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Segment Routing段路由(还不如不翻译~) 思科SP CCIEv4.0片段  

2015-01-13 09:59:01|  分类: 集成ISIS |  标签: |举报 |字号 订阅

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Segment Routing

Segment routing is a technique by which the path followed by a packet is encoded in the packet itself similar to loose or strict source routing. The path is encoded as a list of segments, and each segment is identified by the segment ID (SID) consisting of a flat unsigned 32-bit integer. Interior Gateway Protocols (IGP) segments, a sub-class of segments, identify an IGP forwarding instruction. There are two groups of IGP segments: prefix segments and adjacency segments.

  •  Prefix segments: Prefix segments steer packets along the shortest path to the destination, using all available Equal Cost Multi-Path (ECMP) paths.
  •  Adjacency segments: Adjacency segments steer packets onto a specific link to a neighbor.

Limitations

The segment routing, applied to the MPLS dataplane, offers the ability to tunnel services such as VPN, VPLS, VPWS, etc. from an ingress provider edge (PE) to an egress PE.

  •  The segment routing must be configured on the ISIS instance before configuring prefix SID value.
  •  The prefix SID value must be removed from all the interfaces under the same ISIS instance before disabling segment routing.
  •  The label distribution protocol (LDP) and segment routing are not interoperable. Therefore, the LDP should not be configured on any ISIS instance where segment routing is configured.

Segment Routing Global Block

Segment Routing Global Block (SRGB) is the range of labels reserved for segment routing. SRGB is local property of an segment routing node. In MPLS, architecture, SRGB is the set of local labels reserved for global segments. In segment routing, each node can be configured with a different SRGB value and hence the absolute SID value associated to an IGP Prefix Segment can change from node to node.

The SRGB label value is calculated as follows:

  •  If the platform supports 1000000 labels or more, the SRGB value is from 900000 to 900000 + 2^16.
  •  If the platform supports less than 1000000 labels, the SRGB value is the last 2^16 labels.

Prefix SID

A prefix segment identifier (SID) identifies a segment routing tunnel leading to the destination represented by a prefix. The maximum prefix SID value is 2^16 - 1.

A prefix SID is allocated from the Segment Routing Global Block (SRGB). The prefix SID value translates to a local MPLS label, whose value is calculated as below:
  •  If the platform supports 1000000 labels or more, then the MPLS label corresponding to the prefix SID value is 900000 + sid-value.
  •  If the platform supports less than 1000000 labels, then the MPLS label corresponding to the prefix SID value is maximum-supported-label-value - 2^16 + sid-value.

When a prefix SID value x is configured, the prefix SID translates to a label value equivalent to x + lower boundary of SRGB. For example, in the platform supporting 1000000 MPLS labels or more, configuring a prefix-SID of 10 for interface Loopback 0 with IPv4 address 1.0.0.1/32 results in assigning the label 9000010 to the prefix 1.0.0.1/32.

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