BGP network route-map command

Let's consider following simple network:

Here is related configuration
R1
interface Loopback0
 ip address 1.1.1.1 255.255.255.255

interface FastEthernet0/0
 ip address 192.168.12.1 255.255.255.0
!

router bgp 1

 no synchronization

 bgp log-neighbor-changes

 network 1.1.1.1 mask 255.255.255.255 

 neighbor 192.168.12.2 remote-as 2

 neighbor 192.168.12.2 send-community both

 no auto-summary

!

ip bgp-community new-format

R2

interface FastEthernet0/0

 ip address 192.168.12.2 255.255.255.0

end

interface FastEthernet0/1

 ip address 192.168.23.2 255.255.255.0

end

router bgp 2

 no synchronization

 bgp log-neighbor-changes

 neighbor 192.168.12.1 remote-as 1

 neighbor 192.168.23.3 remote-as 3

 no auto-summary

!

ip bgp-community new-format

R3

interface FastEthernet0/0

 ip address 192.168.23.3 255.255.255.0

!

router bgp 3

 no synchronization

 bgp log-neighbor-changes

 neighbor 192.168.23.2 remote-as 2

 no auto-summary

The prefix 1.1.1.1/32 is advertised to R2 and R3:

R2#sho ip bgp 1.1.1.1

BGP routing table entry for 1.1.1.1/32, version 14

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Flag: 0x820

  Advertised to update-groups:

        1

  1

    192.168.12.1 from 192.168.12.1 (1.1.1.1)

      Origin IGP, metric 0, localpref 100, valid, external, best

Now, let's add community attribute to 1.1.1.1/32 prefix by using network route-map command. First, we create route-map

R1#conf t
R1(config)#route-map LOOP1
R1(config-route-map)#set community 1:100
R1(config-route-map)#exit

Second, apply this route-map to the prefix
R1(config)#router bgp 1
R1(config-router)#network 1.1.1.1 mask 255.255.255.255 route-map LOOP1

We can see on R2 that community 1:100 was indeed added to 1.1.1.1/32

R2#show ip bgp 1.1.1.1

BGP routing table entry for 1.1.1.1/32, version 15

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Flag: 0x8A0

  Advertised to update-groups:

        1

  1

    192.168.12.1 from 192.168.12.1 (1.1.1.1)

      Origin IGP, metric 0, localpref 100, valid, external, best

      Community: 1:100

So far, so good. Now I am going to try well-known community no-export, so R2 does not advertise 1.1.1.1/32 to R3

R1# conf t

R1(config)#route-map LOOP1

R1(config-route-map)#no set community 1:100

R1(config-route-map)#set community no-export

Debug output on R1 indicates that 1.1.1.1/32 is not advertised to R2

*Mar  1 02:37:19.771: BGP(0): sourced route for 1.1.1.1/32 path #0 changed (weight 32768)

*Mar  1 02:37:20.231: BGP(0): nettable_walker 1.1.1.1/32 route sourced locally

*Mar  1 02:37:20.231: BGP(0): 192.168.12.2 send unreachable 1.1.1.1/32

*Mar  1 02:37:20.231: BGP(0): 192.168.12.2 send UPDATE 1.1.1.1/32 -- unreachable

*Mar  1 02:37:20.319: BGP(0): 192.168.12.2 rcv UPDATE about 1.1.1.1/32 -- withdrawn

Sure enough:

R2#show ip bgp 1.1.1.1/32

% Network not in table

That's not what I intended. What happened? The thing is route-map key in BGP network command changes attributes BEFORE affected prefix is inserted into BGP routing table. In this case no-export community is added first, then R1 puts 1.1.1.1/32 in BGP routing table on R1, where R1 sees that this prefix can not be advertised outside its AS. To verify, let's change R1-R2 from eBGP into iBGP:

R1#sho run | sec r b

router bgp 1

 no synchronization

 bgp log-neighbor-changes

 network 1.1.1.1 mask 255.255.255.255 route-map LOOP1

 neighbor 192.168.12.2 remote-as 1

 neighbor 192.168.12.2 send-community both

 no auto-summary

R2#sho run | sec r b

router bgp 1

 no synchronization

 bgp log-neighbor-changes

 neighbor 192.168.12.1 remote-as 1

 neighbor 192.168.23.3 remote-as 3

 no auto-summary

R3#sho run | sec r b

router bgp 3

 no synchronization

 bgp log-neighbor-changes

 neighbor 192.168.23.2 remote-as 1

 no auto-summary

1.1.1.1/32 should appear in R2's BGP routing table:

R2#sho ip bgp 1.1.1.1

BGP routing table entry for 1.1.1.1/32, version 3

Paths: (1 available, best #1, table Default-IP-Routing-Table, not advertised to EBGP peer)

  Not advertised to any peer

  Local

    192.168.12.1 from 192.168.12.1 (1.1.1.1)

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

      Community: no-export

Since R1 and R2 are in the same ASN, no-export community does not affect route advertisement. However,

R3#show ip bgp 1.1.1.1/32

% Network not in table

Important thing to remember is that in this case route-map changes attributes BEFORE prefix added into BGP routing table and advertised to other BGP peers.