Subject: [rbridge] loops in trill networks

From: kreeger at cisco.com (Larry Kreeger (kreeger))
Date: Tue, 11 Oct 2005 16:41:44 -0700

Eric,

Notice that in my example below, I am only talking about broadcast
connectivity - not unicast.  There is not MAC advertisements for
broadcast.  A broadcast must go to every edge port.  In this case, a
spanning tree must exist (across both the RBridge and 802.1d bridged
network) or a broadcast loop will occur.

 - Larry

-----Original Message-----
From: rbridge-bounces at postel.org [mailto:rbridge-bounces at postel.org] On
Behalf Of Gray, Eric
Sent: Tuesday, October 11, 2005 3:45 PM
To: 'Developing a hybrid router/bridge.'
Subject: Re: [rbridge] loops in trill networks

Larry,

	You have to think of bridges and RBridges as existing at two
layers.  How frames are forwarded between RBridges will be determined by
shortest path computation based on MAC advertisements in IS-IS.  How
frames are forwarded across
802.1 bridges between RBridges is based on STP.

--
E

--> -----Original Message-----
--> From: rbridge-bounces at postel.org
--> [mailto:rbridge-bounces at postel.org]On
--> Behalf Of Larry Kreeger (kreeger)
--> Sent: Tuesday, October 11, 2005 5:08 PM
--> To: Developing a hybrid router/bridge.
--> Subject: Re: [rbridge] loops in trill networks
--> 
--> 
--> Radia,
--> 
--> How about the case where the two ports connected to the same bridged

--> domain are on different RBridges?
--> 
--> Consider, the following topology where A,B,C are hosts.
--> 
--> A--RB1--Bridged-Domain--RB2--B
-->               |
-->               |
-->              RB3--C
--> 
--> In this case, would all RB's forward a L2 broadcast to their 
--> respective edge ports?  I was assuming, yes.  If so, then RB1,RB2 
--> and
--> RB3 are all
--> designated RBs in order to have broadcast connectivity between 
--> A,B,C.
--> 
--> Now a new RB4 is added to the topology which connects RB1 and RB3,
--> 
--> A--RB1--Bridged-Domain--RB2--B
-->     |         |
-->     |         |
--> D--RB4-------RB3--C
--> 
--> Are you saying that RB1 and RB3 detect that they now have new 
--> connectivity and battle to be a Designated RB to serve their side of

