TRILL Working Group Radia Perlman
INTERNET-DRAFT Intel Labs
Intended status: Informational Donald Eastlake
Huawei
Anoop Ghanwani
Brocade
Expires: October 4, 2011 April 5, 2011
RBridges: Multilevel TRILL
<draft-perlman-trill-rbridge-multilevel-02.txt>
Abstract
This is an informational document describing issues and comparing
advantages and disadvantages of various possible approaches to
extending TRILL to use multiple levels of IS-IS.
Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. Distribution of this document is
unlimited. Comments should be sent to the TRILL working group
mailing list <rbridge@postel.org>.
Internet-Drafts are working documents of the Internet Engineering
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The list of current Internet-Drafts can be accessed at
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Acknowledgements
The helpful comments of the following are hereby acknowledged: David
Michael Bond and Dino Farinacci.
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Table of Contents
1. Introduction............................................3
1.1 TRILL Scalability Issues...............................3
1.2 Improvements Due to Multilevel.........................4
1.3 More on Areas..........................................4
1.4 Terminology and Acronyms...............................5
2. Multilevel TRILL Issues.................................6
2.1 Non-zero Area Addresses................................6
2.2 Aggregated versus Unique Nicknames.....................7
2.2.1 More Details on Unique Nicknames.....................7
2.2.2 More Details on Aggregated Nicknames.................8
2.2.2.1 Border Learning Aggregated Nicknames...............9
2.2.2.2 Swap Nickname Field Aggregated Nicknames..........11
2.2.2.3 Comparison........................................11
2.3 Building Multi-Area Trees.............................12
2.4 The RPF Check for Trees...............................13
2.5 Area Nickname Acquisition.............................13
2.6 Link State Representation of Areas....................13
3. Area Partition.........................................15
4. Multi-Destination Scope................................16
4.1 Unicast to Multi-destination Conversions..............16
4.2 Selective Broadcast Domain Reduction..................17
5. Co-Existence with Old RBridges.........................18
6. Summary................................................19
7. Security Considerations................................19
8. IANA Considerations....................................19
9. References.............................................20
9.1 Normative References..................................20
9.2 Informative References................................20
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1. Introduction
The IETF TRILL protocol [RFCtrill] provides optimal pair-wise data
frame forwarding without configuration, safe forwarding even during
periods of temporary loops, and support for multipathing of both
unicast and multicast traffic. TRILL accomplishes this by using [IS-
IS] link state routing and encapsulating traffic using a header that
includes a hop count. The design supports VLANs and optimization of
the distribution of multi-destination frames based on VLANs and IP
derived multicast groups. Devices that implement TRILL are called
RBridges.
Familiarity with [RFCtrill] is assumed in this document.
1.1 TRILL Scalability Issues
There are multiple issues that might limit the scalability of a
TRILL-based network:
1. the routing computation load,
2. the volatility of the LSP database creating too much control
traffic,
3. the volatility of the LSP database causing the TRILL network to be
in an unconverged state too much of the time,
4. the size of the LSP database,
5. the hard limit of the number of RBridges, due to the 16-bit
nickname space,
6. the traffic due to upper layer protocols use of broadcast and
multicast, and
7. the size of the end node learning table (the table that remembers
(egress RBridge, VLAN/MAC) pairs).
Extending TRILL IS-IS to be multilevel (hierarchical) helps with some
of these issues.
IS-IS was designed to be multilevel [IS-IS] [RFC1195]. A network can
be partitioned into "areas". Routing within an area is known as
"Level 1 routing". Routing between areas is known as "Level 2
routing". The Level 2 IS-IS network consists of Level 2 routers and
links between the Level 2 routers. Level 2 routers may participate
in one or more areas, in addition to their role as Level 2 routers.
Each area is connected to Level 2 through one or more "border
routers", which participate both as a router inside the area, and as
a router inside the Level 2 "area". Care must always be taken that
it is clear, when transitioning between Level 2 and a Level 1 area in
either direction, which (single) border RBridge will transition the
frame between the levels.
