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Frame Relay Traffic Shapping Terms

I was reading about traffic shaping & policing and the acronyms in the book they were as clear as mud, but now Ive written them down and understand them they are really really simple. I put a few formulas on for them aswell, but havnt checked them so please correct me if im wrong, oh and I have presumed that you are attempting to traffic shape to the CIR.

Tc – This is a time interval in milliseconds when a Committed Burst (Bc) can get sent. Usually Tc = Bc / CIR

Bc – Committed Burst this is the amount of data in bits which can bet sent every Tc. Usually Bc = CIR / Tc

Be – Excess Burst is the number of bits the Bc can be exceed by if no data has been sent if no data has been sent in previous Tcs. EDIT: As commented by Jeriel Atienza the formula is Be = (Ar – CIR) * Tc/1000

CIR – Committed Information Rate this is the bandwidth of a link or VC in bps which the Service Provider guarantees to provide. Quite often the CIR is lower than the full capabilities of a link which is the main reason why traffic should be shaped & policed. CIR = Bc * Tc

Shaped Rate – This is the rate of the traffic which is being shaped in bps, it normally matches the CIR. Usually CIR = Shaped Rate!

Frame Relay LMI - In General

Frame Relay Local Management Interface (LMI) is a set of enchantments to frame relay, originally agreed upon in 1990 by a consortium consisting of Cisco, Digital Equipment Corporation (DEC), Northern Telecom, and StrataCom. Eventually ANSI and CCITT came along created standard versions but most vendors support both standard version alongside the one defined by the consortium.
There are very few differences between them, mainly Cisco LMI type use the DLCI of 1023 and also DLCI number between 16 and 1007 for usable DLCI number and the ANSI & ITU(q933a) standard use a DLCI of 0 and usable DLCI numbers between 16 and 976. The LMI type can be changed on the frame relay enabled interface with the command;

Router(config-if)#frame-relay lmi-type ?
  cisco
  ansi
  q933a

The LMI messages messages sent between Routers and Frame Relay switches provide the the following additional features;
- Inverse ARP – to find out the layer 3 address of device on the other end of the DLCI so you dont need to static map it or similar
- Signalling of Virtual Circuit status messages, therefore if a PVC becomes unreachable all nodes along its path can be aware of this failure so data can be prevented from being sent to indirect failures.
- Multicasting – This extension allows multicast groups to be used over frame relay networks, the higher DLCI numbers reserved by the LMI type are used for this
- Globally signficiant DLCIs – Brilliant!
- Retro flow control with XON/XOFF if the applications using the Frame Relay network know understand FECNs and BECNs
By default LMI messages are sent every 10 seconds, and every sixth message a full status message will be sent which contains more detailed information about each VC, the interface will fail if the interfaces does not receive an LMI message 3 times the hello time, so 30 seconds. You cant actually turn off LMI but you can disable the keepalives with the highly ambiguous command;

Router(config-if)#frame-relay lmi-n391dte ?
  <1-255>  event

Finally you can check the status of LMI with the command;

Router#sh frame lmi

LMI Statistics for interface Serial1/0 (Frame Relay DTE) LMI TYPE = CISCO
  Invalid Unnumbered info 0             Invalid Prot Disc 0
  Invalid dummy Call Ref 0              Invalid Msg Type 0
  Invalid Status Message 0              Invalid Lock Shift 0
  Invalid Information ID 0              Invalid Report IE Len 0
  Invalid Report Request 0              Invalid Keep IE Len 0
  Num Status Enq. Sent 18               Num Status msgs Rcvd 0
  Num Update Status Rcvd 0              Num Status Timeouts 18
  Last Full Status Req 00:00:03         Last Full Status Rcvd never

Finally pretty much every command on the interface beings with frame-relay;

Router(config-if)#frame-relay ?
  accounting             Special accounting instruction
  address-reg            ELMI address registration
  broadcast-queue        Define a broadcast queue and transmit rate
  class                  Define a map class on the interface
  congestion-management  Enable Frame Relay congestion management
  de-group               Associate a DE group with a DLCI
  fragment               Enable end-to-end fragmentation for all PVCs
  fragmentation          Adaptive fragmentation

PROXY ARP - In General

The command no ip proxy-arp was one of those things which I saw in IOS configs and wasn’t to sure what proxy arp is used for or why it exists. Proxy ARP is where a router will respond on behalf of another device, it was used heavily in networks before the days of DHCP & default gateways where a host would ARP for an address that wasn’t on its subnet (modern networks just send the packets to the default gateway instead of arping for the address), the router on the local network would then act as a “proxy” and respond on behalf of the device outside of the subnet.

Proxy ARP isn’t used if hosts are set with default gateways or have routing intelligence, setting a default gateway instead of using proxy ARP is a much better option. Using Proxy ARP instead of a default gateway results in higher ARP traffic & the ARP tables of the hosts get very large as they maintain an IP/MAC binding for every single address the communicate with.

CCIE R&S 4.0 Troubleshooting Section Details

Recently there was video prodcast happened on CCIE R&S 4.0 Troubleshooting Section. So below are the little details announced by Maurilio who is the content designer for CCIE R&S Lab:

    * we may have 9 -12 tickets on troubleshooting
    * each ticket will be independent in configuration prospects. i.e. it will not  
       effect any other trouble tickets configuration or working
    * one can expect from 6, 15 upto 30 routers in troubleshooting section.
    * No layer 2 switching tickets will be presented in troubleshooting section.
    * Troubleshooting section may end up with 20 to 25 points.
    * 2 nos. of layer 2 switching troubleshooting will be there in configuration 

       section.

 It is mandatory to pass each section.....

OEQ == 4 Ques... answer 3 to pass

Troubleshooting == 80 %

Configuration == 80%

Thanks!

Deepak Arora