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Configuration Guide - IP Unicast Routing

CloudEngine 12800 and 12800E V200R002C50

This document describes the configurations of IP Unicast Routing, including IP Routing, Static Route, RIP, RIPng, OSPF, OSPFv3, IPv4 IS-IS, IPv6 IS-IS, BGP, Routing Policy, and PBR.

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Traditional routers select the shortest path as the master route regardless of other factors, such as bandwidth. In this manner, the traffic is not switched to other paths even if a path is congested. MPLS traffic engineering (TE) has advantages in solving the problem of network congestion. With MPLS TE, you can precisely control the traffic path and prevent traffic from passing through congested nodes. Meanwhile, MPLS TE can reserve resources to ensure the quality of services during the establishment of LSPs.

To ensure the continuity of services, MPLS TE introduces the LSP backup and fast reroute (FRR) mechanisms. When faults occur on the link, the traffic can be switched immediately. Through MPLS TE, service providers (SPs) can fully utilize the current network resources to provide diversified services, optimize network resources, and scientifically manage the network.

To achieve the preceding purpose, MPLS needs to learn TE information of all routers in this network. MPLS TE lacks such a mechanism through which each router floods its TE information in the entire network to implement the synchronization of TE information. This mechanism is provided by the IS-IS protocol. Therefore, MPLS TE can advertise and synchronize TE information with the help of the IS-IS protocol.

IS-IS TE is an extension of IS-IS to support MPLS TE and complies with RFC 5305 and RFC 4205. IS-IS TE defines new TLVs in IS-IS LSPs to carry TE information and floods LSPs to flood and synchronize TE information. It extracts TE information from all LSPs and then transmits the TE information to the Constraint Shortest Path First (CSPF) module of MPLS for tunnel path calculation. IS-IS TE plays the role of a porter in MPLS TE. Figure 7-20 shows the relationships between IS-IS TE, MPLS TE, and CSPF.
Figure 7-20 Relationships between MPLS TE, CSPF, and IS-IS TE

New TLVs in IS-IS TE

To carry TE information in LSPs, IS-IS TE defines the following TLVs in RFC 5305:

  • Extended IS reachability TLV

    This TLV takes the place of IS reachability TLV and extends the TLV formats with sub-TLVs. Sub-TLVs are implemented in TLVs in the same manner as TLVs are implemented in LSPs. Sub-TLVs are used to carry TE information configured on physical interfaces.


    Currently, all sub-TLVs defined in RFC 5305 and sub-TLV type 22 defined in RFC 4124 are supported.

    Table 7-5 Sub-TLVs defined in Extended IS reachability TLV



    Length (Byte)


    Administrative Group



    Indicates the administrative group.

    IPv4 Interface Address



    Indicates the IPv4 address of a local interface.

    IPv4 Neighbour Address



    Indicates the IPv4 address of a neighbor interface.

    Maximum Link Bandwidth



    Indicates the maximum bandwidth of a link.

    Maximum Reserved Link Bandwidth



    Indicates the maximum reserved bandwidth of a link.

    Unreserved Bandwidth



    Indicates the unreserved bandwidth.

    Traffic Engineering Default Metric



    Indicates the default metric of TE.

    Bandwidth Constraints sub-TLV



    Indicates the TLV of the bandwidth constraint.

  • Traffic Engineering router ID TLV

    It is of TLV type 134, with a 4-byte Router ID. It is used as the MPLS LSR ID. In MPLS TE, a Router ID uniquely identifies a router. Each router has a Router ID.

  • Extended IP reachability TLV

    This TLV takes the place of IP reachability TLV and carries routing information. It extends the length of the route cost field and carries sub-TLVs.

  • Shared Risk Link Group TLV

    It is of TLV type 138 and used to carry information about the shared risk link group. This TLV can carry information about multiple shared links, each of which is a 4-byte positive integer.

IS-IS TE Implementation

IS-IS TE is implemented in two processes.

  • Process of responding to MPLS TE configurations.

