ERX QoS - PPP: LCP echo requests

12.16.08   |  
‎12-16-2008 12:13 AM

Hello all


We are using ERX for PPP termination in our network with bandwidth oversubscription for the VP and VLAN

that carry DSLAM traffic. We have kept the default keepalive period 30sec for the PPP profile.

As we have seen the CPEs we have, usually send LCP-echo requests to the ERX every 10 secs regardless

of traffic being present or not in the ppp session, and ERX just replies to each of these requests.


Since we use oversubscription we are concerned that some of these LCP echo replies coming from ERX may

be lost (and consequently the corresponding ppp sessions may close due to this congestion)


The question is the following:


Do the ERX module cards (atm and ethernet) by default prioritise the outgoing control traffic that the ERX generates?

(like the PPP-LCP echo replies) over the rest of the internet traffic?


If not do we have always to apply special qos-profile that prioritize PPP control traffic for each VLAN or VP that

we apply traffic shaping to?


thnx for the help




Re: ERX QoS - PPP: LCP echo requests

12.16.08   |  
‎12-16-2008 08:37 AM

There are two keepalive modes of operation: high-density mode andlow-density mode.

In your case it should be high-density keepalive mode. because it is automatically selected if PPP is layeredover ATM, L2TP, or PPPoE. Low-density keepalive mode is selected if PPP is layered over HDLC.


Further, In your case when thekeepalive timer expires, the interface first verifies whether any frames werereceived from the peer in the prior keepalive timeout interval. If so, theinterface does not send an LCP echo request (keepalive). Keepalive packets are sent only if the peer is silent (that is, no traffic was received from the peer during the previous keepalive timeout interval). 


You don't need to apply QoS for LCP echo as long as the traffic is passing.



BLOG: http://weblogs.com.pk/jahil/

Re: ERX QoS - PPP: LCP echo requests

12.17.08   |  
‎12-17-2008 01:29 AM

I have tried to verify the behaviour of the ERX BRAS. You are right the BRAS operates in high-density keepalive mode but

still keepalives at least in my case are exchanged (I have inserted a sniffer for verify this).

The CPEs in our case are configured to send LCP-keepalive requests every 10 seconds even if traffic is present in the line.

(They do not implement some similar to the ERX high density behaviour). The ERX replies to each request but does not

send LCP keepalive requests to the CPE, only replies. At least in my case PPP-LCP-echos are always exchanged for each PPP

session because of the way our DSL CPEs are configured.


 So I still the question remains if ERX by default prioritizes PPP control traffic, when shaping is applied to a VP or VLAN.






I tried to experiment with Control-plane policing in the ERX, for the PPPEchoRequest protocol.

I limited the rate to 1 packet per second but still I did not see any effects to the PPP sessions


#sh control-plane policer protocol PppEchoRequest
                           Rate     Size     Packets    Packets
   Protocol      Enabled   (pps)   (pkts)   Committed   Exceeded
--------------   -------   -----   ------   ---------   --------
PppEchoRequest      true      1        1           0          0


The commited rate always is 0 even if the BRAS has PPP subscribers.


Is this because the PPPEchoRequests are handled by the line-cards and not the SRP that the

plicing tries to protect??


Has anyone successfully used the control-plane policing for PPP?


Thnx for the input