One document matched: draft-ietf-idr-dpa-application-02.txt
Differences from draft-ietf-idr-dpa-application-01.txt
INTERNET-DRAFT Enke Chen
<draft-ietf-idr-dpa-application-02.txt> Tony Bates
Expires in six months MCI
January 1996
Application of the BGP Destination Preference Attribute
in Implementing Symmetric Routing
<draft-ietf-idr-dpa-application-02.txt>
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
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Internet Drafts are draft documents valid for a maximum of six
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Abstract
This paper presents applications of the proposed Destination
Preference Attribute (DPA) for BGP. It shows how the DPA attribute
can aid in the implementation of symmetric inter-domain routing in
the multi-provider Internet.
1. Introduction
The Destination Preference Attribute (DPA) is proposed in [4] for
BGP. This attribute can be used by an autonomous system (AS) to
specify a globally transitive preference in its routing announcement
via BGP so that the upstream BGP speakers can use the preference to
favor certain path for return traffic. This paper presents a typical
application of this attribute. It illustrates how the DPA attribute
facilitates the implementation of symmetric inter-domain routing and
load-sharing for the the typical cases presented in [3].
Chen & Bates [Page 1]
INTERNET-DRAFT Application of DPA January 1996
This paper assumes that in general an ISP treats other ISPs equally
(in terms of the "LOCAL_PREF" parameter) in the route selection
process. It also assumes the following order of preference is
followed for the purpose of route selection: first the "LOCAL_PREF"
parameter, followed by the DPA attribute, the shortest AS-path, the
MED and the IGP metric.
2. Application of the DPA Attribute
In [3] we present several typical topologies of Internet connections,
their inter-domain routing requirements, and the current practice to
implement these routing policies. This section illustrates how the
DPA attribute can be used to facilitate the implementation.
2.1 An Entity with a Single Direct Provider
+-----+ +-----+ +-----+
| ISP | | ISP | | ISP |
+-----+ +-----+ +-----+
| | |
+-----+ +-----+ +-----+
| AS1 | | RSP | | RSP |
+-----+ +-----+ +-----+
|
+-----+
| AS1 |
+-----+
(a) (b) (c)
Figure 1
The routing is always symmetric at the inter-domain level. The DPA
attribute should not be set as it is not needed.
AS1 can either take full routing or use default.
Chen & Bates [Page 2]
INTERNET-DRAFT Application of DPA January 1996
2.2 Backup of Entities with Different Direct Providers
+-----+ +------+ +------+
| ISP | | ISP3 | | ISP3 |
+-----+ +------+ +------+
/ \ / \ / \
/ \ / (NAP) \ / (NAP) \
+-----+ +-----+ +------+ +------+ +------+ +------+
| AS1 |----| AS2 | | ISP1 |------| ISP2 | | ISP1 |-------| ISP2 |
+-----+ +-----+ +------+ +------+ +------+ +------+
| | | |
+------+ +------+ +------+ +------+
| AS1 |------| AS2 | | RSP1 | | RSP2 |
+------+ +------+ +------+ +------+
| |
+------+ +------+
| AS1 |------| AS2 |
+------+ +------+
(a) (b) (c)
Figure 2
In all cases of Figure 2, in order to provide for backup, AS1 shall
permit the acceptance of AS2's routes from both AS2 and AS1's direct
providers, and permits their announcement to its direct providers.
Similar configuration for AS2. As presented in [3], the "LOCAL_PREF"
configuration is sufficient to implement the routing requirements.
It is not necessary to use the DPA attribute.
However, the DPA attribute would simplify the implementation as shown
in the following.
Policy 1: Used solely as a backup link.
AS1 can simply announce all its routes with a higher DPA value to
its direct provider, and with a lower DPA value to AS2. AS1 can
either carry full routing or only take partial routing (AS2's
routes) from both its direct provider and AS2, and configure
default routes. Similar configuration for AS2.