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1.2 Improvements Due to Multilevel
Partitioning the network into areas reduces the size of the LSP
database in each router, and stops volatility of the topology in one
area from disrupting other areas. Allowing TRILL to utilize IS-IS's
hierarchy solves issues 1 through 4 above, but does not necessarily
help the other 3 issues (hard limit of 16-bit RBridge nicknames,
traffic due to upper layer protocols using multicast, and size of end
node learning table). However, partitioning the network into areas
facilitates a technique described in Section 4 to limit the broadcast
domain for some traffic, thus reducing problem 6 (traffic due to
upper layer protocols use of broadcast and multicast).
We propose two alternatives of hierarchical or multilevel TRILL. One
we call the "unique nickname" alternative. The other we call the
"aggregated nickname" alternative. In the aggregated nickname
alternative, border RBridges replace either the ingress or egress
nickname field in the TRILL header of unicast frames with an
aggregated nickname representing an entire area.
The aggregated nickname alternative has the following advantages:
o it solves the 16-bit RBridge nickname limit,
o it lessens the amount of inter-area routing information that must
be passed in IS-IS,
o it greatly reduces the RPF information (since only the area
nickname needs to appear, rather than all the ingress RBridges in
that area), and
o it enables computation of trees such that the portion computed
within a given area is rooted within that area.
The unique nickname alternative has the advantage that border
RBridges are simpler and do not need to do TRILL Header nickname
modification.
1.3 More on Areas
Each area is configured with an "area address", which is advertised
in IS-IS messages, so as to avoid accidentally interconnecting areas.
Note that, although the area address had other purposes in CLNP, (IS-
IS was originally designed for CLNP/DECnet), for TRILL the only
purpose of the area address would be to avoid accidentally
interconnecting areas.
Currently, the TRILL specification says that the area address must be
zero. If we change the specification so that the area address value
of zero is just a default, then most of IS-IS multilevel machinery
works as originally designed. However, there are TRILL-specific
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issues, which we address below in this document.
1.4 Terminology and Acronyms
This document uses the acronyms defined in [RFCtrill] and the
following additional acronym:
DBRB - Designated Border RBridge
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2. Multilevel TRILL Issues
The TRILL-specific issues introduced by hierarchy include the
following:
a. Configuration of non-zero area addresses, encoding them in IS-IS
PDUs, and possibly interworking with old RBridges that do not
understand nonzero area addresses.
b. Nickname management.
c. Advertisement of pruning information (VLAN reachability, IP
multicast addresses) across areas.
Distribution tree pruning information is only an optimization,
as long as multi-destination frames are not prematurely pruned.
For instance, border RBridges could advertise they can reach
all possible VLANs, and have an IP multicast router attached.
This would cause all multi-destination traffic to be
transmitted to border RBridges, and possibly pruned there, when
the traffic could have been pruned earlier based on VLAN or
multicast group if border RBridges advertised more detailed
VLAN and/or multicast listener and multicast router attachment
information.
d. Computation of trees across areas for multi-destination frames.
e. Computation of RPF information for those trees.
g. Compatibility, as much as practical, with existing, unmodified
RBridges.
The most important form of compatibility is with existing TRILL
fast path hardware. Changes that require upgrade to the slow
path firmware/software are more tolerable. Compatibility for
the relatively small number of border RBridges is less
important than compatibility for non-border RBridges.
2.1 Non-zero Area Addresses
The current TRILL base protocol specification [RFCtrill] says that
the area address in IS-IS must be zero. The purpose of the area
address is to ensure that different areas are not accidentally hooked
together. Furthermore, zero is an invalid area address for layer 3
IS-IS, so it was chosen as an additional safety mechanism to ensure
that layer 3 IS-IS would not be confused with TRILL IS-IS. However,
TRILL uses a different multicast address and an Ethertype to avoid
such confusion, so it is not necessary to worry about this.
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Since current TRILL RBridges will reject any IS-IS messages with
nonzero area addresses, the choices are as follows:
a.1 upgrade all RBridges to understand non-zero area addresses,
a.2 neighbors of old RBridges must remove the area address from IS-IS
messages when talking to an old RBridge (which might break IS-IS
security and/or cause inadvertent merging of areas),
a.3 ignore the problem of accidentally merging areas entirely, or
a.4 keep the fixed "area address" field as 0 in TRILL, and add a new,
optional TLV for "area name" that, if present, could be compared,
by new RBridges, to prevent accidental area merging.
