The article demonstrates how an AS can utilize a combination of aggregation and more-specific prefixes to influence how a neighboring AS reaches the networks reside in the AS.
AS 200 advertises the aggregate 192.168.0.0/16 and the more-specific prefix 192.168.1.0/24 through the RT2-RT4 link; and the aggregate 192.168.0.0/16 and the more-specific prefixes 192.168.2.0/24 and 192.168.3.0/24 through the RT1-RT3 link.
Below shows the BGP tables on RT1 and RT2 prior to implementing any BGP aggregation configuration:
RT1#sh ip bgp BGP table version is 6, local router ID is 13.13.13.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path * i192.168.1.0 12.12.12.2 0 100 0 200 i *> 13.13.13.3 0 0 200 i *> 192.168.2.0 13.13.13.3 0 200 i * i 12.12.12.2 0 100 0 200 i *> 192.168.3.0 13.13.13.3 0 200 i * i 12.12.12.2 0 100 0 200 i RT1# -------------------------------------------------------------------------------- RT2#sh ip bgp BGP table version is 5, local router ID is 24.24.24.2 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 192.168.1.0 24.24.24.4 0 200 i * i 12.12.12.1 0 100 0 200 i * i192.168.2.0 12.12.12.1 0 100 0 200 i *> 24.24.24.4 0 0 200 i * i192.168.3.0 12.12.12.1 0 100 0 200 i *> 24.24.24.4 0 0 200 i RT2#
The suppress map is a form of route maps that can be used to indicate the more-specific prefixes to be suppressed or the more-specific prefixes to be allowed.
When a prefix is permitted through the suppress map, the prefix is suppressed.
When a prefix is not permitted (denied) through the suppress map, the prefix is not suppressed – allowed.
The deny logic does not prevent the prefix from being advertised; but prevents it from being suppressed.
A suppress map called SUPPRESS is implemented on RT3 to prevent it from advertising 192.168.1.0/24. As a result, RT3 will advertise the aggregate 192.168.0.0/16, and the more-specific prefixes 192.168.2.0/24 and 192.168.3.0/24. Notice how suppressed entries are indicated with the s status code.
Note: In certain situations, some neighbors may require to receive some of the more-specific prefixes that have been suppressed. An unsuppress map is a form of route maps that is applied on a per-neighbor basis to allow previously suppressed prefixes to be advertised.
In this case, RT3 is configured to advertise the suppressed 192.168.1.0/24 prefix to RT4 for it to advertise to RT2 in AS 100.
RT3#conf t Enter configuration commands, one per line. End with CNTL/Z. RT3(config)#access-list 1 permit 192.168.1.0 0.0.0.255 RT3(config)#access-list 1 deny any RT3(config)# RT3(config)#access-list 2 permit 192.168.1.0 0.0.0.255 RT3(config)# RT3(config)#route-map SUPPRESS permit 10 RT3(config-route-map)#match ip address 1 RT3(config-route-map)#exit RT3(config)# RT3(config)#route-map ALLOW-IBGP permit 10 RT3(config-route-map)#match ip address 2 RT3(config-route-map)#exit RT3(config)# RT3(config)#router bgp 200 RT3(config-router)#aggregate-address 192.168.0.0 255.255.0.0 suppress-map SUPPRESS RT3(config-router)#neighbor 34.34.34.4 unsuppress-map ALLOW-IBGP RT3(config-router)# RT3(config-router)#do sh ip bgp BGP table version is 6, local router ID is 192.168.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 192.168.0.0/16 0.0.0.0 32768 i s> 192.168.1.0 0.0.0.0 0 32768 i *>i192.168.2.0 34.34.34.4 0 100 0 i *>i192.168.3.0 34.34.34.4 0 100 0 i RT3(config-router)# -------------------------------------------------------------------------------- RT4#sh ip bgp BGP table version is 7, local router ID is 192.168.3.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *>i192.168.0.0/16 34.34.34.3 0 100 0 i *>i192.168.1.0 34.34.34.3 0 100 0 i *> 192.168.2.0 0.0.0.0 0 32768 i *> 192.168.3.0 0.0.0.0 0 32768 i RT4#
RT1#sh ip bgp BGP table version is 10, local router ID is 13.13.13.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path * i192.168.0.0/16 12.12.12.2 0 100 0 200 i *> 13.13.13.3 0 0 200 i *>i192.168.1.0 12.12.12.2 0 100 0 200 i *> 192.168.2.0 13.13.13.3 0 200 i * i 12.12.12.2 0 100 0 200 i *> 192.168.3.0 13.13.13.3 0 200 i * i 12.12.12.2 0 100 0 200 i RT1# -------------------------------------------------------------------------------- RT2#sh ip bgp BGP table version is 9, local router ID is 24.24.24.2 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 192.168.0.0/16 24.24.24.4 0 200 i * i 12.12.12.1 0 100 0 200 i *> 192.168.1.0 24.24.24.4 0 200 i * i192.168.2.0 12.12.12.1 0 100 0 200 i *> 24.24.24.4 0 0 200 i * i192.168.3.0 12.12.12.1 0 100 0 200 i *> 24.24.24.4 0 0 200 i RT2#
RT4 is configured with the similar logic as with RT3 to advertise the aggregate 192.168.0.0/16 and its directly connected networks – 192.168.2.0/24 and 192.168.3.0/24 to AS 100 through RT3.
