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New Member

How DE bit works in frame-relay

Hi all,

How De bit works in Frame-relay if i am connecting my Fr-switches over IP.


In this situation how i will use De bit for congession control

Please Help

Thanks in advance

  • WAN Routing and Switching
VIP Purple

Re: How DE bit works in frame-relay

Hello Kamlesh,

I am not sure if I understand fully where in your network you want to set the DE, but consider the following example: assume R3 and R4 have a frame relay connection, and you want to set the DE bit for all non-essential traffic, in this case let´s suppose that is TELNET and FTP traffic. You would configure a frame-relay map-class and then a policy-map:


class-map SET_FR_DE

match access-group 101


policy-map DE_SET

class SET_FR_DE

set fr-de


map-class frame-relay DLCI_102

frame-relay cir 256000

frame-relay bc 2560

service-policy output DE_SET


interface Serial0

frame-relay interface-dlci 102

class DLCI_102


access-list 101 permit tcp any any eq 21

access-list 101 permit tcp any any eq 25

This configuration example configures DLCI 102 wit a CIR of 256K, and sets de DE bit for TELNET and FTP traffic, which indicate that these frames have lower importance than other frames, and are Discard Eligible, that is, they can be dropped in case of congestion...

Is that what you are looking for ?



Re: How DE bit works in frame-relay


DE=1 means out of contract (above CIR) and DE=0 means within contract (below CIR).

As far as I understand you do FR switching over GRE? Well you could match with a class-map on DE bits and use these classes for output QOS (maybe IP precedence settings). Queueing however only occurs when you overload an interface. and I do not see how traffic from a serial would overload the ethernet. So presumably you do not need this at all in your setup.

Hope this helps.


New Member

Re: How DE bit works in frame-relay


Thanks to all

Manually setting up a DE bit for some specific traffic is OK but what if normal traffic is comming and it is more than CIR but less than EIR.

My ethernet can be overload if i am getting traffic more than it can handle.

You mean i have to set De bit on that traffic and use map class for marking the traffic for QoS with Ip precedence bits.

Thanks in advance

Re: How DE bit works in frame-relay


for queueing you need to specify which traffic is getting what amount of bandwidth. In your setup presumably GRE (you didn´t answer that) is sent over the Ethernet and that is what you can specify. To differentiate between DE=1 and DE=0 in the FR frames you have to map them to some value visible in GRE, i.e. IP precedence.

By the way, in your setup drawn the ethernet can not be overloaded. But I assume now that there are other interfaces which can contribute to the load on the ethernet drawn. Then QoS based on IP precedence (or DSCP) makes sense. CBWFQ would be advisable.

Hope this helps


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