"frame-relay fragment" causes link to stop working

Question

Hi people!
I have a strange problem regarding the "frame-relay fragment xyz" command. I have the following configuration:
interface Serial0/0
description R3926704: phy. FrameRelay-Interface (Se0/0)
bandwidth 512
no ip address
max-reserved-bandwidth 95
encapsulation frame-relay IETF
no ip mroute-cache
load-interval 30
no snmp trap link-status
no fair-queue
cdp enable
frame-relay traffic-shaping
frame-relay lmi-type q933a
end
interface Serial0/0.1 point-to-point
description YYY: FR-Verbindung zum YY(Se0/0.1)
bandwidth 512
ip address A.A.A.A B.B.B.B
frame-relay class egress_queue_512k
frame-relay interface-dlci 16
end
map-class frame-relay egress_queue_512k
frame-relay cir 486000
frame-relay bc 48600
frame-relay be 0
frame-relay mincir 486000
service-policy output egress_queue
... if i change the map-class to:
map-class frame-relay egress_queue_512k_voice
frame-relay cir 486000
frame-relay bc 4860
frame-relay be 0
frame-relay mincir 486000
service-policy output egress_queue
frame-relay fragment 53
... no traffic goes over the link any more. no telnet no other traffic. the strange thing is, that i can ping the router!??
does anyone know what's the problem here??
thanks for any help!!
Mario

Answer 1

Mario,
You need to base the fragment size on the lowest speed link between the 2 routers. If the lowest speed link is 512k, the recommended fragment size is 640 bytes.This is to achieve a 10ms serialization delay. The formula to compute this is Max Delay X Bandwidth. Please see Table 5 in the following link for recommended fragment sizes for 10ms serialization delay for different speed links.
frame-relay fragment command
http://www.cisco.com/en/US/products/sw/iosswrel/ps5187/products_command_reference_chapter09186a008017cf52.html#wp1034268
Hope this helps. If so, please rate the post.
Brandon