A suppress map called ALLOW that allows the prefix 192.168.1.0/24 (deny logic; not being suppressed) and suppresses everything else (permit any logic; suppress everything else) is implemented on RT4.
The naming of the suppress maps SUPPRESS and ALLOW on RT3 and RT4 respectively reflects the main function of the route maps.
In RT3’s configuration, it is more appropriate to suppress a specific entry – 192.168.1.0/24 and allow others as there are much more prefixes to be allowed – 192.168.2.0/24 and 192.168.3.0/24.
In RT4’s configuration, it is more appropriate to allow a specific entry – 192.168.1.0/24 and suppress others as there are much more prefixes to be suppressed – 192.168.2.0/24 and 192.168.3.0/24.
RT4#conf t Enter configuration commands, one per line. End with CNTL/Z. RT4(config)#access-list 1 deny 192.168.1.0 0.0.0.255 RT4(config)#access-list 1 permit any RT4(config)# RT4(config)#access-list 2 permit 192.168.2.0 0.0.0.255 RT4(config)#access-list 2 permit 192.168.3.0 0.0.0.255 RT4(config)# RT4(config)#route-map ALLOW permit 10 RT4(config-route-map)#match ip address 1 RT4(config-route-map)#exit RT4(config)# RT4(config)#route-map ALLOW-IBGP permit 10 RT4(config-route-map)#match ip address 2 RT4(config-route-map)#exit RT4(config)# RT4(config)#router bgp 200 RT4(config-router)#aggregate-address 192.168.0.0 255.255.0.0 suppress-map ALLOW RT4(config-router)#neighbor 34.34.34.3 unsuppress-map ALLOW-IBGP RT4(config-router)# RT4(config-router)#do sh ip bgp BGP table version is 10, local router ID is 192.168.3.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 192.168.0.0/16 0.0.0.0 32768 i * i 34.34.34.3 0 100 0 i *>i192.168.1.0 34.34.34.3 0 100 0 i s> 192.168.2.0 0.0.0.0 0 32768 i s> 192.168.3.0 0.0.0.0 0 32768 i RT4(config-router)# -------------------------------------------------------------------------------- RT3(config-router)#do sh ip bgp BGP table version is 10, local router ID is 192.168.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path * i192.168.0.0/16 34.34.34.4 0 100 0 i *> 0.0.0.0 32768 i s> 192.168.1.0 0.0.0.0 0 32768 i *>i192.168.2.0 34.34.34.4 0 100 0 i *>i192.168.3.0 34.34.34.4 0 100 0 i RT3(config-router)#
RT1#sh ip bgp BGP table version is 14, local router ID is 13.13.13.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path * i192.168.0.0/16 12.12.12.2 0 100 0 200 i *> 13.13.13.3 0 0 200 i *>i192.168.1.0 12.12.12.2 0 100 0 200 i *> 192.168.2.0 13.13.13.3 0 200 i *> 192.168.3.0 13.13.13.3 0 200 i RT1# -------------------------------------------------------------------------------- RT2#sh ip bgp BGP table version is 16, local router ID is 24.24.24.2 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 192.168.0.0/16 24.24.24.4 0 0 200 i * i 12.12.12.1 0 100 0 200 i *> 192.168.1.0 24.24.24.4 0 200 i *>i192.168.2.0 12.12.12.1 0 100 0 200 i *>i192.168.3.0 12.12.12.1 0 100 0 200 i RT2#
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