MPLS : loop-detect=yes

RouterOS Forwarding Protocols
1

Hi Guys

When would we use “loop-detect=yes” in MPLS setups?

2

Well, normally you do not need to use it. The main intention of loop detection in MPLS architecture is to have some means to survive loops when hardware is not able to do TTL decrement on forwarding (some ATM switches, perhaps). Other than that - you may want to use loop detection if you are using static routing - then loop detection may help you to diagnose routing problems (e.g. if you will set up routing loop, LSP will not be established). If you use some IGP, it will deal with loops anyway and not having loop detection will save some resources on LSRs.

3

Thanks for clarifying that. I use OSPF so I will keep it disabled

4

Another 2 quickies:

  1. Regarding “Use explicit NULL”

When would I use it?

Correct me if i’m wrong, would I enable “Use explicit NULL” on router R3? Assuming I want to preserve CoS

http://www.doyleassociates.net/Blog/Labels/Figure2.gif


2) Regarding “Distribute for default route”

Is this the same as “redistribute default route” under /routing OSPF instances?
Should I only have this setting enabled at a single egress point in my MPLS cloud or can I have it at multiple breakouts?

Thanks again

5

In this setup you should enable it on R4 - this will make R4 to distribute “explicit null” (that will make R3 do SWAP) instead of “implicit null” (makes R3 do POP).

  1. Regarding “Distribute for default route”

Is this the same as “redistribute default route” under /routing OSPF instances?
Should I only have this setting enabled at a single egress point in my MPLS cloud or can I have it at multiple breakouts?

Depends on your network (and what you use MPLS for), but it is always safer to not enable this setting, before you understand the effect of having LSP for default route prefix. When using routing, every router does routing lookup and will choose the most specific route (so if one router routes packet and uses default route, next hop router may use more specific route, if it has any). LDP establishes LSP on per route prefix basis. So that when packet “enters” LSP established for default route, it will get switched to the end of that LSP, not taking into account that some intermediate hop has more specific route.

6

Thanks..As a rule of thumb, should I enable “Use explicit NULL” on all my egress routers?

7

Only if it is necessary to receive EXP bits at the egress router (meaning - you use this information on egress router in some way). Otherwise it is not necessary and actually having penultimate hop popping is more efficient - egress router can work on packet right away (e.g. route it or inspect next label), w/o having to remove explicit null label first.