Re: MPLS LDP IGP Sync HOLDDOWN?

The feature can only save you so much. If you left "mpls ip" off then it's your fault not the feature's fault. The ideal situation is that all links run LDP IGP auto config and LDP IGP sync, which by design tries to prevent both a config and software problem.

In the MPLS core in a real design all links run IGP/LDP so you would want this protection. If a link runs IGP but not LDP you simply turn LDP auto config off at the link level for that particular interface.

Brian McGahan, CCIE #8593 (R&S/SP/Security), CCDE #2013::13
bmcgahan_at_INE.com<mailto:bmcgahan_at_INE.com>

Internetwork Expert, Inc.
http://www.INE.com<http://www.ine.com/>

On Jul 3, 2013, at 11:19 PM, "Tom Kacprzynski" <tom.kac_at_gmail.com<mailto:tom.kac_at_gmail.com>> wrote:

Thanks Yuri!!...that's exactly what I was looking for. I could not generate
an example where the IGP session would stay down. I was only able to get
the max cost advertised...This brings me to another questions. If the rules
for LDP IGP Sync say that if the LDP peer is reachable but the LDP session
is not up, by default the IGP connections will be in down state
indefinitely (unless you configure the holddown timers). But my question
is if the LDP session is down how is the router going to know what's the
LDP peer's transport IP to check if it is reachable? If LDP hellos are
never sent they will not know about each other. I am missing another rule
in the LDP IGP Sync feature?

For example I referenced this topology at
http://kemot-net.wdfiles.com/local--files/mpls-ldp-igp-synchronization/mpls%20ldp%20igp%20sync%20001.JPG

R4's Gi2/0 is down, because R3's Gi2/0 does not have "mpls ip" configured.
IGP and LDP are not synchronized.

R4#sh ip ospf mpls ldp interface gi2/0
GigabitEthernet2/0
 Process ID 1, Area 0
 LDP is configured through LDP autoconfig
 LDP-IGP Synchronization : Required
 Holddown timer is not configured
 Interface is down and pending LDP <<------------interface down

Router R4 does not see the transport IP of the peer, since it doesn't get
any LDP hellos.

R4#sh mpls ldp discovery detail
Local LDP Identifier:
   4.4.4.4:0
   Discovery Sources:
   Interfaces:
       GigabitEthernet1/0 (ldp): xmit/recv
               ....
       GigabitEthernet2/0 (ldp): *xmit*
           Enabled: IGP config;
           Hello interval: 5000 ms; Transport IP addr: 4.4.4.4
           Clients: IPv4, mLDP

...so how is R4 able to detect the reachability of R3's peer IP address per
the feature's rules?

Thank you,

Tom

On Wed, Jul 3, 2013 at 4:25 PM, Yuri Bank <yuribank_at_gmail.com<mailto:yuribank_at_gmail.com>> wrote:

What if you're using LDP autoconfiguration? You may have some links to non
MPLS enabled routers/devices, that are running a IGP (Such as a Load
balancer, firewall etc...) Without specifying a holddown time the IGP
adjacency would never come up. Although in that situation it would be best
to disable mpls entirely on those interfaces.

-YuriB

On Tue, Jul 2, 2013 at 6:45 PM, Tom Kacprzynski <tom.kac_at_gmail.com<mailto:tom.kac_at_gmail.com>> wrote:

Hi,
I'm trying to figure out why would anyone use the holddown option for IGP
Sync? Does is not advertise the max metric after the timers expires for
the
link having synchronization issue? If that's the case why would you enable
the sync feature in the first time if it will be bypassed after the
holddown timer expires? Am I missing something?

I have a topology I'm testing this feature. Instead of filling up the
email
with attachments I put the image at
http://wiki.kemot-net.com/mpls-ldp-igp-synchronization

Thank you,

Tom Kacprzynski

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Received on Thu Jul 04 2013 - 01:56:24 ART