BGP Route Refresh Capability

Lessons Discussion
1

This topic is to discuss the following lesson:

https://networklessons.com/bgp/bgp-route-refresh-capability/

2

Hi Rene,

I really enjoyed the presentation of your book called how to master ccna. This is the kind of presentation im looking for but most of these books out there have too much detail. Do you have a book for CCENT or atleast can you recommend a similar book for CCENT?

Thanks
Holmes

3

Your discussion on BGP synchronization is excellent. Really explains the topic. Many Thanks

4

Glad to hear that you enjoyed it!

5

Thank you for this clear demo.

Anything about Enhanced Route Refresh ?
Thanks,

6

Hi Navid,

Glad to hear you like it.

I don’t have anything on Enhanced Route Refresh at the moment. I’ll add it to my list, might be nice for the future.

Rene

7

question:

why we need “R2#clear ip bgp 192.168.12.1 in” here to make R2 sent Route-refresh? it should send automatically if route-refresh is supported and negotiated, thanks

8

Hi Xin,

Using the clear ip bgp in command is a nice way to manually request a route refresh from your neighbor.

Once you do this, you can see it in the debug that R2 is requesting R1 for a refresh:

R2#
BGP: 192.168.12.1 sending REFRESH_REQ(5) for afi/safi: 1/1

Rene

9

Hi Rene,

In summary …

HARD RESET : It will tear down TCP session as well as BGP session.Establishing new BGP session, will send Route refresh request to neighbor and learn all prefix again also network interuption will occured.

Soft Reconfiguration : Need More Memory due to store RAW Prefix in seperate table. No network interuption and will not send any Route refresh request.

Route Refresh : This is the most suitable method.No network interuption , no extra memory needed.Just send a Route refresh request.

Please correct me if I am wrong with my understanding .Thx

br/zaman

10

Hi NL Team,

If using the route refresh option inbound, I assume any filters we have Inbound will be processed before routes are installed in the actual routing table? Is there any reason one would use soft reconfiguration over route refresh apart from it either being supported / not supported on the platform?
Lastly, for route refresh to work do we need the peer router to be able to support this or only our own local router?
Thanks :grin:

11

Hello Robert,

That’s right. When you receive a refreshed route, filters are applied and then it gets installed in the BGP table, and in the routing table.

Today, there is no reason to use soft reconfiguration instead of route refresh. Soft reconfiguration was a bit of a workaround so you didn’t have to do a hard reset. Nowadays, all routers support route refresh so there’s no reason not to use it. Both peers need to support this.

Rene

1 Like
12

Thanks Rene,

Do we have a way of checking if the remote peer supports route refresh by any commands? Or would it be a case of having to ask the Provider of the peer link if their device supports route refresh?
Thanks
Robert

13

Hello Robert,

There is a quick way to see the capabilities when you have established a neighbor adjacency:

R1#show ip bgp neighbors | begin capa   
    1 active, is not multisession capable (disabled)
  Neighbor capabilities:
    Route refresh: advertised and received(new)
    Four-octets ASN Capability: advertised and received
    Address family IPv4 Unicast: advertised and received
    Enhanced Refresh Capability: advertised and received
    Multisession Capability: 
    Stateful switchover support enabled: NO for session 1

Rene

14

Hi Rene,
Do we need to always clear ip bgp <> in if we make a change to the inbound policy? I do not see the change takes effect unless I perform that. My routers are having route-refresh capability enabled

Thanks,
Madhu

15

Hello Madhu

If you make changes to the inbound or outbound policy, you will still require the clear ip bgp command. However, notice that by adding the keyword in, the reset is not a hard reset, but it is a soft reconfiguration that is implemented, as the options for the command indicate. So this is indeed a command that uses the route refresh capability.

So yes, you do need the clear ip bgp command but in order to verify that this command gives you the route refresh capability, you need to use the in, out or soft keywords.

I hope this has been helpful!

Laz

16

when you use clear ip bgp in, out or soft, this tells router to send a route refresh? so if I forgot to put the in, out or soft command at the end it would hard reset for that neighbor?

17

Hello Justin

Only the clear ip bgp soft command will initiate a BGP reset without tearing down the session. If you use clear ip bgp in or clear ip bgp out, it will still be a hard reset. If you use clear ip bgp it will do a hard reset on both inbound ant outbound sessions.

You can find out more info about this command at this Cisco command reference.

I hope this has been helpful!

Laz

18

Hi Rene,
I understand that I can change the attributes of the prefixes that I receive by doing a soft reset (clear ip bgp * soft / clear ip bgp * in), which requires the Route Refresh Capability, or configuring soft reconfiguration (neighbor 1.1.1.1 soft-reconfiguration inbound).
If I do not make a soft clear and I do not configure soft reconfiguration, my inbound routes will never be updated with the new attributes?
Does BGP send an update message if there are no changes like a new network command?

19

Hello Jennifer

If you make a change to a routing policy (policy attributes or filters) then in order to make those policies take effect you must apply one of the three solutions stated in the lesson. If you do not, these policies will never be applied.

Note that the changing of the policies is different than the updating of prefixes. Prefixes are exchanged between BGP routers, and when changes do take affect, based on the BGP timer, BGP will converge. Specifically, BGP uses a keepalive message sent every 60 seconds, and a holdtime timer of 180 seconds. Additionally, uses a scanner that checks all routes in the BGP table every 60 seconds. These mechanisms are used for sharing prefixes and routing convergence.

However, for attribute changes, that is, changes to the way that the router will interpret and handle BGP information received, these won’t take effect unless one of the three methods in the lesson are employed.

I hope this has been helpful!

Laz

1 Like
20

Hi All,

It’s my understanding that if a BGP peer supports the route refresh capability, the peer re-advertises the prefixes to the requesting router allowing for the inbound policy to process using the new policy changes.

We have two methods of clearing the BGP session, the hard reset which is disruptive and the soft reset which request a full advertisement from its BGP peer without tearing down the TCP session.

My questions are:

  1. How do we perform/issue a route refresh? via the soft reset method?
  2. Route refresh is a dynamic method, requires no configuration, right?
  3. If route refresh is not supported, which method will the router use?
  4. When troubleshooting, sometimes it’s very helpful to see all the inbound routes, will this require to configure the soft-reconfig inbound command?

Regards,