Notes on Frame Relay and ATM Configuration

Notes on Frame Relay

• Encaps the physical interface as frame-relay
• On the physical interface:
• Remove inverse arp “no frame inverse-arp” to prevent the interface from auto-discovery
• Prevent interface responding to discovery requests by turning off “no arp frame”

Point to Point FR Interfaces

• Configure the sub interface but remember to define point to point on the end
• Assign IP to sub interface
• Create DLCI map using “frame-relay interface-dlci xxx”

Point to Multi-Point Interfaces

• Configure the sub interface but remember to define point to multipoint on the end
• Assign IP to sub interface
• Map frame relay dlci’s “frame-relay map ip x.x.x.1 103 broadcast” broadcast is required for routing protocols.

Need to bear in mind that when running a point to multipoint in a hub and spoke topology, spokes are not going to be able to communicate with each other by default as the frame relay map only maps the outgoing DLCI to the IP address of the hub router.

To get to a spoke, an additional map using the same DLCI but pointing to the IP address of the other spoke is required i.e.

• “frame-relay map ip x.x.x.3 103 broadcast”. This needs to be done on ALL spokes that need to communicate with each other”

Other Useful Commands

• “clear frame-relay map” – removes any cached LMI entries
• “show frame-relay map” shows status of maps:
     active – all is good
     inactive – this end is working, the other isn’t
     deleted – this end isn’t working

With a point to multipoint environment, spokes will advertise their local networks to the hub via a routing protocol (EIGRP) but because of split horizon (not advertising something back through the interface it was received on) the advertisements will not be passed back through the interface to the spokes. Entering the following command from the multipoint sub-interface will solve the problem:

“no ip split-horizon eigrp as-num”

Frame Relay Traffic Shaping and ATM

• CIR    Committed information rate
• Tc     Timing Interval (usual 1/8 sec)
• Bc    Commited Burst
• Be    Excess Burst

CIR
—  = Bc
Tc

In the event of 64K this would be:

64000 (bits)
————  = 8000bps per time interval (clock cycle)
8

Data can only be sent through an interface at its maximum rate so if an interface is 64k, data will be sent at 8000 bps but will be forced to stop for the remaining timing intervals until its clear to send again. if there isn’t 8000bps to send, amount of data that wasn’t sent can be “banked” for use as excess burst up to the negotiated level on the next time timing interval.

Problems can arise with Excess Burst however because it can be marked by the provider as Discard Eligable (DE) which means it may be discarded during periods of provider congestion.

• BECN – Backwards Explicit Congestion Notification - will be tagged on to a return packet from the destination router by the service provider to a router that is sending data into the Frame Relay cloud at levels above their CIR and excess burst as a warning to slow down.
• FECN – Forwards Explicit Congestion Notification – A FECN is a message that is sent to a destination network by the service provider requesting the destination router to send a generic packet back to a router that is congesting the service provider network so that the service provider can tag it with a BECN flag so the sending router will slow down.

BECNS and FECNS are ignored by default.

Traffic Shaping Configuration steps

1. Create Map-Class and define parameters
2. Interface Configuration “Frame traffic-shaping”
3. Apply Map-Class to interface.

From global config “map-class frame-relay SLOW”
“frame-relay adaptive-shaping becn” – set router to respond to BECN notifications
“frame-relay min-cir 24400″ (bps) – minimum speed line will ever drop to
“frame-relay cir 28800″ – sets the CIR
“frame-relay bc 3600″ – sets the committed burst rate at 28800/8 which means 3600 bits will be sent per timing interval
“frame-relay be 200″ – sets the excess burst to 200 bits
“frame-relay tc 125″ – defaults to 125 ms which is 1/8 of a second.

“frame-relay traffic-rate 28000 200″ – does the same as above but the router will calculate the Tc Bc etc automatically.

Go into the physical interface:
“frame-relay traffic-shaping” – enables interface to support map classes (the big ON button effectively)

Then need to go into the sub interface to apply the map to the specific interface otherwise it will apply to all PVC’s
“int s0/0.1″

“frame-relay class SLOW” – apply the class

or

“frame-relay interface-dlci xxx”
“class SLOW”

“show traffic-shape”

ATM

No error checking
Has no sequence numbers
Frame size is identical for all packets – (53 bytes – 48 data/5 header)
Could be known as Cell Relay
Integrates well with other network types
Used by 80% of service providers
Variety of implementations
very Fast

“int atm0/1″
“atm pvc 56 0 115 aal5snap”

56 is a label
0 is the VPI
115 is the VCI
aal5snap is the encapsulation

Both values are locally significant.