How to configure prefix summarization with aggregate address BGP

Design & Configure

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 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 ?

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 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 ?

Author: Marcin Bialy