 send-community
    R1 (config-router)# aggregate-address 50.0.0.0 255.255.0.0 summary-only as-set attribute-map ATT_MAP
    R1 (config-router)# exit

     Verification commands:

    R1# show bgp ipv4 unicast
    Network Next Hop Metric LocPrf Weight Path
    s> 50.0.0.0/24 0.0.0.0 0 32768 ?
    *> 50.0.0.0/16 0.0.0.0 100 32768 ?
    s> 50.0.1.0/24 0.0.0.0 0 32768 ?
    *> 80.50.0.0/24 0.0.0.0 0 32768 ?

    R1# show bgp ipv4 unicast neighbors 80.50.0.2 advertised-routes
    Network Next Hop Metric LocPrf Weight Path
    *> 50.0.0.0/16 0.0.0.0 100 32768 ?
    *> 80.50.0.0/24 0.0.0.0 0 32768 ?

    R2# show bgp ipv4 unicast neighbors 80.50.0.1 routes
    Network Next Hop Metric LocPrf Weight Path
    *>i50.0.0.0/16 80.50.0.1 0 100 0 ?
    r>i80.50.0.0/24 80.50.0.1 0 100 0 ?

    R2# show bgp ipv4 unicast 50.0.0.0/16
    BGP routing table entry for 50.0.0.0/16, version 21
    Paths: (1 available, best #1, table Default-IP-Routing-Table)
    Flag: 0x820
    Not advertised to any peer
    Local, (aggregated by 100 1.1.1.1)
    80.50.0.1 from 80.50.0.1 (1.1.1.1)
    Origin incomplete, metric 0, localpref 100, valid, internal, best
    Community: 100:100

     

    advertise-map – to choose specific prefixes for composing new set of attributes such as AS_SET for aggregate prefix.


    R1# configure terminal
    R1 (config)# ip prefix-list ADVERT permit 50.0.1.0/24
    R1 (config)# route-map ADVERT_MAP permit 10
    R1 (config-route-map)# match ip address prefix-list ADVERT
    R1 (config)# router bgp 100
    R1 (config-router)# aggregate-address 50.0.0.0 255.255.0.0 summary-only as-set advertise-map ADVERT_MAP
    R1 (config-router)# exit

    Verification commands:


    R1# show bgp ipv4 unicast
    Network Next Hop Metric LocPrf Weight Path
    s> 50.0.0.0/24 0.0.0.0 0 32768 ?
    *> 50.0.0.0/16 0.0.0.0 100 32768 ?
    s> 50.0.1.0/24 0.0.0.0 0 32768 ?
    *> 80.50.0.0/24 0.0.0.0 0 32768 ?

    R1# show bgp ipv4 unicast neighbors 80.50.0.2 advertised-routes
    Network Next Hop Metric LocPrf Weight Path
    *> 50.0.0.0/16 0.0.0.0 100 32768 ?
    *> 80.50.0.0/24 0.0.0.0 0 32768 ?

    R2# show bgp ipv4 unicast neighbors 80.50.0.1 routes
    Network Next Hop Metric LocPrf Weight Path
    *>i50.0.0.0/16 80.50.0.1 0 100 0 ?
    r>i80.50.0.0/24 80.50.0.1 0 100 0 ?

    R2# show bgp ipv4 unicast 50.0.0.0/16
    BGP routing table entry for 50.0.0.0/16, version 21
    Paths: (1 available, best #1, table Default-IP-Routing-Table)
    Flag: 0x820
    Not advertised to any peer
    Local, (aggregated by 100 1.1.1.1)
    80.50.0.1 from 80.50.0.1 (1.1.1.1)
    Origin incomplete, metric 0, localpref 100, valid, internal, atomic-aggregate, best

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