In principal, different solutions could be used in different areas
but it would be much simpler to adopt one of these choices uniformly.
2.2 Aggregated versus Unique Nicknames
In the unique nickname alternative, all nicknames across the campus
must be unique. In the aggregated nickname alternative, RBridge
nicknames are only of local significance within an area, and the only
nickname externally (outside the area) visible is the "area
nickname", which aggregates all the internal nicknames.
The aggregated nickname approach eliminates the potential problem of
nickname exhaustion, minimizes the amount of nickname information
that would need to be forwarded between areas, minimizes the size of
the forwarding table, and simplifies RPF calculation and RPF
information.
2.2.1 More Details on Unique Nicknames
With unique cross-area nicknames, it would be intractable to have a
flat nickname space with RBridges in different areas contending for
the same nicknames. Instead, each area would need to be configured
with a block of nicknames. Either some RBridges would need to
announce that all the nicknames other than that block are taken (to
prevent the RBridges inside the area from choosing nicknames outside
the area's nickname block), or a new TLV would be needed to announce
the allowable nicknames, and all RBridges in the area would need to
understand that new TLV.
Currently the encoding of nickname information in TLVs does not allow
any aggregation. The information could be encoded as ranges of
nicknames to make this somewhat manageable; however, a new TLV for
announcing nickname ranges would not be intelligible to old RBridges.
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There is also an issue with the unique nicknames approach in building
distribution trees, as follows:
With unique nicknames in the TRILL campus and TRILL header nicknames
not rewritten by the border RBridges, there would have to be globally
known nicknames for the trees. Suppose there are k trees. For all
of the trees with nicknames located outside an area, the trees would
be rooted at a border RBridge or RBridges. Therefore, there would be
either no splitting of multi-destination traffic with the area or
restricted splitting of multi-destination traffic between trees
rooted at a highly restricted set of RBridges.
2.2.2 More Details on Aggregated Nicknames
The aggregated nickname approach enables passing far less nickname
information and works as follows:
Each area would be assigned a 16-bit nickname. This would not be the
nickname of any actual RBridge. Instead, it would be the nickname of
the area itself. Border RBridges would know the area nickname for
their own area(s).
The TRILL Header nickname fields in TRILL Data frames being
transported through a multilevel RBridge campus with aggregated
nicknames are as follows:
- When being transported in Level 2, the ingress nickname is the
nickname of the ingress RBridge's area while the egress nickname
is either the nickname of the egress RBridge's area or a tree
nickname
- When being transported in Level 1 to Level 2, the ingress
nickname is the nickname of the ingress RBridge itself while the
egress nickname is either the nickname of the area of the egress
RBridge or a tree nickname.
- When being transported from Level 2 in Level 1, the ingress
nickname is the nickname of the ingress RBridge's area while the
egress nickname is either the nickname of the egress RBridge
itself or a tree nickname.
- When both the ingress and egress RBridges are in the same area,
there need be no change from the existing base TRILL protocol
standard in the TRILL Header nickname fields.
There are two variation of the aggregated nickname approach. The
first is the Border Learning approach, which is described in Section
2.2.2.1. The second is the Swap Nickname Field approach, which is
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described in Section 2.2.2.2. Section 2.2.2.3 compares the advantages
and disadvantages of these two variations.
2.2.2.1 Border Learning Aggregated Nicknames
This section provides an illustrative example and description of the
border learning variation of aggregated nicknames.
In the following picture, R2 and R3 are area border RBridges. A
source S is attached to R1. The two areas have nicknames 15961 and
15918, respectively. R1 has a nickname, say 27, and R4 has a
nickname, say 44 (and in fact, they could even have the same
nickname, since the RBridge nickname will not be visible outside the
area).