    IS-IS TE functions only after MPLS TE is enabled.

    IS-IS TE updates the TE information in IS-IS LSPs based on MPLS TE configurations.

    IS-IS TE transmits MPLS TE configurations to the CSPF module.

  • Process of handling TE information in LSPs.

    IS-IS TE extracts TE information from IS-IS LSPs and transmits the TE information to the CSPF module.

In typical applications, IS-IS TE helps MPLS TE set up TE tunnels. As shown in Figure 7-21, a TE tunnel is set up between RouterA and RouterD.

Figure 7-21 IS-IS TE networking

The networking configuration is as follows:

  • Enable MPLS TE on RouterA, RouterB, RouterC, and RouterD and enable MPLS TE CSPF on RouterA to calculate the tunnel path.

  • Run IS-IS and enable IS-IS TE on RouterA, RouterB, RouterC, and RouterD to implement communication between the four routers.

After the preceding configuration is complete, IS-IS on RouterA, RouterB, RouterC, and RouterD sends LSPs carrying TE information configured on each router. RouterA then obtains the TE information of RouterB, RouterC, and RouterD from the received LSPs. The CSPF module can calculate the path required by the TE tunnel based on the TE information on the entire network.

Route Calculation on TE Tunnel Interfaces

IS-IS Shortcut (AA) and IS-IS Advertise (FA) calculate routes through TE tunnel interfaces. For the traffic transmitted through a specific route, MPLS guarantees the forwarding comparing with IP, which is unreliable. When IS-IS Shortcut (AA) and IS-IS Advertise (FA) are configured, MPLS forwarding is achieved with TE tunnel interfaces involving in route calculation and being the outbound interfaces of specific routes.

Figure 7-22 Principle of IS-IS Shortcut (AA) and Advertise (FA)

IS-IS Shortcut (AA) and IS-IS Advertise (FA) have the following differences:

  • IS-IS Advertise (FA) advertises TE tunnel information to other ISs, whereas IS-IS Shortcut (AA) does not.

    As shown in Figure 7-22, if the TE tunnel is enabled with IS-IS Advertise (FA), RouterA advertises information indicating that RouterC is its neighbor. The neighbor information is carried in TLV type 22 with no sub-TLVs. That is, no TE information is carried. If the TE tunnel is enabled with IS-IS Shortcut (AA), RouterA does not advertise such information.

  • IS-IS Advertise (FA) affects the SPF tree of other routers, whereas IS-IS Shortcut (AA) does not.

    IS-IS Shortcut (AA) does not affect the original structure of the IS-IS SPF tree, irrespective of whether a TE tunnel exists or not. Apart from the link from RouterA to RouterB, and that from RouterB to RouterC, a link marked with an Shortcut from RouterA to RouterC is added. The link marked with an Shortcut participates in route calculation.

    If the TE tunnel is enabled with IS-IS Advertise (FA), RouterA advertises the message that "RouterC is a neighbor of RouterA" to other routers on the network. Other routers then consider RouterC a neighbor of RouterA and add RouterC to the SPF tree without marking it with an Shortcut.

  • IS-IS Advertise (FA) does not support a relative metric, whereas IS-IS Shortcut (AA) supports.

    IS-IS Shortcut (AA) supports an absolute metric and a relative metric.

    If you use an absolute metric, the metric value of TE tunnels in IS-IS is fixed. If you use a relative metric, the metric value of TE tunnels in IS-IS is the sum of the physical link cost and relative metric. As shown in Figure 7-22, if the relative metric is set to 1, the cost of the path from SwitchA to SwitchC through the TE tunnel is 21 (10+10+1). If the relative metric is set to 0, the TE tunnel and physical link are of equal-cost on the outbound interface. If the relative metric is less than 0, the TE tunnel interface is preferred as the outbound interface.

  • IS-IS Advertise (FA) requires bidirectional TE tunnels, whereas IS-IS Shortcut (AA) requires only unidirectional tunnels.

Updated: 2019-03-21

Document ID: EDOC1000166601

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