Riverstone Networks WICT1-12 - Manual (Page 315)

Riverstone Networks RS Switch Router User Guide Release 8.0 17-7
MPLS Configuration MPLS Architecture Overview
Label Advertising Mode
An LSR can advertise label bindings to its peers in one of two modes:
• It can make an independent decision to bind a label to an FEC and distribute that binding to its
peers—this is called independent mode.
• It can bind a label to a particular FEC if it is the egress LSR for that route or if it has already received
a label binding for that FEC from its next hop for the route—this is called ordered mode.
The RS supports only ordered label advertising.
Label Retention Mode
An LSR can store the label bindings in one of two modes:
• Only label bindings received from next hop downstream LSR peers are stored—this is called
conservative label retention.
• All label bindings received from peer LSRs are stored, even if the peer is not the next hop for a
route—this is called liberal label retention.
The RS supports only liberal label retention.
17.1.4 Penultimate Hop Popping
The next-to-last LSR in an LSP, or the penultimate LSR, can pop the label stack. This process, called penultimate hop
popping (PHP), has the following advantage: If the penultimate LSR does not pop the label stack, the egress LSR must
do two table lookups to process the packet: first, it must look at the top label in the stack and pop the stack. Then it
must either look at the next label in the stack, or if there is no other label in the stack, look up the packet’s destination
address to forward it. By having the penultimate LSR pop the label stack, there is only a single table lookup performed
at each of the egress and penultimate LSRs. By default, RS routers that are PHP LSRs pop the label stack. You can
configure the RS egress router to notify the PHP router to not pop the label stack.
17.1.5 MPLS Tunnels
You can use MPLS label stacks, instead of network layer encapsulation, to tunnel packets across a backbone MPLS
network. Tunneling allows shared resources, such as public networks, to be used to carry private communications. For
example, companies can use VPN tunnels to send intranet traffic over the Internet. The advantage that MPLS offers is
that the contents of the tunneled data packets do not need to be examined as they proceed through an LSP, as the
forwarding of the packets is based only on the attached labels.
Multiple LSPs traveling between the same LSRs across a network can be sent together on a higher-level tunnel LSP.
The packets to be sent through the tunnel LSP are considered to be a single FEC by the LSR at the tunnel entrance.
This LSR pushes an additional label onto the packets that enter the tunnel LSP before sending the labeled packets to
the next hop in the tunnel. When the packets emerge from the tunnel LSP, the top label is popped.
In Figure 17-6 , R1 is the ingress LSR for an LSP to the egress LSR, R4. R2 is the entry point for the tunnel LSP and
R3 is the exit point for the tunnel LSP.