Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 124)

100 Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
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Configuring MPLS
1
At the MPLS router that pops the label (either the penultimate LSR or the egress LER), the incoming
packet’s MPLS TTL value is copied to the packet’s IP TTL field, the IP TTL field is decremented by 1,
and the checksum is recalculated. The result is that each LSR in the MPLS domain is counted as
one hop. This is the default behavior.
Optionally, the user can configure TTL propagation so that the entire MPLS domain appears as a
two hops. In this case, the ingress LER decrements the IP packet’s TTL value by one and then
places a value of 255 in the packet’s MPLS TTL field. The MPLS TTL value is decremented by 1 as
the MPLS packet passes through each LSR in the MPLS domain. When the label is popped, the
value in the MPLS TTL field is discarded, not copied to the packet’s IP TTL field. The unlabeled IP
packet’s TTL is then decremented by one as it passes through the egress LER. This means that the
packet’s IP TTL is decremented twice from the time it enters the ingress LER to the time it exits the
egress LER, making the MPLS domain appear as two hops.
To configure TTL propagation so that the entire MPLS domain appears as two hops, enter the
following commands on both the ingress LER and the MPLS router that pops the label (either the
penultimate LSR or the egress LER).
Brocade(config-mpls)# policy
Brocade(config-mpls-policy)# no propagate-ttl
Syntax: [no] propagate-ttl
When no propagate-ttl is configured, the ingress LER places a value of 255 into the packet’s MPLS
TTL field, regardless of the TTL value in the packet’s IP header. The packet’s IP TTL value is
decremented twice: once at the ingress LER and once at the egress LER. With this option, the
entire MPLS domain (regardless of the number of transit LSR hops) counts as two hops for the IP
TTL value.
NOTE
When the user chooses to configure TTL propagation in this way, it is important that the user enters
the no propagate-ttl command at both the ingress LER and the MPLS router that pops the label.
When the user omits the no propagate-ttl command at the MPLS router that pops the label, the value
in the packet’s MPLS TTL field would be copied into the packet’s IP TTL field. This value could be as
high as 255.
Enabling LSP accounting
The LSP accounting feature provides the ability to count the number of traffic bytes and packets
forwarded through a specified LSP. The LSP accounting feature is supported for the following:
• RSVP-signaled LSPs
• LDP signaled LSPs
When the command vlan-counter exclude-overhead is configured or removed, the LSP counters in
software and hardware are flushed, and accounting starts fresh. In summary:
• Ingress-tunnel accounting without exclude-ethernet-overhead:
• With vlan-counter exclude-overhead not configured: the size includes 20-byte Ethernet
overhead (IFG+ Preamble) and 4-byte CRC.
• With vlan-counter exclude-overhead configured: excludes 20-byte per-packet Ethernet
overhead from byte counting.