Area 15961 level 2 Area 15918
+-------------------+ +-----------------+ +-------------+
| | | | | |
| S--R1---Rx--Rz-----R2----Rb---Rc--Rd---Re--R3---Rk--R4---D |
| 27 | | | | 44 |
| | | | | |
+-------------------+ +-----------------+ +-------------+
Let's say that S transmits a frame to destination D, which is
connected to R4, and let's say that D's location is learned by the
relevant RBridges already. The relevant RBridges have learned the
following:
1) R1 has learned that D is connected to nickname 15918
2) R3 has learned that D is attached to nickname 44.
The following sequence of events will occur:
- S transmits an Ethernet frame with source MAC = S and destination
MAC = D.
- R1 encapsulates with a TRILL header with ingress RBridge = 27, and
egress = 15918.
- R2 has announced in the Level 1 IS-IS instance in area 16961, that
it is attached to all the area nicknames, including 15918.
Therefore, IS-IS routes the frame to R2. (Alternatively, if a
distinguished range of nicknames is used for Level 2, Level 1
RBridges seeing such an egress nickname will know to route to the
nearest border router, which can be indicated by the IS-IS
attached bit.)
- R2, when transitioning the frame from Level 1 to Level 2, replaces
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the ingress RBridge nickname with the area nickname, so replaces
27 with 15961. Within Level 2, the ingress RBridge field in the
TRILL header will therefore be 15961, and the egress RBridge field
will be 15918. Also R2 learns that S is attached to nickname 27 in
area 15961 to accommodate return traffic.
- The frame is forwarded through Level 2, to R3, which has
advertised, in Level 2, reachability to the nickname 15918.
- R3, when forwarding into area 15918, replaces the egress nickname
in the TRILL header with R4's nickname (44). So, within the
destination area, the ingress nickname will be 15961 and the
egress nickname will be 44.
- R4, when decapsulating, learns that S is attached to nickname
15961, which is the area nickname of the ingress.
Now suppose that D's location has not been learned by R1 and/or R3.
What will happen, as it would in TRILL today, is that R1 will forward
the frame as a multi-destination frame, choosing a tree. As the
multi-destination frame transitions into Level 2, R2 replaces the
ingress nickname with the area nickname. If R1 does not know the
location of D, the frame must be flooded, subject to possible
pruning, in Level 2 and, subject to possible pruning, from Level 2
into every Level 1 area.
Now suppose that R1 has learned the location of D (attached to
nickname 15918), but R3 does not know where D is. In that case, R3
must turn the frame into a multi-destination frame within area 15918.
In this case, care must be taken so that, in case R3 is not the
Designated transitioner between Level 2 and its area for that multi-
destination frame, but was on the unicast path, that another border
RBridge in that area not forward the now multi-destination frame back
into Level 2. Therefore, it would be desirable to have a marking,
somehow, that indicates the scope of this frame's distribution to be
"only this area" (see also Section 4).
The issue described at the end of Section 2.2.1 with trees in the
unique nickname alternative is eliminated with aggregated nicknames.
With aggregated nicknames, each border RBridge that will transition
multi-destination frames can have a mapping between Level 2 tree
nicknames and Level 1 tree nicknames. There need not even be
agreement about the total number of trees; just that the border
RBridge have some mapping, and replace the egress RBridge nickname
(the tree name) when transitioning levels.
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2.2.2.2 Swap Nickname Field Aggregated Nicknames
As a variant, two additional fields could exist in TRILL Data frames
we call the "ingress swap nickname field" and the "egress swap
nickname field". The changes in the example above would be as
follows:
- R1 will have learned the area nickname of D and the RBridge
nickname of R4 to which D is attached. In encapsulating a frame to
D, it puts the area nickname of D (15918) in the egress nickname
field of the TRILL Header and puts the nickname of R3 (44) in a
egress swap nickname field.
- R2 moves the ingress nickname to the ingress swap nickname field
and inserts 15961, the area nickname for S, into the ingress
nickname field.
- R3 swaps the egress nickname and the egress swap nickname fields,
which sets the egress nickname to 44.
- R4 learns the correspondence between the source MAC/VLAN of S and
the { ingress nickname, ingress swap nickname field } pair as it
decapsulates and egresses the frame.
2.2.2.3 Comparison
The Border Learning variant described in Section 2.2.2.1 above
minimizes the change in non-border RBridges but imposes the burden on
border RBridges of learning and doing lookups in all the end station
MAC addresses within their area(s) that are used for communication
outside the area. The burden could be somewhat reduced by decreasing
the area size and increasing the number of areas.
