[Xorp-hackers] OSPFv2 P2P Links

Bharadwaj Kalahasti b_kalahasti at yahoo.com
Mon Aug 27 14:59:38 PDT 2007


Erlend,
  Generally speaking, one set of implementations for non-broadcast neighbors such as
those of P2MP, P2P and NBMA seems to be to configure the neighbors as 'STATIC'
neighbors. 
  For P2P Neighbors, the Hellos are multicast. The Intf state for the OSPF interface
 should be P2P after the IP interface comes up. And the Nbr state should finally be
Full on the local end assuming that the Router ID of the neighbor is seen by the
local router. 

  These are the steps that should happen:
-- So assuming that your neighbor has gone Down at the remote end for the reasons - 
Admin down, Link transmission failure but Link is still Up etc, then this should
cause Inactivity Timer to be fired on the local end. 
-- Expiry of this timer should cause a new Router LSA to be generated and
area-flooded but without the Neighbor's Router ID in its Neighbor list (since it
failed to have bi-directional communication). 
-- Since the new Router-LSA would be different in its LS Header from the previous
one for the same local router's Router ID, that should cause its installation on all
of the area's routers and kick off a new SPF run once the installation of the
Router-LSA in the LSDB is done. 
-- The SPF run should take care of deleting the P2P host route in the Routing Table.

  
  I think the implementation of a P2P Nbr is upto interpretation. The catch here is
that a static configuration of the Nbr would mean that the Hello packet can always
have the Neighbor router listed in it indicating bi-directional communication and
full adjacency though the link may actually be down (if knowing the existence of a
Neighbor implies including it in the sent Hello). The other way to interpret the RFC
is that configuration is good enough to know its existence but its inclusion in the
Hello is subject to event occurence on the link. 

  I do not know the OSPFv2 code in XORP but if Static neighbors are not supported
yet, then may be it is a good idea to support it. 

Regards,
Kalahasti

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


Hi all,

 

It seems like XORP OSPFv2 does not fully manage to update the routing
table

when a neighbor connected through a p2p link goes down, e.g. when an
xorp 

OSPFv2 neighbour is stopped. The neighbour state is set correctly to
down,

but the routes advertised by/through the neighbour are not removed from
the 

rest of the network. The LS DB is identical before and after the
neighbour 

has been stopped (only the age field differs, but is not set to MaxAge).

 

I have tested this in a very simple scenario with 3 routers R1-R2-R3 up
and

running with p2p links configured over Ethernet. I have for example
stopped

ospf on R2 and then observed that R1 and R3 still have routes to each
other.

However, when the same routers run with Broadcast links the routes are 

correctly removed when one of the other routers goes down.

 

I can see from the source code and debug output that the difference in 

handling of the InactivityTimer Event (i.e. Hello timeout) between
Broadcast

and p2p links seems to start in the method:
Neighbour<A>::tear_down_state;

This method does not do any work for p2p (_peer.adjacency_change

only works for DR or BDR). I therefore simply tried to add the call:

_peer.get_area_router()->refresh_router_lsa()

to the end Neighbour<A>::tear_down_state. This causes a total recompute

of the routing but the routes from/via the router that has been shut
down are

not removed.

 

It may be that p2p links should have some kind of special static state, 

but to me, it does not seem sane that the links to/via a router that has

gone down are not removed. The RFC 2178 is somewhat diffuse when

it comes to p2p and Hello timeout. (It may seem somewhat strange

to run p2p over Ethernet but this is a requirement.)

 

Anyway I hope someone can give me some hints on how to work further in
this.

 

I can send debug logs and config setup if necessary.

Thanks in advance! 

 

Regards,

Erlend


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