BGP Conditional Routing Advertise Map

 

image

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

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