--> the network?  Assuming RB3 wins, all broadcasts must enter and leave

--> the Bridged-Domain via RB3 to get between hosts A,C,D and B.
--> RB2 must still
--> be the Designated RB for traffic to reach B.
--> 
--> I didn't think routers needed to deal with this since they don't 
--> forward broadcasts.  Are you saying that routing protocols already 
--> deal with these types of scenarios?
--> 
--> Thanks, Larry
--> 
--> -----Original Message-----
--> From: rbridge-bounces at postel.org
--> [mailto:rbridge-bounces at postel.org] On Behalf Of Radia Perlman
--> Sent: Tuesday, October 11, 2005 12:46 PM
--> To: Developing a hybrid router/bridge.
--> Subject: Re: [rbridge] loops in trill networks
--> 
--> The intention is that deployment not have to be careful. If two of 
--> an RBridge's ports are connected to the same bridged broadcast 
--> domain (i.e., there is a bridge-connected path between the two 
--> ports), then the RBridge will think it has two ports connected to 
--> the same link. Which is OK.
--> It won't forward any packets, including not BPDUs, between those 
--> ports.
--> At most one of
--> its ports will become "designated" for sourcing/sinking 
--> unencapsulated traffic to/from that link.
--> 
--> It would be just like a router having two ports connected to the 
--> same link, which should work with no problem.
--> 
--> Radia
--> 
--> 
--> 
--> Gray, Eric wrote:
--> 
--> >Radia,
--> >
--> >	If I am understanding what you are saying correctly, there are
--> some
--> >deployment issues with Rbridges into a 802.1 bridged environment.
--> >
--> >	The deployment issues arise when individual RBridges are
--> installed, if
--> >this is not carefully planned in advance.
--> >If an Rbridge is installed on a link where it is connected
--> to two or
--> >more 802.1 bridges and there is another L2 path between those two 
--> >bridges, the RBridge will either have to forward BPDUs or - like a 
--> >router - recognize that multiple ports are connected to
--> the same LAN.
--> >
--> >	Is there an intention that the latter would be the default case
--> until
--> >and unless the installed RBridge is able to discover a connected 
--> >RBridge peer?
--> >
--> >--
--> >Eric Gray
--> >
--> >--> -----Original Message-----
--> >--> From: rbridge-bounces at postel.org 
--> >--> [mailto:rbridge-bounces at postel.org]On
--> >--> Behalf Of Radia Perlman
--> >--> Sent: Tuesday, October 11, 2005 12:44 PM
--> >--> To: Developing a hybrid router/bridge.
--> >--> Subject: Re: [rbridge] loops in trill networks
--> >--> 
--> >--> 
--> >--> There's nothing an RBridge can do about loops
--> consisting of just
--> >--> bridges.
--> >--> Which is why it's good to partition the spanning trees
--> formed by
--> >--> the bridges so that the remaining spanning trees are
--> as small, and
--> >--> therefore as stable, as possible.
--> >--> 
--> >--> RBridges are like routers. They sit at a layer above
--> bridges. The
--> >--> bridges form what looks like a single link to the RBridges. If 
--> >--> there are a bunch of bridges connecting a bunch of
--> segments, all
--> >--> the RBridges attached to that portion of the network see that 
--> >--> bridged segment as a single link.
--> >--> 
--> >--> Radia
--> >--> 
--> >--> Loa Andersson wrote:
--> >--> 
--> >--> >Oh, yes it is true ...
--> >--> >
--> >--> >but I thought that a trill network could consist of a
--> mixed of
--> >--> >rbridges and the type of bridges that exist today, if
--> that is the
--> >--> >case you can possibly form the loop over only non-rbridges
--> >--> >
--> >--> >but I'm not up to speed on this, there might be a way
--> to handle
--> >--> >this also
--> >--> >
--> >--> >/Loa
--> >--> >
--> >--> >Joe Touch wrote:
--> >--> >  
--> >--> >
--> >--> >>There is, and that's what it's there for.
--> >--> >>
--> >--> >>Joe
--> >--> >>
--> >--> >>Mike Hughes wrote:
--> >--> >>
--> >--> >>    
--> >--> >>
--> >--> >>>--On 11 October 2005 15:23 +0200 Loa Andersson <loa at pi.se>
--> wrote:
--> >--> >>>
--> >--> >>>
--> >--> >>>
--> >--> >>>      
--> >--> >>>
--> >--> >>>>this might be an old discussion, but it is some
--> time since it
--> >--> >>>>look at it. What do we do in trill networks about
--> transient
--> >--> >>>>loops, i.e. if use link state protocols there is a
--> possibility
--> >--> >>>>that we have transient loops. With no TTL mechanisms
--> >--> those looping
--> >--> >>>>frames could cause quite a bit of problem.
--> >--> >>>>        
--> >--> >>>>
--> >--> >>>Unless I've missed something, or things have changed
--> >--> overnight, there is a
--> >--> >>>TTL provided in the SHIM header.
--> >--> >>>
--> >--> >>>Mike
--> >--> >>>
--> >--> >>>
--> >--> >>>---------------------------------------------------------
--> >--> ---------------
--> >--> >>>
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--> >--> >>>rbridge mailing list
--> >--> >>>rbridge at postel.org
--> >--> >>>http://www.postel.org/mailman/listinfo/rbridge
--> >--> >>>      
--> >--> >>>
--> >--> >
--> >--> >
--> >--> >  
--> >--> >
--> >--> 
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--> >
--> 
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