Dec 2 2008
How does TE detect a neighbor-down
There are three “down notifications” which cause MPLS nodes to switch traffic onto their FRR backup tunnels.
1-Interface “down notification”
2-RSVP Hello neighbor “down notification”
3-BFD neighbor “down notification”
Some interfaces such as POS have very quick interface down notification ie around 50ms. If a router detects a link down it reroutes the traffic via a backup tunnel.
There are some instances where the router may not experience an interface down due to an intermediary device such as an ethernet switch. In these cases the router can rely on either RSVP hello neighbor “down notifications” or BFD “down notifications”.
RSVP hello protocol mechanics
An RSVP hello enabled router sends out RSVP hellos every interval. An RSVP hello enabled neighbor responds with an RSVP hello Ack. If the sending node does not receive an RSVP hello Ack for four intervals it declares the neighbor as down and notifies FRR.
There are two configurable paramaters for RSVP hellos.
1-Hello interval
2-Number of Ack misses before declaring a neighbor as down.
The hello interval is configured using the following command.
ip rsvp signalling hello fast-reroute refresh interval
The default value is 10000 milliseconds (10 seconds), which is hardly fast convergence. The interval values which can be configured are from 1000 to 30000 (1 to 30 seconds). Which even if use 1 second is not that fast.
The number of Ack misses before declaring a neighbor down can be configured using.
ip rsvp signalling hello fast-reroute refresh misses msg-count
The default for the msg-count argument is 4.
BFD protocol mechanics
BFD establishes a session with neighboring routers. BFD then sends accelerated control messages similar to IGP hellos.
To enable BFD for use with FRR there are two configuration steps.
1-Enable BFD globally
2-Enable BFD at interface level.
To enable BFD globally use the command.
ip rsvp signalling hello bfd
To enable BFD at interface level use the command.
interface gig1/0
ip rsvp signalling hello bfd
bfd interval milliseconds min_rx milliseconds multiplier interval-multiplier
BFD has the advantage over RSVP hellos in that it can acheive sub-second convergence.
Nov 25, 2010 @ 03:10:13
I tried this on Dynamips running on a macbook pro i7, ios version c7200-SPSERVICEK9-M 12.2(33) SRD.
Bfd neighbors won’t come up and debug bfd shows nothing.
but i can ping each routers interface.
did a packet capture too and no good.
where have you tried this ?
Nov 25, 2010 @ 05:01:11
Hi Douglas
Although I mentioned the bfd feature I didnt get around to testing it. However if the cli took the commands but its still not coming up then this may either be linked to dynamips issue or there may be some other config issue.
HTH
Zarar