Chapter 4: Cisco MPLS Traffic Engineering

Cisco Press

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Figure 4-1 shows the default bandwidth utilization thresholds that influence link flooding in both operating systems. The figure highlights with a circle the changes in bandwidth reservation that trigger the flooding of the link state. Initially, the link has all its bandwidth available. Two TE LSPs reserve 5 percent of the link bandwidth each. Those reservations do not trigger any flooding. The next five reservations cross one of the default thresholds and cause the advertising of the updated link information. At this point, seven reservations have consumed 88 percent of the link bandwidth. After reaching this maximum, five TE LSPs are torn down. All but the fourth TE LSP introduce a change in bandwidth reservation that forces flooding. The link remains with two TE LSPs that hold 10 percent of the link bandwidth.

Configuring Link Attributes

A node will advertise MPLS TE attributes for a link when you configure the IGP with the respective extensions. As described in Chapter 2, those link characteristics include the TE administrative weight, attribute flags, and bandwidth. All links in your MPLS TE network have these three parameters regardless of your IGP choice (IS-IS or OSPF) and regardless of whether you configure them explicitly. This behavior includes all nodes whether they act as TE LSP headends, midpoints, or tailends.

Not all links on a node have to be part of your MPLS TE network. You need to configure at least one link connecting to another MPLS TE node. If you do not configure the link parameters explicitly, a link will use default values as soon as you enable MPLS TE and RSVP on the respective interface.

Figure 4.1

Figure 4-1

Bandwidth Utilization Thresholds for Flooding Link Updates


Note - When you configure the link characteristics under an interface, you are configuring one end of the link. The node at the other end of the link is responsible for the configuration in the other direction. The configuration on both sides does not necessarily have to match.


Cisco IOS relies on MPLS TE and RSVP interface commands to set the link characteristics. You use the mpls traffic-eng administrative-weight, mpls traffic-eng attribute-flags, and ip rsvp bandwidth commands. As a prerequisite, you must enable MPLS TE on the interface using the mpls traffic-eng tunnels command. Unless configured explicitly, the administrative weight defaults to a value of the IGP metric, the attribute flags defaults to 0, and the RSVP bandwidth defaults to 75 percent of the interface bandwidth. If you do not configure the ip rsvp bandwidth command, the RVSP bandwidth defaults to 0. You can still signal TE LSPs through that link if those TE LSPs do not try to reserve any bandwidth. You can configure the RSVP bandwidth above or below the actual link capacity. The section "Signaling of TE LSPs" elaborates on the RSVP commands.

Cisco IOS XR uses the interface submodes under the mpls traffic-eng and rsvp configuration modes to define the link characteristics. You use the admin-weight and attribute-flags commands within the mpls traffic-eng configuration mode. In addition, you configure the bandwidth commands within the rsvp mode. All three link characteristics have the same defaults as in Cisco IOS.

Example 4-11 shows the configuration of link characteristics in Cisco IOS. The example includes two interfaces enabled for MPLS TE. Interface POS0/1/0 contains explicit configuration for the attribute flags, administrative weight, and RSVP bandwidth. Interface POS1/0/0 has an explicit administrative weight and implicit attribute flags and RSVP bandwidth.

Example 4-11 Definition of Link Characteristics for MPLS TE in Cisco IOS

interface POS0/1/0
 ip address 172.16.0.0 255.255.255.254
 mpls traffic-eng tunnels
 mpls traffic-eng attribute-flags 0xF
 mpls traffic-eng administrative-weight 20
 ip rsvp bandwidth 100000
!
interface POS1/0/0
 ip address 172.16.0.2 255.255.255.254
 mpls traffic-eng tunnels
 mpls traffic-eng administrative-weight 30
 ip rsvp bandwidth
!

Example 4-12 shows the configuration of link attributes for two interfaces in Cisco IOS XR. Notice that the interface configuration resides under the rsvp and mpls traffic-eng configuration modes. Interface POS0/3/0/0 uses an explicit bandwidth, administrative weight, and attribute flags. Interface POS0/3/0/1 only defines the RSVP bandwidth and the attribute flags explicitly.

Example 4-12 Definition of Link Characteristics for MPLS TE in Cisco IOS XR

rsvp
 interface POS0/3/0/0
  bandwidth 100000
 !
 interface POS0/3/0/1
  bandwidth 155000
 !
!
mpls traffic-eng
 interface POS0/3/0/0
  admin-weight 5
  attribute-flags 0x8
 !
 interface POS0/3/0/1
  attribute-flags 0xf
 !
!

