BGP No-Advertise Community

Contents

Intro

The BGP NO-ADVERTISE community is a “well-known” community that prevents a route from being advertised to any peers, including both iBGP and eBGP. BGP routers that receive prefixes with this community attached understand not to send it to any iBGP or eBGP peers.

Topology

BGP NO_ADVERTISE community tutorial topology with four routers: R1 in AS1, R2 in AS23, and R3 and R4 in AS34, connected in a linear configuration for EVE-NG simulation.
The last octet is the router number unless specified otherwise. Example: R1's G0/2 is 10.10.13.1/24. Don't forget to download the EVE-NG topology file for this tutorial below.

Initial Configs

Configuration Steps

1. Configure Basic BGP

I’ll start by getting all of the BGP peerings up using their respective AS numbers and R2’s Lo2 prefix advertised into BGP.

R1:

router bgp 1
 neighbor 10.10.13.3 remote-as 34

R2:

router bgp 2
 neighbor 10.10.23.3 remote-as 34
 network 2.2.2.2 mask 255.255.255.255

R3:

router bgp 34
 neighbor 10.10.13.1 remote-as 1
 neighbor 10.10.23.2 remote-as 2
 neighbor 10.10.34.4 remote-as 34

R4:

router bgp 34
 neighbor 10.10.34.3 remote-as 34

If you want to verify that BGP neighbors are up and prefixes are being received you can do that using the show ip bgp summary command on all routers. 

R1:

R1#show ip bgp summ | begin Neighbor
Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
10.10.13.3      4           34      11      10        2    0    0 00:07:09        1

R1’s peering with R3 is up and one prefix is being received.

R2:

R2#show ip bgp summ | begin Neighbor
Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
10.10.23.3      4           34      11      11        2    0    0 00:07:00        0

R2’s peerings with R3 is up.

R3:

R3#show ip bgp summ | begin Neighbor
Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
10.10.13.1      4            1      10      11        2    0    0 00:07:09        0
10.10.23.2      4            2      11      11        2    0    0 00:06:59        1
10.10.34.4      4           34       9      10        2    0    0 00:05:01        0

R3’s peerings with R1, R2 and R4 are up. You can also see R3 is receiving one prefix from R2.

R4:

R4#show ip bgp summ | begin Neighbor
Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
10.10.34.3      4           34      10       9        1    0    0 00:05:01        1

R4’s peering with R3 is up as well and one prefix is being received. 

Let me make sure that R1, R3 and R4 is receiving the correct prefix. I’ll use show ip bgp for this.

R1#show ip bgp | begin Network
     Network          Next Hop            Metric LocPrf Weight Path
 *>   2.2.2.2/32       10.10.13.3                             0 34 2 i
R3#show ip bgp | begin Network
     Network          Next Hop            Metric LocPrf Weight Path
 *>   2.2.2.2/32       10.10.23.2               0             0 2 i

R4#show ip bgp | begin Network
     Network          Next Hop            Metric LocPrf Weight Path
 * i  2.2.2.2/32       10.10.23.2               0    100      0 2 i

So far I’ve confirmed that the BGP peerings are up and the correct prefix from R2 is being received throughout the network. Now I’ll make sure that when R2 sends prefixes to R3, the No-Advertise community is attached. When R3 receives this prefix from R2, it will see this community and not advertise that prefix to any BGP peers.

2. Enable the sending of communities.

In Cisco IOS, BGP communities are not sent by default. You need to enable it on a per-neighbor.

R2:

router bgp 2
 neighbor 10.10.23.3 send-community

3. Attach the No-Advertise community to R2's prefix.

Now you need to specifiy on R2 which prefixes the community should be attached to. I’ll use a route-map for this.

R2:

route-map R2-NO-ADVERTISE permit 10
set community no-advertise

By not having a match statement that references a prefix-list or ACL I’ll be applying the No-Advertise community on all prefixes. The only thing needed is to “set” the community to No-Advertise. 

The route-map is configured but it won’t do anything until I apply it to a neighbor.

R2:

router bgp 2
 neighbor 10.10.23.3 route-map R2-NO-ADVERTISE out

The route-map is applied towards the neighbor R3 in the outbound direction. This means the community is applied to all prefixes R2 sends outbound to R3. If I wanted, I could’ve applied this in the inbound direction on R3 instead. Let’s see the details of R2’s BGP table.

If the policy takes a while to kick in, do a clear ip bgp * soft on R2 to speed things up. 
R3#sh ip bgp 2.2.2.2
BGP routing table entry for 2.2.2.2/32, version 3
Paths: (1 available, best #1, table default, not advertised to any peer)
  Not advertised to any peer
  Refresh Epoch 1
  2
    10.10.23.2 from 10.10.23.2 (2.2.2.2)
      Origin IGP, metric 0, localpref 100, valid, external, best
      Community: no-advertise
      rx pathid: 0, tx pathid: 0x0

In the above output you can see that for the 2.2.2.2/32 prefix on R3, the No-Advertise community is applied. Here’s what it looked like before the route-map was applied.

R3#sh ip bgp 2.2.2.2
BGP routing table entry for 2.2.2.2/32, version 4
Paths: (1 available, best #1, table default)
  Advertised to update-groups:
     1          2         
  Refresh Epoch 1
  2
    10.10.23.2 from 10.10.23.2 (2.2.2.2)
      Origin IGP, metric 0, localpref 100, valid, external, best
      rx pathid: 0, tx pathid: 0x0

Now I’ll verify that R1 and R4 don’t have these prefixes in their BGP tables. 

R1#show ip bgp 2.2.2.2
% Network not in table

R4#show ip bgp 2.2.2.2
% Network not in table

The above output shows that R1 and R4 aren’t recieving the 2.2.2.2/32 prefix anymore. What if you don’t have access to R1 or R4? How would you verify this is working? Let me show you a command you can use on R3. 

R3#sh ip bgp neighbors 10.10.13.1 advertised-routes 

Total number of prefixes 0 

R3#sh ip bgp neighbors 10.10.34.4 advertised-routes 

Total number of prefixes 0

The show ip bgp neighbors advertised-routes command shows me what prefixes the router is sending to specific neighbors. This is a great way to verify from the local routers perspecitve what exactly is being sent. In this case nothing thanks to the No-Advertise community which prevents prefixes from being advertised to iBGP or eBGP peers.  

Take a look at the BGP UPDATE message in Wireshark. This is what R2 sent to R3 when the No-Advertise community was applied. Notice under the Path Attribute it says Communities: NO_ADVERTISE

Wireshark capture screenshot displaying BGP NO_ADVERTISE community attribute in packet details.

EVE-NG Lab File

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Full Configs

Here are the full configs from all routers if you want to try it out yourself.

Discussion

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