The Swap Nickname Field variant described in Section 2.2.2.2
eliminates the extra address learning burden on border RBridges but
requires more extensive changes to non-broader RBridges. In
particular they must learn to associate both an RBridge nickname and
an area nickname with end station MAC/VLAN pairs (except for
addresses that are local to their area).
The Swap Nickname Field alternative is more scalable but less
backward compatible for non-broder RBridges.
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2.3 Building Multi-Area Trees
It is easy to build a multi-area tree by building a tree in each area
separately, (including the Level 2 "area"), and then having only a
single border RBridge, say R1, in each area, attach to the Level 2
area. R1 would forward all multi-destination frames between that
area and Level 2.
People might find this unacceptable, however, because of the desire
to path split (not always sending all multi-destination traffic
through the same border RBridge).
Having multiple border RBridges introduces some complexities:
a) calculating the RPF check when a multi-destination frame
originates outside the area (which border RBridge injected the
frame into the area?)
b) calculating the filtering information (which border RBridge will
transition the frame into Level 2?)
This might be solvable if all RBridges are multilevel aware, however
it is difficult to imagine how to ensure that old RBridges would
calculate RPF and filtering information sensibly.
Ignoring old RBridges for now, various possible solutions are as
follows:
a) elect one border RBridge for transitioning all multi-destination
frames between levels (call that the Designated Border RBridge
(DBRB))
b) allow the DBRB to appoint other border RBridges to forward some
subset of the inter-level frames. (as the DRB does, on a per-VLAN
basis, on a link). Make the appointment information visible to
the other RBridges in the area so that they can calculate their
RPF and filtering information.
If b), then on what basis would the appointment be made? Various
possibilities are as follows:
o based on VLAN
o based on tree root
o based on ingress RBridge nickname
The more flexibility that is allowed, the more complex the
announcement of information becomes, and the more complex the tree
database becomes. If appointment is made based on VLAN, then the RPF
check would need to be based on (tree, VLAN, ingress nickname),
rather than simply (tree, ingress nickname) as it is today.
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2.4 The RPF Check for Trees
For multi-destination frames originating locally in R1's area,
computation of the RPF check is done as today. For multi-destination
frames originating outside R1's area, computation of the RPF check
must be done based on which one of the border RBridges (say R1, R2,
or R3) injected the frame into the area.
An RBridge, say R4, located inside an area, must be able to know
which of R1, R2, or R3 transitioned the frame into the area from
Level 2. (or into Level 2 from an area).
This could be done based on having the DBRB announce the transitioner
assignments to all the RBridges in the area.
2.5 Area Nickname Acquisition
In the aggregated nickname alternative, each area must acquire a
unique area nickname. It is probably simpler to allocate a block of
nicknames (say, the top 4000) to be area addresses, and not used by
any RBridges.
The area nicknames need to be advertised and acquired through Level
2.
Within an area, all the border RBridges must discover each other
through the Level 1 link state database, by advertising, in their LSP
"I am a border RBridge".
Of the border RBridges, one will have highest priority (say R7). R7
can dynamically participates, in Level 2, to acquire a nickname for
the area. R7 could give the area a pseudonode name, such as R7.5,
within Level 2. So an area would appear, in Level 2, as a pseudonode
and the pseudonode can participate, in Level 2, to acquire a nickname
for the area.
Within Level 2, all the border RBridges [for an area] can advertise
reachability to the pseudonode, which would mean connectivity to the
area nickname.
2.6 Link State Representation of Areas
Within an area, say area A, there is an election for the DBRB,
(Designated Border RBridge), say R1. This can be done through LSPs
within area A. The border RBridges announce themselves, together
with their DBRB priority. (Note that the election of the DBRB cannot
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be done based on Hello messages, because the border RBridges are not
necessarily physical neighbors of each other. They can, however,
reach each other through connectivity within the area, which is why
it will work to find each other through Level 1 LSPs.)
R1 acquires the area nickname (in the aggregated nickname approach),
gives the area a pseudonode name (just like the DRB would give a
pseudonode name to a link). R1 advertises, in area A, what the
pseudonode name for the area is (and the area nickname that R1 has
acquired).
