3com 3031 Instruccion De Instalación
672
C
HAPTER
52: MPLS A
RCHITECTURE
the returned label map message from its downstream LSR. Usually, the upstream
LSR selects the downstream LSR according to the information in its routing table.
In , the sequential label control mode has been used by the LSRs on the way along
LSP1, and the independent label control mode has been used by for the LSRs on
LSP2.
LSR selects the downstream LSR according to the information in its routing table.
In , the sequential label control mode has been used by the LSRs on the way along
LSP1, and the independent label control mode has been used by for the LSRs on
LSP2.
In DU (downstream unsolicited) mode, the label is distributed in the following
way: when LDP session is established successfully, the downstream LSR will actively
distribute label map message to its upstream LSR. And the upstream LSR saves the
label map information and processes the received label map information according
to the routing table information.
way: when LDP session is established successfully, the downstream LSR will actively
distribute label map message to its upstream LSR. And the upstream LSR saves the
label map information and processes the received label map information according
to the routing table information.
Constrain-based Routing LDP
MPLS also supports Constrain-based Routing LDP mechanism (CR-LDP). CR-LDP
implies that, while originating the establishment of LSP, the ingress node adds
some constrain information for LSP routing in the label request message. The
constrain information may be either exact designation for the LSRs along the way
or unspecific restriction for selecting downstream LSR, and in the former case it is
referred to as strict explicit routing and in the latter case it is referred to as loose
explicit routing.
implies that, while originating the establishment of LSP, the ingress node adds
some constrain information for LSP routing in the label request message. The
constrain information may be either exact designation for the LSRs along the way
or unspecific restriction for selecting downstream LSR, and in the former case it is
referred to as strict explicit routing and in the latter case it is referred to as loose
explicit routing.
LSP loop control
While establishing an LSP in an MPLS domain, it is also necessary to prevent path
loop from happening. To avoid the LSP path loop, two methods: maximum hop
count and path vector, can be used.
loop from happening. To avoid the LSP path loop, two methods: maximum hop
count and path vector, can be used.
The maximum hop count method uses the hop-count information that is
contained in the message bound with the forwarding label, this value is
incremented by one for each hop. When the value exceeds the specified maximum
value, it determines that a loop happened, and the process for establishing LSP is
terminated.
contained in the message bound with the forwarding label, this value is
incremented by one for each hop. When the value exceeds the specified maximum
value, it determines that a loop happened, and the process for establishing LSP is
terminated.
The path vector method uses the path information recorded in the message
bound with the forwarding label, and, for every hop, the corresponding router
checks if its ID is contained in this record. If not, the router adds its ID into the
record; and if its ID is contained in the record, it indicates that a loop has occurred
and the process for establishing LSP is terminated.
bound with the forwarding label, and, for every hop, the corresponding router
checks if its ID is contained in this record. If not, the router adds its ID into the
record; and if its ID is contained in the record, it indicates that a loop has occurred
and the process for establishing LSP is terminated.
LSP Tunnel and
Hierarchy
LSP Tunnel
MPLS supports LSP tunnel technology. On an LSP path, LSR Ru and LSR Rd are
upstream and downstream of each other. However, the path between LSR Ru and
LSR Rd may not be part of the path provided by routing protocol. MPLS allows
establishing a new LSP path <Ru R1...Rn Rd> between LSR Ru and LSR Rd, and LSR
Ru and LSR Rd are the starting point and ending point of this LSP respectively. The
LSP between LSR Ru and LSR Rd is referred to as the LSP tunnel, which avoids the
traditional encapsulated tunnel on the network layer. If the route along which the
tunnel passes and the route obtained hop by hop from routing protocol is in
consistent, this tunnel is called hop-by-hop routing tunnel. And if the two routes
are not consistent, then the tunnel of this type is called explicit routing tunnel.
upstream and downstream of each other. However, the path between LSR Ru and
LSR Rd may not be part of the path provided by routing protocol. MPLS allows
establishing a new LSP path <Ru R1...Rn Rd> between LSR Ru and LSR Rd, and LSR
Ru and LSR Rd are the starting point and ending point of this LSP respectively. The
LSP between LSR Ru and LSR Rd is referred to as the LSP tunnel, which avoids the
traditional encapsulated tunnel on the network layer. If the route along which the
tunnel passes and the route obtained hop by hop from routing protocol is in
consistent, this tunnel is called hop-by-hop routing tunnel. And if the two routes
are not consistent, then the tunnel of this type is called explicit routing tunnel.