RSVP - Explicit Route Object (ERO)

Default RSVP Behavior

By default, RSVP follow the IGP best path to establish the MPLS LSP.

Here the RSVP followed the IGP best path to establish a LSP between PE1 and PE2 routers. The LSP path is PE1-P3-P4-P5-PE2, with lowest OSPF metric cost that is four.

We can verify this LSP path in following snippet 

IGP path for PE2 (192.168.1.2)

In above two outputs we can clearly see that the LSP using IGP best path. 

 

Explicit Route Object (ERO)

With ERO (Explicit Route Object) we can influence the RSVP default path selection process and force RSVP to establish the the LSP path according to the defined ERO. ERO contains one or more user defined constraints that instructs the ingress router to establish the LSP on a particular path.

The ERO can be defined with two parameters

a.       Loose hop

b.       Strict hop

When a loose hop is configured, it identifies one or more transit LSRs through which the LSP must be routed. The RSVP use the IGP to determines the exact route from the ingress router to the first loose hop, or from one loose hop to the next, or from last loose hop to the egress. The loose hop specifies only that a particular LSR be included in the LSP and path must go through from that LSR.

Strict constraint must be defined as very next hop of loose hop or very next hop to ingress LSR. The strict hop ERO will process hop by hop. it identifies an exact path through which the LSP must be routed. If a LSR receive a PATH message with Strict Hop it verify the strict constraint and if it verify successfully then the constraint will removed from ERO and PATH message forward to downstream router.

 

In above diagram RSVP follow the IGP best path and establish the LSP along that path. The best IGP path from PE1 to PE2 is PE1-P1-P2-PE2 with total metric cost is 3. However, with ERO we can force RSVP to establish the path from PE1 to PE2 that must be routed through P4 transit LSR. For that purpose, in ERO we defined P4 as the loose hop.

root@PE-1# run show configuration protocols mpls

no-cspf;

label-switched-path pe1-to-pe2 {

    from 192.168.1.1;

    to 192.168.1.2;

}

interface ge-0/0/8.0;

interface ge-0/0/9.0;

[edit]

root@PE-1# show protocols mpls

no-cspf;

label-switched-path pe1-to-pe2 {

    from 192.168.1.1;

    to 192.168.1.2;

    primary ERO-1;

}

path ERO-1 {

    192.168.5.4 loose;

}

interface ge-0/0/8.0;

interface ge-0/0/9.0;

Following diagram shows the LSP path with transit LSR P4 as loose hop.

RSVP use IGP best path to establish LSP from ingress LSR to loose hop P4, and from P4 to egress LSR.

We can verify it in following snippet

 

Now we add the strict hop P5 after the loose hop. This will force RSVP to establish the LSP via P4 to P5 and then P5 to PE2 egress LSR instead of P4 to PE2 directly. You can configure the next-hop interface or next-hop loop-back as the strict constraint. The only requirement for strict hop is that it must be the next-hop of a link state router.

root@PE-1# show protocols mpls

no-cspf;

label-switched-path pe1-to-pe2 {

    from 192.168.1.1;

    to 192.168.1.2;

    primary ERO-1;

}

path ERO-1 {

    192.168.5.4 loose;

    192.168.5.5 strict;

}

interface ge-0/0/8.0;

interface ge-0/0/9.0;

The following diagram shows the LSP with transit LSR P4 as loose hop and transit LSR P5 as strict hop.


We can verify the path in following snippet

~~~ The End ~~~