Policy 2: Used for traffic between AS1 and AS2, and as backup in
general
As with Policy 1, AS1 can simply announce all its routes with a
higher DPA value to its direct provider, and with a lower DPA
Chen & Bates [Page 3]
INTERNET-DRAFT Application of DPA January 1996
value to AS2. AS1 also needs to configure proper "LOCAL_PREF"
value for AS2's routes. This is to override the higher DPA value
for AS2's routes received from the direct provider. AS1 can
either carry full routing or only take partial routing (AS2's
routes) from both its direct provider and AS2, and configure
default routes. Similar configuration for AS2.
2.3 An Entity with Multiple Direct Providers
This is where the DPA attribute would be most useful. As shown in
Figure 3, AS1 has two direct providers. X and Y are routes of AS1.
Note that AS1 could be an RSP.
+------+ +-----+ +------+
| ISP3 | | ISP | | ISP3 |
+------+ +-----+ +------+
/ \ / \ / \
/ (NAP) \ / \ / (NAP)\
+------+ +------+ +------+ +------+ +------+ +-----+
| ISP1 |------| ISP2 | | RSP1 | | RSP2 | | ISP1 |-----| ISP2|
+------+ +------+ +------+ +------+ +------+ +-----+
\ / \ / | |
\L1 /L2 \ / | |
+------+ +------+ +------+ +-----+
| AS1 | | AS1 | | RSP1 | | RSP2|
|X Y| |X Y| +------+ +-----+
+------+ +------+ \ /
\ /
+------+
| AS1 |
|X Y|
+------+
(a) (b) (c)
Figure 3
Policy 1: One link is used as primary, the other as pure backup
AS discussed in [3], the routing policy can be implemented by
coordinating the "LOCAL_PREF" parameter with direct providers. It
is not necessary to use the DPA attribute.
However, the DPA attribute would simplify the implementation as
detailed in the following.
Chen & Bates [Page 4]
INTERNET-DRAFT Application of DPA January 1996
AS1 can simply announce all its routes with a higher DPA value to
the primary provider, and with a lower DPA value to the other
provider.
AS1 can either carry full routing or use default. If AS1 takes
full routing, then AS1 also need to configure "LOCAL_PREF" so that
the primary path is preferred.
Policy 2: Each link is used for traffic with the respective direct
provider. In general one link is used as primary, the other as
backup.
As with Policy 1, AS1 can simply announce all its routes with a
higher DPA value to the primary provider, and with a lower DPA
value to the other provider.
AS1 can be configured:
o either with partial routing (only routes of the direct
providers and their customers) and configure default routes
with different weights.
o or with full routing and configure "LOCAL_PREF" values. The
AS-list would still need to be updated and maintained, as
discussed in [3].
Policy 3: Partial load-sharing among these links
That is, the direct link is used for traffic between AS1 and its
direct providers including its customers. However, the closest
exit point would be taken for traffic beyond these direct
providers and their customers.
AS1 shall categorize its networks into two categories (say X and
Y). Then, X routes shall be configured with higher DPA value when
they are being announced to one direct provider, and with lower
DPA values when they are being announced to the other direct
provider. Similar configuration for Y routes.
In addition, AS1 also needs to configure "LOCAL_PREF" so that the
direct link is taken between the AS and its direct providers.
AS1 can take full routing. It can also take partial routing
(routes of direct providers and their customers), and configure
equal-weight default routes at its border routers and propagate
them into its AS.
Chen & Bates [Page 5]
INTERNET-DRAFT Application of DPA January 1996
Policy 4: Complete load-sharing among these links
That is, each network in AS1 sends packets to the closer (in terms
of internal route preference) border router that peers with a
direct provider. The return traffic is expected to take a symmet-
ric path.
AS1 shall categorize its networks into two categories (say X and
Y). Then, X routes shall be configured with higher DPA value
when they are being announced to one direct provider, and with
lower DPA values when they are being announced to the other direct
provider. Similar configuration for Y routes.
It is much simpler if AS1 does not take full routing. Then AS1
can configure default routes at its border routers and propagate
them into its AS (via iBGP).
If AS1 still prefers to take full routing, this policy can only be
achieved if AS1 manipulates the AS-path length so that routes
received from the direct providers would have equal AS-path
length. However, the routes with their AS paths manipulated would
not be propagated upstream. That is, the propagation of the
superfluous information in the AS path would be limited to the AS
and possibly its downstream ASs, rather than the whole Internet.