Verifying Link Information Distribution

You can examine the link information that a node advertises by using the show mpls traffic-eng link-management advertisements command. The output provides details on all the MPLS TE link attributes and the IGP configuration. The same command syntax is available in Cisco IOS and Cisco IOS XR.

Example 4-13 shows the command output in Cisco IOS. The output corresponds to the configuration of interface POS0/1/0 in Example 4-11. Example 4-14 contains the equivalent command output in Cisco IOS XR. The command output corresponds to interface POS0/3/0/0 in Example 4-6. Both Cisco IOS and Cisco IOS XR provide almost identical information with slightly different formats. Both examples highlight the flooding protocol, the MPLS TE router ID, TE metric, the reservable (global) bandwidth, the amount of available bandwidth at each of the eight priority levels, and the link attribute flags.

Example 4-13 Examining MPLS TE Link Advertisements in Cisco IOS

Router#show mpls traffic-eng link-management advertisements 
Flooding Status:   ready
Configured Areas:   1
IGP Area[1] ID:: isis level-2
 System Information::
  Flooding Protocol:  ISIS
 Header Information::
  IGP System ID:    1720.1625.5001.00
  MPLS TE Router ID:  172.16.255.1
  Flooded Links:    1
 Link ID:: 0
  Link Subnet Type:   Point-to-Point
  Link IP Address:   172.16.0.0
  IGP Neighbor:     ID 1720.1625.5129.00, IP 172.16.0.1
  TE metric:      20
  IGP metric:      10
  SRLGs:        None
  Physical Bandwidth:  155000 kbits/sec
  Res. Global BW:    100000 kbits/sec
  Res. Sub BW:     0 kbits/sec
  Downstream::
                             Global Pool    Sub Pool 
                             -----------    ----------
   Reservable Bandwidth[0]:       100000      0 kbits/sec
   Reservable Bandwidth[1]:       100000      0 kbits/sec
   Reservable Bandwidth[2]:       100000      0 kbits/sec
   Reservable Bandwidth[3]:       100000      0 kbits/sec
   Reservable Bandwidth[4]:       100000      0 kbits/sec
   Reservable Bandwidth[5]:       100000      0 kbits/sec
   Reservable Bandwidth[6]:       100000      0 kbits/sec
   Reservable Bandwidth[7]:       100000      0 kbits/sec
  Attribute Flags:    0x0000000F
! Output omitted for brevity

Example 4-14 Examining MPLS TE Link Advertisements in Cisco IOS XR

RP/0/4/CPU0:Router[P]#show mpls traffic-eng link-management advertisements 

 Flooding Status            : ready
 Last Flooding              : 904 seconds ago
 Last Flooding Trigger      : Link BW changed
 Next Periodic Flooding In  : 132 seconds 
 Diff-Serv TE Mode          : Not enabled 
 Configured Areas           : 1

 IGP Area[1]:: isis DEFAULT level-2
   Flooding Protocol  : ISIS
   IGP System ID      : 1720.1625.5129.00
   MPLS TE Router ID  : 172.16.255.129
   Flooded Links      : 2

   Link ID:: 0 (POS0/3/0/0)
     Link IP Address    : 172.16.0.1
     O/G Intf ID        : 1
     Neighbor           : ID 1720.1625.5001.00, IP 172.16.0.0
     SRLGs              :
     TE Metric          : 5
     IGP Metric         : 10
     Physical BW        : 155520 kbits/sec
     BCID               : RDM 
     Max Reservable BW  : 100000 kbits/sec
     Res Global BW      : 100000 kbits/sec
     Res Sub BW         : 0 kbits/sec

     Downstream::
                        Global Pool  Sub Pool  
                        -----------  -----------
      Reservable BW[0]:      100000            0 kbits/sec
      Reservable BW[1]:      100000            0 kbits/sec
      Reservable BW[2]:      100000            0 kbits/sec
      Reservable BW[3]:      100000            0 kbits/sec
      Reservable BW[4]:      100000            0 kbits/sec
      Reservable BW[5]:      100000            0 kbits/sec
      Reservable BW[6]:      100000            0 kbits/sec
      Reservable BW[7]:      100000            0 kbits/sec

     Attribute Flags: 0x00000008

! Output omitted for brevity

Examining the MPLS TE topology database is an indispensable task to troubleshoot a vast number of problems. The show mpls traffic-eng topology command displays the entire MPLS TE topology database. The output can be rather lengthy, so make sure you get familiar with the command options. They enable you to narrow down the amount of information to display. The topology database includes information about all the links in the MPLS TE network according to the local configuration and the IGP advertisements from other nodes in the same area or level.

