Extreme Networks BlackDiamond 6800 MPLS, Installation And User Manual

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MPLS Module Installation and User Guide 6-5
LSPs and IBGP Next Hops
TLS tunnels use a two-label stack to tunnel Layer 2 traffic across an IP MPLS domain. If
multiple equal-cost LSPs exist to the egress tunnel LSR, TLS tunnel traffic is distributed
across the LSPs using multiple two-label stack MPLS headers. Each two-label stack
MPLS header has a different outer label, each outer label representing a different
NHLFE, with the same inner label representing the TLS VLAN. TLS tunnels can be
logically bound to multiple equal-cost LSPs.
As stated earlier, up to four equal-cost LSPs are supported per FEC. Non-IP ingress
tunnel traffic is distributed across TLS tunnel LSPs based on the MAC addresses of the
packet. Ingress IP tunnel traffic is distributed based on the IP addresses of the packet.
The distribution hash algorithms are similar to those previously discussed.
Overriding IBGP Metrics for RSVP-TE LSPs
By default, RSVP-TE LSPs inherit the underlying IGP path cost. You can override the
path cost by configuring the LSP IGP metric. The IGP metric can only be specified for
RSVP-TE LSPs. RSVP-TE LSPs can be assigned a fixed cost metric, independent of the
actual topological IGP cost metric. By controlling the path cost for RSVP-TE LSPs, you
can manipulate how different traffic flows are tunneled across an MPLS domain. For
example, if the RSVP-TE IGP path cost is set higher than its actual IGP metric, the LSP
is not used to transport IP routed traffic, but can still be used to transport TLS VLAN
traffic.
LSPs and IBGP Next Hops
You can also use indirect LSPs to reach BGP next hops. For example, an IBGP session is
established across the OSPF/MPLS backbone, and the communicating routers run both
OSPF and IBGP. When an IBGP route is installed, MPLS determines whether a direct
LSP exists to the destination and whether an indirect LSP exists to the BGP next hop. If
an indirect LSP exists to the BGP next hop, the LSP is included in the indirect LSP field
of the route table entry. If an LSP to an EBGP next hop is not available, a check is made
for an LSP to the ASBR used to reach the BGP next hop.
The recalculation requirements for BGP are similar to those for OSPF; when an indirect
LSP to an ASBR (corresponding to a BGP next hop router) changes state; the BGP
routing table entries must be checked to ensure their LSP information is still valid.