The pseudonode LSP initiated by R1 includes any information
extraneous to area A that should be input into area A (such as area
nicknames of external areas, or perhaps (in the unique nickname
variant), all the nicknames of external RBridges in the TRILL campus
and filtering information such as IP multicast groups and VLANs).
All the other border RBridges for the area announce (in their LSP)
attachment to that pseudonode.
Within Level 2, R1 generates a Level 2 LSP on behalf of the area,
also represented as a pseudonode. The same pseudonode name could be
used within Level 1 and Level 2, for the area. (There does not seem
any reason why it would be useful for it to be different, but there's
also no reason why it would need to be the same). Likewise, all the
area A border RBridges would announce, in their Level 2 LSPs,
connection to the pseudonode.
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3. Area Partition
It is possible for an area to become partitioned, so that there is
still a path from one section of the area to the other, but that path
is via the Level 2 area.
An area will naturally break into two areas in this case.
An area address might be configured to ensure two areas are not
inadvertently connected. That area address appears in Hellos and
LSPs within the area. If two chunks, connected only via Level 2,
were configured with the same area address, this would not cause any
problems. (They would just operate as separate Level 1 areas.)
A more serious problem occurs if the Level 2 area is partitioned in
such a way that it could be healed by using a path through a Level 1
area. TRILL will not attempt to solve this problem. Within the
Level 1 area, a single border RBridge will be the DBRB, and will be
in charge of deciding which (single) RBridge will transition any
particular multi-destination frames between that area and Level 2.
If the Level 2 area is partitioned, this will result in multi-
destination frames only reaching the portion of the TRILL campus
reachable through the partition attached to the RBridge that
transitions that frame. It will not cause a loop.
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4. Multi-Destination Scope
There are at least two reasons it would be desirable to be able to
mark a multi-destination frame with a scope that indicates this frame
should not exit the area, as follows:
1. To address an issue in the border learning variant of the
aggregated nickname alternative, when a unicast frame turns into a
multi-destination frame when transitioning from Level 1 to Level
1, as discussed in Section 4.1.
2. To constrain the broadcast domain for certain discovery,
directory, or service protocols as discussed in Section 4.2.
Multi-destination frame distribution scope restriction could be done
in a number of ways. For example, their could be a flag in the frame
that means "for this area only". However, the technique that might
require the least change to RBridge fast path logic would be to
indicate this in the egress nickname that designates the distribution
tree being used. There could be two general tree nicknames for each
tree, one being for distribution restricted to the area and the other
being for multi-area trees. Or, alternatively, there would be a set
of N (perhaps 16) special currently reserved nicknames used to
specify the N highest priority trees but with the variation that if
the special nickname is used, the frame is not transitioned between
areas.
4.1 Unicast to Multi-destination Conversions
In the border learning variant of the aggregated nickname
alternative, a unicast frame might be known at the Level 1 to Level 2
transition, be forwarded as a unicast frame to the least cost border
RBridge advertising connectivity to the destination area, but turn
out to have an unknown destination MAC/VLAN pair when it arrives at
that border RBridge.
In this case, the frame must be converted into a multi-destination
frame and flooded in the destination area. However, if the border
RBridge doing the conversion is not the border RBridge designated to
transition the resulting multi-destination frame, there is the danger
that the designated transitioner may pick up the frame and flood it
back into Level 2 from which it may be flooded into multiple areas.
This danger can be avoided by putting any multi-destination frame
that results from such a conversion on a distribution tree that is
restricted to the area.
Alternatively, a multi-destination frame intended only for the area
could be tunneled (within the area) to the RBridge Rx, that is the
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appointed transitioner for that form of frame (say, based on VLAN),
with instructions that Rx only transmit the frame within the area,
and Rx could initiate the multi-destination frame within the area.
Since Rx introduced the frame, and is the only one allowed to
transition that frame to Level 2, this would accomplish scoping of
the frame to within the area. Since this case only occurs when
unicast frames need to be turned into multi-destination as described
above, the suboptimality of tunneling between the border RBridge that
receives the unicast frame and the appointed level transitioner for
that frame, would not be an issue.