Examples 4-15 and 4-16 show an output fragment of this command in Cisco IOS and Cisco IOS XR, respectively. The output highlights the MPLS TE router ID, the TE metric, the attribute flags, the maximum (global) reservable bandwidth, and the amount of available bandwidth at each of the eight priority levels. In both examples, RSVP has not allocated any link bandwidth.

Example 4-15 Examining the MPLS TE Topology Database in Cisco IOS

Router#show mpls traffic-eng topology 
My_System_id: 172.16.255.1 (ospf 100 area 0)

Signalling error holddown: 10 sec Global Link Generation 51

IGP Id: 172.16.255.129, MPLS TE Id:172.16.255.129 Router Node (ospf 100 area 0)
   link[0]: Point-to-Point, Nbr IGP Id: 172.16.255.1, nbr_node_id:10, gen:51
     frag_id 0, Intf Address:172.16.0.1, Nbr Intf Address:172.16.0.0
     TE metric:1, IGP metric:1, attribute_flags:0x0
     SRLGs: None 
     physical_bw: 155520 (kbps), max_reservable_bw_global: 155000 (kbps)
     max_reservable_bw_sub: 0 (kbps)

                            Global Pool    Sub Pool
           Total Allocated  Reservable     Reservable
           BW (kbps)        BW (kbps)      BW (kbps)
           ---------------  -----------    ----------
    bw[0]:            0          155000             0
    bw[1]:            0          155000             0
    bw[2]:            0          155000             0
    bw[3]:            0          155000             0
    bw[4]:            0          155000             0
    bw[5]:            0          155000             0
    bw[6]:            0          155000             0
    bw[7]:            0          155000             0
     
! Output omitted for brevity

Example 4-16 Examining the MPLS TE Topology Database in Cisco IOS XR

RP/0/4/CPU0:Router#show mpls traffic-eng topology 
My_System_id: 172.16.255.129 (ospf  area 0)

Signalling error holddown: 10 sec Global Link Generation 173

IGP Id: 172.16.255.129, MPLS TE Id: 172.16.255.129 Router Node (ospf  area 0)

 Link[0]:Point-to-Point, Nbr IGP Id:172.16.255.1, Nbr Node Id:36, gen:173
   Frag Id:0, Intf Address:172.16.0.1, Intf Id:0
    Nbr Intf Address:172.16.0.0, Nbr Intf Id:0
   TE Metric:1, IGP Metric:1, Attribute Flags:0x0
   Switching Capability:, Encoding:
   Physical BW:155520 (kbps), Max Reservable BW Global:155000 (kbps)
   Max Reservable BW Sub:0 (kbps)
                            Global Pool    Sub Pool
           Total Allocated  Reservable     Reservable
           BW (kbps)        BW (kbps)      BW (kbps)
           ---------------  -----------    ----------
    bw[0]:             0      155000              0
    bw[1]:             0      155000              0
    bw[2]:             0      155000              0 
    bw[3]:             0      155000              0
    bw[4]:             0      155000              0
    bw[5]:             0      155000              0
    bw[6]:             0      155000              0
    bw[7]:             0      155000              0

! Output omitted for brevity

Tip - The use of command-line interface (CLI) output filtering can be of great help to easily extract important information from the topology database. You filter the command output using the pipe operand (|) and the begin, exclude, or include keywords followed by a regular expression. You can save your most useful filters using the alias configuration command. As an example, the following filter enable you to quickly identify those links that have an IGP or TE metric of 10 on Cisco IOS or Cisco IOS XR nodes:

show mpls traffic-eng topology | include [Ff]rag | [Mm]etric:10,

Consult the Cisco IOS and Cisco IOS XR configuration guides for further detail about filtering CLI output.


Path Computation

A headend performs path computation for a TE LSP. The MPLS TE tunnel interface defines a destination for the TE LSP and a list of path options to reach that destination. These path options may reference an explicit path or request the computation of a dynamic path to the destination. Even if you configure a tunnel with an explicit path, the node verifies the path using the MPLS TE topology database unless you disable this check explicity.

Configuring the TE LSP Path

You can specify multiple path options for a TE LSP. These options can define a complete or partial (loose) explicit path. In addition, you can configure the node with an option to compute dynamically the complete path to the destination. As the section "Defining a TE Tunnel Interface" described, you configure the TE LSP destination with its own command independent of the path configuration. In Cisco IOS, you configure the TE LSP destination with the tunnel destination command and specify a path option with the tunnel mpls traffic-eng path-option command. Cisco IOS XR uses the destination command and the path-option command. You can configure multiple path options for a tunnel. Each one has a sequence number, and the node evaluates them in increasing order.

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