BGP Conditional Routing Advertise Map

 

R1 configuration

interface Loopback0 ip address 1.1.0.1 255.255.255.0 ! interface Loopback1 ip address 1.1.1.1 255.255.255.0 ! interface FastEthernet0/0 ip address 10.1.12.1 255.255.255.0 duplex auto speed auto ! interface FastEthernet0/1 ip address 10.1.13.1 255.255.255.0 duplex auto speed auto ! router bgp 100 no synchronization bgp log-neighbor-changes network 1.1.0.0 mask 255.255.255.0 network 1.1.1.0 mask 255.255.255.0 network 10.1.13.0 mask 255.255.255.0 neighbor 10.1.12.2 remote-as 100 neighbor 10.1.13.3 remote-as 300 no auto-summary

R2 configuration

interface FastEthernet0/1  ip address 10.1.12.2 255.255.255.0  duplex auto  speed auto ! router bgp 100  no synchronization  bgp log-neighbor-changes  neighbor 10.1.12.1 remote-as 100  no auto-summary

R3 configuration

interface Loopback0  ip address 3.3.0.3 255.255.255.0 interface Loopback1  ip address 3.3.1.3 255.255.255.0 interface FastEthernet0/1  ip address 10.1.13.3 255.255.255.0  duplex auto  speed auto ! router bgp 300  no synchronization  bgp log-neighbor-changes  network 3.3.0.0 mask 255.255.255.0  network 3.3.1.0 mask 255.255.255.0  neighbor 10.1.13.1 remote-as 100  no auto-summary

Using advertise map configure R1 to advertise 1.1.0.0/24 prefix only if 3.3.0.0/24

exists in R1 BGP table

To achieve this we will use Advertise map with exist map

The advertise map matches the prefix that we wan to advertise incase the condition is met in the exist map (route exist in the BGP table)

let’s see the configuration on R1

ip prefix-list R1_ADV seq 5 permit 1.1.0.0/24 ! ip prefix-list R3_EXIST seq 5 permit 3.3.0.0/24 ! route-map ADVERTISE permit 10  match ip address prefix-list R1_ADV ! route-map EXIST permit 10  match ip address prefix-list R3_EXIST router bgp 100 neighbor 10.1.12.2 advertise-map ADVERTISE  exist-map EXIST

let’s see the BGP table on R2 before we shut down lo0 on R3

R2(config-if)#do sh ip bgp BGP table version is 6, local router ID is 2.2.1.2 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 *>i1.1.0.0/24       10.1.12.1                0    100      0 i *>i1.1.1.0/24       10.1.12.1                0    100      0 i *>i3.3.0.0/24       10.1.13.3                0    100      0 300 i *>i3.3.1.0/24       10.1.13.3                0    100      0 300 i *>i10.1.13.0/24     10.1.12.1                0    100      0 i

we can see that 1.1.0.0/24 and 3.3.0.0/24 exist in R2 BGP table

now let’s shut down lo0 on R3 and see the BGP on R2

R2(config-if)#do sh ip bgp BGP table version is 4, local router ID is 2.2.1.2 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 *>i1.1.1.0/24       10.1.12.1                0    100      0 i *>i3.3.1.0/24       10.1.13.3                0    100      0 300 i *>i10.1.13.0/24     10.1.12.1                0    100      0 i

we can see that R2 is not receiving the prefix 1.1.0.0/24 as expected

also we can check that on R1

R1(config-router)#do sh ip bgp ne 10.1.12.2 | b Outbound                                                               Outbound    Inbound   Local Policy Denied Prefixes:    --------    -------     advertise-map:                                1        n/a     Total:                                                    1          0

Now let’s try the opposite, so we will advertise 1.1.0.0/24 only 3.3.0.0 doesn’t exist

route-map NON_EXIST permit 10  match ip address prefix-list R3_ADV router bgp 100 neighbor 10.1.12.2 advertise-map ADVERTISE no-exist-map NON_EXIST

No let’s check the BGP on R2 when lo0 is up on R3

R2(config-if)#do sh ip bgp BGP table version is 13, local router ID is 2.2.1.2 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 *>i3.3.0.0/24       10.1.13.3                0    100      0 300 i *>i3.3.1.0/24       10.1.13.3                0    100      0 300 i

as we can see we don’t receive 1.1.0.0/24 on R2 because R1 has 3.3.0.0/24 prefix from R3

now let’s shutdown lo0 on R3 and see the changes on R2

R2(config-if)#do sh ip bgp BGP table version is 24, local router ID is 2.2.1.2 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 *>i1.1.0.0/24       10.1.12.1                0    100      0 i *>i1.1.1.0/24       10.1.12.1                0    100      0 i * i3.3.1.0/24       10.1.13.3                0    100      0 300 i *>i10.1.13.0/24     10.1.12.1                0    100      0 i

As we can see on R2 when R1 doesn’t have 3.3.0.0/24 we start receiving 1.1.0.0/24

R1(config-router)#do sh ip bgp BGP table version is 5, local router ID is 1.1.1.1 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 *> 1.1.0.0/24       0.0.0.0                  0         32768 i *> 1.1.1.0/24       0.0.0.0                  0         32768 i *> 3.3.1.0/24       10.1.13.3                0             0 300 i *> 10.1.13.0/24     0.0.0.0                  0         32768 i