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Chapter 10
Wide Area Networking Protocols
Notice that we changed the encapsulation type for the first mapping. The
frame
map command is the only way to configure multiple frame encapsu-
lation types on an interface.
The broadcast keyword at the end of the map command tells the router to
forward broadcasts for this interface to this specific virtual circuit. Remember
that Frame Relay is a nonbroadcast multiaccess (NBMA) encapsulation
method, which will not broadcast routing protocols. You can either use the
map
command with the broadcast keyword or the neighbor command
within the routing process.
Instead of putting in map commands for each virtual circuit, you can use
the inverse-arp function to perform dynamic mapping of the IP address
to the DLCI number. This makes our configuration look like this:
RouterA(config)#int s0.16 multipoint
RouterA(config-subif)#encap frame-relay ietf
RouterA(config-subif)#ip address 172.16.30.1
255.255.255.0
Yes, this configuration is a whole lot easier to do, but it's not as stable as
using the map command. Why? Sometimes, when using the inverse-arp
function, configuration errors occur because virtual circuits can be insidi-
ously and dynamically mapped to unknown devices.
Frame Relay Congestion Control
In this section, we will define how the Frame Relay switch handles conges-
tion problems.
DE
When a Frame Relay router detects congestion on the Frame
Relay network, it will turn the Discard Eligibility (DE) bit on in a
Frame Relay packet header. If the switch is congested, the Frame Relay
switch will discard the packets with the DE bit set first. If your band-
width is configured with a committed information rate (CIR) of zero,
the DE will always be on.
FECN When the Frame Relay network recognizes congestion in the
cloud, the switch will set the Forward-Explicit Congestion Notification
(FECN) bit to 1 in a Frame Relay packet header. This will indicate to the
destination DCE that the path just traversed is congested.
BECN When the switch detects congestion in the Frame Relay network,
it will set the Backward-Explicit Congestion Notification (BECN) bit in
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