Tuesday, 21 April 2015

Remarking Calls over Two CoS MPLS WAN

I had a scenario where the client bought two CoS L3 MPLS links for all his offices (HQs & BRs). 20% CoS1 is used for voice calls while 80% CoS3 for data.

As you know, MPLS providers will drop excess packets on CoS1. The excess packets from CoS1 won't be remarked. For other CoSes excess packets are remarked to default CoS.

Initially, we deployed E-LCAC for his network to make sure that once CoS1 is full calls are blocked. The client decided not to go for AAR.

The demand for On-Net calls increased and the users started getting "Not Enough Bandwidth" message very frequent.

Increasing E-LCAC location bandwidth wasn't an option as this will degrade overall calls performance due to packets getting dropped at CoS1 (remember excess packets at CoS1 are dropped).

The client didn't want to invest on upgrading CoS1 and wanted to pass excess calls over CoS3.


- Network devices won't be able to do this as they mark packets not calls. This means that packets for a single call can have different marks which is very bad.
- CUCM won't be able to do this as CUCM doesn't set in the call path. It does signaling only.


The solution was to combine CUCM and Network devices with an RSVP E-LCAC deployment. This will utilize the fact that RSVP CAC will remark the calls to default DSCP in case RSVP reservation fails.

1. CUCM will try to reserve bandwidth using static location 
2. Next CUCM will try to allocate RSVP agents for the call
3. In case RSVP bandwdith is available, reservation is made and the RSVP agent will pass the calls as CoS1 (DSCP=EF)
4. In case RSVP bandwidth isn't available (fully consumed), reservation will failed. CUCM will still allocate RSVP agent without RSVP reservation. The RSVP agent will pass the calls as CoS3 (DSCP=0 by default)

Configuration Steps

I won't be detailing the step by step config as this can be lengthy. However, I will list summary steps and some relevant config.

1. In CUCM create locations for each office and a transit location called MPLS.
2. Connect the offices locations to MPLS location using links.
3. Link bandwidth should equal the total bandwidth of the office (CoS1 + CoS3).
4. In your E-LCAC location configuration, configure RSVP settings to optional (video desired) 
5. Configure software MTP at the gateway of each location. The MTP codec should match your inter-region codec and the maximum number of sessions should accommodate for the entire office bandwidth (CoS1 + CoS3)

e.g. if you are using G729 and your office bandwidth is 2 Mbps, then the max number of sessions should be > 86.

dspfarm profile 3 mtp 
 description **** RSVP CAC MTP ***
 codec g729r8
 maximum sessions software 100
 associate application SCCP

6. Create MRGs per office with the software MTPs associated and MRGLs per office. Assign the MRGLs to offices SIP/H323 Trunks/Gateways

7. Configure RSVP on the office routers. The amount of RSVP bandwidth should equal CoS1 and the max per-RSVP session bandwidth should be for 1 call + buffer (for G729 the max per-RSVP session bandwidth should be 40kbps).

 interface GigabitEthernet0/1
  ip rsvp bandwidth 400 40
  ip rsvp data-packet classification none

8. You can change the default DSCP marking for failed RSVP reservations from service parameters
Note: By default, RSVP attempts to process every packet not just provide CAC. Turn this off with the ip rsvp data-packet classification none interface command.

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