3. Configure Preference for Routes with the DPA Attribute
It is possible, although not common, that the DPA attribute has been
set by one AS (say AS1), and another AS (say AS2) desires further
preference between its direct providers. The following options are
available for AS2:
(1) AS2 uses the DPA attributes to do load sharing for routes other
than AS1's. That is, AS2 does not include AS1's routes in load shar-
ing with respect to AS2's direct providers. Instead, AS2 can coordi-
nate with its direct providers to configure the proper "LOCAL_PREF"
values so that one provider is used as the primary, the other as the
backup, for all of AS1's routes. This is to make sure that routing
symmetry is maintained for routing to AS1. If there are multiple ASs
that have configured the DPA attributes, then AS2 can perform load
sharing by distribute (on per-AS basis) routes evenly with respect to
its direct providers.
(2) AS1 chooses to re-set the DPA attribute for route announcements
including AS1's routes. This may well cause the DPA attributes set
by AS1 not to be used by upstream BGP speakers (due to non-comparable
DPA attributes).
Chen & Bates [Page 6]
INTERNET-DRAFT Application of DPA January 1996
In many cases, Option (1) is probably preferred. However, Option (1)
may not be able to maintain routing symmetry, either. It should be
emphasized that when dealing with the complicated topologies of
Internet connections, one needs to take into account its internal
network topology, its connection to direct providers and to the major
interconnection points. Coordination between providers is strongly
recommended.
The following example illustrates Option (1). In Figure 4, AS1 has
two direct providers RSP1 and RSP2. AS1 does load sharing by setting
DPA attributes for routes W and Z. RSP1 has direct providers ISP1
and ISP2, and wishes to do load sharing.
+------+
| ISP3 |
+------+
/ \
/ (NAP) \
+------+ +------+
| ISP1 |-----| ISP2 |
+------+ +------+
| / |
| / |
+------+ +------+
| RSP1 | | RSP2 |
|X Y| | |
+------+ +------+
\ /
\ /
+------+
| AS1 |
|W Z|
+------+
Figure 4
In this example, RSP1 can use the DPA attributes to do load sharing
for routes without the DPA attributes. For AS1's routes (such as W
and Z) that are already configured with the DPA attribute, RSP1 can
coordinate, with ISP1 or ISP2, to configure the proper "LOCAL_PREF"
value so that one acts as primary to reach routes of AS1. For
instance, ISP1 configures lower "LOCAL_PREF" value for all of AS1's
routes so that the ISP1 - ISP2 link is preferred to reach AS1's
routes. This would also ensure that ISP3 would use ISP2 to reach
AS1's routes.
Chen & Bates [Page 7]
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4. Security Considerations
Security considerations are not discussed in this memo.
5. Acknowledgments
The authors would like to thank Yakov Rekhter of Cisco for his help-
ful comments and suggestions.
6. References
[1] Rekhter, Y., and Li, T., "A Border Gateway Protocol 4 (BGP-4)",
RFC1771, March 1995.
[2] Y. Rekhter, and P. Gross, "Application of the Border Gateway Pro-
tocol in the Internet", RFC1772, March 1995.
[3] Chen, E., and Bates, T., "Current Practice of Implementing Sym-
metric Routing and Load Sharing in the Multi-Provider Internet",
INTERNET-DRAFT, <draft-ietf-idr-symm-multi-prov-02.txt>, January
1996.
[4] Chen, E., and Bates, T., "Destination Preference Attribute for
BGP", INTERNET-DRAFT, <draft-ietf-idr-bgp-dpa-04.txt>, January 1996.
6. Author's Addresses
Enke Chen
MCI
2100 Reston Parkway
Reston, VA 22091
phone: +1 703 715 7087
email: enke@mci.net
Tony Bates
MCI
2100 Reston Parkway
Reston, VA 22091
phone: +1 703 715 7521
email: Tony.Bates@mci.net
Chen & Bates [Page 8]
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