BGP order of preference

[Roel Reyes]

Hi All, 

Im confused with the following documents on how bgp process updates and result on my lab

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/5816-bgpfaq-5816.html#one

For outbound updates the order of preference is:

1. filter-list

2. route-map | unsuppress-map

3. advertise-map (conditional-advertisement)

4. prefix-list|distribute-list

5. ORF prefix-list (a prefix-list the neighbor sends us)

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/5816-bgpfaq-5816.html#one

When i tried on my lab the result is, it updates outbound using route-map

My route-map is used to filter network 100.100.100.0

and

My filter list to advertised all local routes on my AS (1.1.1.0   &   10.10.10.0   &  100.100.100.0 )

router bgp 1111
bgp log-neighbor-changes
network 1.1.1.0 mask 255.255.255.0
network 10.10.10.0 mask 255.255.255.0
network 100.100.100.0 mask 255.255.255.0
neighbor 192.168.1.2 remote-as 1111
neighbor 192.168.1.2 route-map Local_route-out out
neighbor 192.168.1.2 filter-list 1 out
neighbor 192.168.1.6 remote-as 1111
neighbor 192.168.1.6 route-map Local_route-out out
!
ip forward-protocol nd
!
ip as-path access-list 1 permit ^$
!
no ip http server
no ip http secure-server
!
!
ip prefix-list Local_routes seq 5 permit 1.1.1.0/24
ip prefix-list Local_routes seq 10 permit 10.10.10.0/24
!
route-map Local_route-out permit 10
match ip address prefix-list Local_routes
!

R1# clear ip bgp * soft out
R1#show ip bgp neighbors 192.168.1.2 advertised-routes
BGP table version is 8, local router ID is 10.10.10.10
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.0/24 0.0.0.0 0 32768 i
*> 10.10.10.0/24 0.0.0.0 0 32768 i

[Francesco Molino]

Hi

That's normal you think it advertises prefixes based on route-map because:
- your as-path access-list filters locally originated prefixes (^$) which means only the 3 subnets as network statement (based on your bgp config)
- then a second policy filters only 2 subnets from those 3 based on your route-map/prefix-list
- finally the result you got is the sum of the previous 2 policies.

[Roel Reyes]

Hi,

So you mean, all filter(filter-list,route-map, prefix list) applied on neighbor will be executed? I thought only filter list or the one who is in first order of the preference.

Thanks,

[Jon Marshall · Accepted solution]

The order of preference tells you which order they are applied to your route advertisements but they are all applied. 

So as Francesco says the filter list is applied first and allows all prefixes through and then the route map is applied. 

Jon

[Francesco Molino · Accepted solution]

@jon answered this question.
All are applied but they're applied in the order you have seen in the reference document you gave.

[Hemant Sharma]

Francesco's answer is correct.

The routes pass through the filter-list first and all three are allowed, but then the route-map filter is configured to allow only two, which is where the third route gets dropped.

Regards,

Hemant

[Vishnu Vardhan S]

We are confusing here with the routes and the actual traffic. 

The policy we are applying manipulates when the route is learnt in inbound meaning for the out traffic. and viceversa. 

I think its working as expected. 

R1-1.1.1.1-----------b-R2-a--------------R3 3.3.3.3

If my internet is at 3.3.3.3 and if I apply policy on "a" with route map so my traffic from R1 to R3 will be manipulated. 

[Francesco Molino]

Not sure i follow you. What policy do you apply?

Also not sure it's related to the original question. Can you create a new post?

[Vishnu Vardhan S]

He is just confused between the function of dataplane and the control plane as we know the policy or filter list etc we apply we do it control the control plane updates to our Routing table from RIB in table, so the traffic from inside network will get impacted.