Unlike RSVP, SR-TE we can configure headend router by imposing the labels which can be node or link alternatively controller push the SR-TE paths. No need to maintain the path status end to end like RSVP
They call it as Distributed or centralised. When we use distributed means router will refer the labels advertised by the IGP or BGP.
If we use centralised controller then the controller calculates the path and push it to the Router using PCEP or SR-TE policy address family via BGP.
While selecting the SR to implement in the network it is advisable to learn about the Number of labels supported by the each hardware. As of today nokia FP based chipsets support 12 Labels, and for other products it varies.
Having talked about label, consider that not all the 12 labels will be consumed by the transport label. Need to reserve for service label, oam label, FRR label and entropy label will also consume the label stack.
Need to configure the label stack size, max-sr label and additional-frr labels together define the overall label stake.
By default , node will automatically calculate the label stack for trasnport based on default program but in sometime we don’t require them for that we need configure dynamic-egress-label-limit command set to true or false.
Distributed SR-TE
SR-TE headend responsible for computing the path to the destination. SR supports 2 methods, one is local cspf and second it explicitly path using an IP address or a SID same as RSVP TE.
I am going to use the below topology for configuration the distributed SR-TE.
Let’s configure SR-TE in our LAB setup.
MPLS label range used for SR:
mpls-labels
sr-labels start 20000 end 20099
exit
PE1- index 11, PE2 – Index 12 and vice versa
ASBR -index 1, ASBR2 – index 2, ASBR3 – index 3, ASBR4 – index 4. So, PE1 node SID is 20011, PE2 node SID – 20012 and vice versa
Local CSPF
When the local cspf is used, router will calculate the path entirely with the adj sid using TE database. Local cspf is limited to single domain or area cannot span across different area as it reliance on TE data for calculation.
Constraints like igp delay, metric admin-group and hop count can be added but SR-TE don’t have the ability to reserve the bandwidth like RSVP. If we use bandwith reservation it wont take into effect, and in controller Bandwidth is pass on to PCE and it use that value in calculating the path and apply in its topology even it wont reserve it.Let’s configure the SR-TE LSP between PE5 towards PE3
A:PE5>config>router>mpls# info
----------------------------------------------
resignal-timer 30
resignal-on-igp-event
resignal-on-igp-overload
path "local-cspf"
no shutdown
exit
lsp "PE5toPE3" sr-te
to 1.0.0.3
from 1.0.0.5
path-computation-method local-cspf
max-sr-labels 5 additional-frr-labels 2
primary "local-cspf"
exit
no shutdown
exit
no shutdown
First I configured the path but I forgot to call it under the lsp so the lsp was down. When I debug further identified that there is no path populated. Then I checked the configuration and fixed the error later the LSP is come up.
Lets explore the each command I entered under the sr-te LSP.
PE5to PE3 LSP path in detail
we can see the local-cspf uses Adj label for end to end LSP Path in SR-TE.
I used few configuration in inside mpls contexts are resignal timer and resignal on igp event and resignal-on-igp-overload.
Resignal Timer:
When configured resignal timer expires mpls check with CSPF table for better cost path. If its available, then it performs MBB to switch the traffic to the new path without any traffic distortion.
resignal-on-igp-event:
This command only valid for lsp initiated by router using cspf. Not work if this lsp path is controlled by controller.
Any igp events occurred, the it tracks the ted database and expire the resignal timer to trigger the MBB for new path calculation
Current LSP path. Lets trigger the IGP events and capture the output.
I did shut the interface between the ASBR1 and ASBR3 but the LSP didn’t come up and down with no cspf route to the destination. Later after doing
Investing some time in the topology and other routers identified that to reach LSP from PE5 to PE3 after shutting the link between ASBR it require more than 5 hops but we configured only 5 hops in the label stake. After reconfiguring it the LSP comeup
*A:PE5>config>router>mpls>lsp# info
----------------------------------------------
to 1.0.0.3
from 1.0.0.5
path-computation-method local-cspf
max-sr-labels 9 additional-frr-labels 2
primary "local-cspf"
exit
no shutdown
----------------------------------------------
After that when i unshut the interface didnt help to resignal the LSP.
Again i did shut the link to trigger the igp event then the LSP was not switching, later identified i have done the wrong configuration there is special command for sr-te for igp-event.
After the reconfiguration it worked perfectly as expected.
*A:PE5>config>router>mpls# info sr-te-resignal
----------------------------------------------
resignal-timer 30
resignal-on-igp-overload
resignal-on-igp-event
resignal-timer 30
resignal-on-igp-event
resignal-on-igp-overload
exit
The reason why we need the configuration because routers in the path doesnot maintain the state path so if the remote link goes down then the LSP path doesnot change it will blackhole the traffic
Alternatively in production environment we use S-BFD for SR-TE Tunnels with BFD action to avoid traffic blackhole
resignal-on-igp-overload
if the system in the topology goes into overload status it trigger the resignal of the LSP.
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