MPLS LDP Loop-Detection

[amit.bhagat]

Hi Guys,

Just wondering if MPLS LDP Loop-Detection is completely supported in IOS. I can see with the following command that it is enabled.

R3#show mpls ldp parameters

LDP Feature Set Manager: State Initialized

  LDP features:

    Auto-Configuration

    Basic

    ICPM

    IP-over-MPLS

    IGP-Sync

    LLAF

    TCP-MD5-Rollover

    TDP

Protocol version: 1

Session hold time: 180 sec; keep alive interval: 60 sec

Discovery hello: holdtime: 15 sec; interval: 5 sec

Discovery targeted hello: holdtime: 90 sec; interval: 10 sec

Downstream on Demand max hop count: 255

LDP for targeted sessions

LDP initial/maximum backoff: 15/120 sec

LDP loop detection: on

But the Label Mapping messages do not carry any Hop-Count and Path-Vector TLVs.

Any ideas?

Regards,

Amit.

[Vivek Ruhil]

Hi Amit

MPLS relies on IGP to detect loops in the network and has no inbuilt mechanism to detect loops other than the fact that it has a TTL field and if that value decrements to 0 all the way down from 255 then the packet is dropped.

[amit.bhagat]

Hi Vivek,

Thanks for replying but I disagree with your comment. LDP has a mechanism for Loop Detection defined in RFC 5036. In Cisco IOS, this is configured globally using the command "mpls ldp loop-detection". This feature is advertised in LDP Initialization messages in Common Session Parameters TLV using "D" flag.

As per RFC, if this feature is enabled, the downstream LSR must add Hop-Count TLV and Path-Vector TLV in Label Mapping and Label Request messages to detect a loop.

If IOS doesn't send these TLVs for frame-mode MPLS, and relies on IGP like you mentioned, then it is acceptable.

Regards,

Amit.

[Vivek Ruhil]

Hi Amit

I understand what you are saying, however, my reply was from the point of view that this was never implemented in Cisco IOS and hence my reply.

Actually, to me also it doesn't make sense for MPLS to have its own loop detection when for MPLS traffic to work it uses the IGP path, Why overload the router by doing the same thing in different protocols when only one protocol can give us the result. Only true in case of MPLS + IGP becuase they both completely rely on each other to function.

[amit.bhagat]

Okay, that's weird. Then why allow implementers to enable the feature in the first place using the command "mpls ldp loop-detection"?

[Vivek Ruhil]

Hi

So, let me re-phrase

This command is required in Cell Based MPLS Networks and not frame-mode mpls networks, and today I don't remember any ISP using Cell mode MPLS,in fact, it never got deployed widely.

check the below link

http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/fsldp22.html#wp1359012

[amit.bhagat]

Yep, that's what I thought.

Thanks Vivek.