have to carry a small amount of non-IP traffic, as in the example shown in Figure 115 on
page 516.
Figure 115: Common ISP Network
In this scenario, it is not economical to have special ATM switches in front of E Series
routers just to accommodate the small percentage of non-IP traffic. The ATM layer 2
services over MPLS feature lets you replace the ATM traffic selector switch with an
E Series router, as shown in Figure 116 on page 516. The two routers pass traffic between
two interfaces through an MPLS tunnel using Martini encapsulation, regardless of the
contents of the packets.
Figure 116: E Series Router Replacing Remote ATM Switch
ATM layer 2 services over MPLS supports two encapsulation methods on E Series routers:
•
AAL5 relay encapsulation
•
VCC cell relay encapsulation
The following sections describe each of these encapsulation methods.
AAL5 Encapsulation
JunosE Software supports the AAL5 relay method of encapsulation that is specified in
the Martini draft. This method is also referred to as AAL5 service data unit (SDU)
encapsulation.
ATM Martini encapsulation emulates ATM switch behavior by creating a pseudowire
between pairs of ATM virtual circuits. When the router receives AAL5 packets on one of
those circuits, it reassembles them, encapsulates them using Martini encapsulation, and
forwards them to an MPLS tunnel. At the end of the tunnel, the packet is de-encapsulated,
segmented back, and sent to a selected ATM VC.
In Figure 117 on page 517, an MPLS tunnel connects two E Series routers, and ATM
cross-connects provide a pseudowire between the ATM VCs on the two routers. All AAL5
Copyright © 2010, Juniper Networks, Inc.516
JunosE 11.2.x BGP and MPLS Configuration Guide
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