4.2 Selective Broadcast Domain Reduction
There are a number of service, discovery, and directory protocols
that, for convenience, are accessed via multicast or broadcast
frames. Examples are DHCP and the NetBIOS Service Location Protocol.
Some such protocols provide some means to restrict distribution to an
IP subnet or equivalent to reduce size of the broadcast domain they
are using and then provide a proxy that can be placed in that subnet
to use unicast to access a service elsewhere. In cases where a proxy
mechanism is not currently defined, it may be possible to create one
that references a central server or cache. With multilevel TRILL, it
is possible to construct very large IP subnets which could become
saturated with multi-destination traffic of this type unless frames
can be further restricted in their distribution. Such restricted
distribution can be accomplished for some protocols, say protocol P,
as follows:
- Either (1) at all ingress RBridges in an area place all protocol P
multi-destination frames on a distribution tree restricted to the
area or (2) at all border RBridges between that area and Level 2,
detect protocol P multi-destination frames and do not transition
them.
- Place one (or more for back-up) protocol P proxies inside each
area. These proxies can than unicast protocol P requests or other
messages to the actual campus servers for P and receive responses
or other messages from those servers and deliver them within the
area via unicast, multicast, or broadcast as appropriate. Such
proxies would not be needed if it was acceptable for all protocol
P traffic to be restricted to an area.
While it might seem logical to connect the campus servers to RBridges
in Level 2, they could be placed within one or more areas so that, in
some cases, those areas might not require a local proxy server.
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INTERNET-DRAFT RBridges: Multilevel TRILL
5. Co-Existence with Old RBridges
RBridges that are not multilevel aware may have a problem with
calculating RPF check and filtering information, since they would not
be aware of assignment of border RBridge transitioning.
A possible solution, as long as any old RBridges exist within an
area, is to have the border RBridges elect a single DBRB (Designated
Border RBridge), and have all inter-area traffic go through the DBRB
(unicast as well as multi-destination). If that DBRB goes down, a
new one will be elected, but at any one time, all inter-area traffic
(unicast as well as multi-destination) would go through that one
DRBR. However this eliminates load splitting at level transition.
R. Perlman, et al [Page 18]
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6. Summary
This draft outlines the issues and possible approaches to multilevel
TRILL. The alternative involving area nicknames for aggregation has
significant advantages in terms of scalability over using campus wide
unique nicknames; not just of avoiding nickname exhaustion, but
allowing, for instance, RPF checks to be aggregated based on an
entire area.
Some issues are not difficult, such as dealing with partitioned
areas. Some issues are more difficult, especially dealing with old
RBridges.
7. Security Considerations
TBD
8. IANA Considerations
This document requires no IANA actions. RFC Editor: Please delete
this section before publication.
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9. References
Normative and Informational references for this document are listed
below.
9.1 Normative References
[IS-IS] - ISO/IEC 10589:2002, Second Edition, "Intermediate System to
Intermediate System Intra-Domain Routing Exchange Protocol for
use in Conjunction with the Protocol for Providing the
Connectionless-mode Network Service (ISO 8473)", 2002.
[RFC1195] - Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFCtrill] - Perlman, R., D. Eastlake, D. Dutt, S. Gai, and A.
Ghanwani, "RBridges: Base Protocol Specification", draft-ietf-
trill-rbridge-protocol-16.txt, in RFC Editor's queue.
9.2 Informative References
None.
R. Perlman, et al [Page 20]
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Authors' Addresses
Radia Perlman
Intel Labs
2200 Mission College Blvd.
Santa Clara, CA 95054-1549 USA
Phone: +1-408-765-8080
Email: Radia@alum.mit.edu
Donald Eastlake
Huawei Technologies
155 Beaver Street
Milford, MA 01757 USA
Phone: +1-508-333-2270
Email: d3e3e3@gmail.com
Anoop Ghanwani
Brocade Communications Systems
130 Holger Way
San Jose, CA 95134 USA
Phone: +1-408-333-7149
Email: anoop@brocade.com
R. Perlman, et al [Page 21]
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R. Perlman, et al [Page 22]
