BGP traffic priority

[mediaworksnz]

Hello, I had a case whereby user traffic passing through my router saturated the link and caused the BGP peering to drop. 

What mechanism does a Cisco 1921 router to prioritise it's BGP keepalive traffic etc over normal user traffic.

[Francesco Molino]

Hi

You can use QoS to do that. 

By default, all routing protocol is tagged with DSCP CS6. On several platforms, there's a hidden system queue for routing protocol to take care of this issue (not flapping when large amount of traffic goes over interfaces). I don't recall if this is the case on 1921.

However, you can create a qos policy to make sure this traffic (CS6) has a reserved bandwidth for example, to make sure it won't flap next time.

[mlund]

Hi

Fransesco is completely correct, the hidden queue he is mentioned is called "pak priority". Unfortunately bgp is not using this feature as other routing protocol is. So a qos policy needs to be configured to handle the bgp traffic.

/Mikael

[mediaworksnz]

Thank you So how would you recommend I adjust my QoS policy-maps.

i.e. should I simply add CS6 to my 'Multi-Media' class map ?

Thank you kindly for any advice.

Currently I have the following:

class-map match-any IA-High
match ip dscp cs3
match ip dscp af31
match ip dscp af32
match ip dscp af33
class-map match-any Multi-Media
match ip dscp ef
match ip dscp cs4
match ip dscp af41
match ip dscp af42
match ip dscp af43
class-map match-any DataTrans
match ip dscp af11
match ip dscp af12
match ip dscp af13
class-map match-any IA-Low
match ip dscp cs2
match ip dscp af21
match ip dscp af22

policy-map QUEUES
class Multi-Media
priority level 1

class IA-Low
bandwidth remaining percent 10
class DataTrans
bandwidth remaining percent 60
class IA-High
bandwidth remaining percent 15
class class-default
bandwidth remaining percent 15

policy-map WAN
class class-default
shape peak 30000000
service-policy QUEUES

[Francesco Molino]

You can add it wherever you want.
Usually i prefer creating a distinct class to reserve a small amount of bandwidth dedicated for cs6 tagged traffic.

[Dennis Mink]

I your  class-map match-any IA-High.

you will need to add a match on a class-map.

this class map should contain an access list matching tcp port 179 (which is what BGP uses to establish peers and send updates).

[mediaworksnz]

Hello, so to clarify, do I need to match TCP 179 or can I simply match CS6. I was thinking the simplest method for me would be to add CS6 to the following class map:

class-map match-any Multi-Media
match ip dscp cs6 *******

match ip dscp ef
match ip dscp cs4
match ip dscp af41
match ip dscp af42
match ip dscp af43

[Francesco Molino · Accepted solution]

You don't need to match tcp-179 as it should already be tagged. You can do a quick tcpdump to validate it.

[mediaworksnz]

Thank you Francesco, so to confirm, would you agree that doing the following will address the issue:

i.e. to add CS6 to the following class map:

class-map match-any Multi-Media
match ip dscp cs6 *******

match ip dscp ef
match ip dscp cs4
match ip dscp af41
match ip dscp af42
match ip dscp af43

[Francesco Molino]

Yes this will work.

Have you done a tcpdump to confirm that your traffic is already marked with CS6?

[mediaworksnz]

Thanks, yes a tcpdump confirms that BGP traffic is being marked CS6.

Therefore, would you agree that doing the following will address the issue:

i.e. to add CS6 to the following class map:

class-map match-any Multi-Media
match ip dscp cs6 *******

match ip dscp ef
match ip dscp cs4
match ip dscp af41
match ip dscp af42
match ip dscp af43

policy-map QUEUES
class Multi-Media
priority level 1

class IA-Low
bandwidth remaining percent 10
class DataTrans
bandwidth remaining percent 60
class IA-High
bandwidth remaining percent 15
class class-default
bandwidth remaining percent 15

policy-map WAN
class class-default
shape peak 30000000
service-policy QUEUES

[Francesco Molino]

Yes it will solve your issue as it will goes to priority queue