📄 rfc3345.txt
字号:
| | Sub-AS 65000 | | Sub-AS 65001 | |
| | | | | |
| | | *1 | | |
| | Ra . . . . . . . . . . . . . . . . . Rd | |
| | . . | | . | |
| | .*3 .*2 | | .*6 | |
| | . . | | . | |
| | Rb . . . . . Rc | | Re | |
| | . *5 . | | . | |
| \ . . / \ . / |
| ---.------------.--- ---------.---------- |
\ .(10) .(1) AS1 .(0) /
-------.------------.---------------------------.--------------
. . .
------ . ------------ .
/ \ . / \ .
| AS10 | | AS6 |
\ / \ /
------ ------------
. .
. .
. --------------
. / \
| AS100 |- 10.0.0.0/8
\ /
--------------
Figure 2: Example AS Confederations Topology
The number contained in parentheses on each AS1 EBGP peering session
represents the MED value advertised by the peer to be associated with
the 10.0.0.0/8 network reachability advertisement.
McPherson, et al. Informational [Page 7]
RFC 3345 BGP Persistent Route Oscillation Condition August 2002
The number following each '*' on the IBGP peering sessions represents
the additive IGP metrics that are to be associated with the BGP
NEXT_HOP attribute for the concerned route.
For example, the Ra IGP metric value associated with a NEXT_HOP
learned via Rb would be 3; while the metric value associated with the
NEXT_HOP learned via Re would be 6.
Table 2 depicts the 10.0.0.0/8 route attributes as seen by routers
Rb, Rc and Re, respectively. Note that the IGP metrics in Figure 2
are only of concern when advertising the route to an IBGP peer.
Router MED AS_PATH
--------------------
Rb 10 10 100
Rc 1 6 100
Re 0 6 100
Table 2: Route Attribute Table
For the following steps 1 through 6 the best route will be marked
with an '*'.
1) Ra has the following BGP table:
NEXT_HOP
AS_PATH MED IGP Cost
-------------------------------
* 10 100 10 3
(65001) 6 100 0 7
6 100 1 2
The '10 100' route is selected as best and is advertised to Rd,
though this is not the cause of the persistent route
oscillation.
2) Rd has the following in its BGP table:
NEXT_HOP
AS_PATH MED IGP Cost
-------------------------------
6 100 0 6
* (65000) 10 100 10 4
The '(65000) 10 100' route is selected as best because it has
the lowest IGP metric. As a result, Rd sends an
UPDATE/withdraw to Ra for the '6 100' route that it had
previously advertised.
McPherson, et al. Informational [Page 8]
RFC 3345 BGP Persistent Route Oscillation Condition August 2002
3) Ra receives the withdraw from Rd. Ra now has the following in
its BGP table:
NEXT_HOP
AS_PATH MED IGP Cost
-------------------------------
* 10 100 10 3
6 100 1 2
Ra received a withdraw for '(65001) 6 100', which changes what
is considered the best route for Ra. Ra does not compute the
best path for a prefix unless its best route was withdrawn.
This is why Ra has the '10 100, 10, 3' route selected as best,
even though the '6 100, 1, 2' route is better.
4) Ra's periodic BGP scanner runs and realizes that the '6 100'
route is better because of the lower IGP metric. Ra sends an
UPDATE/withdraw to Rd for the '10 100' route since Ra is now
using the '6 100' path as its best route.
Ra's BGP table looks like this:
NEXT_HOP
AS_PATH MED IGP Cost
-------------------------------
10 100 10 3
* 6 100 1 2
5) Rd receives the UPDATE from Ra and now has the following in its
BGP table:
NEXT_HOP
AS_PATH MED IGP Cost
-------------------------------
(65000) 6 100 1 3
* 6 100 0 6
Rd selects the '6 100, 0, 6' route as best because of the lower
MED value. Rd sends an UPDATE message to Ra, reporting that '6
100, 0, 6' is now the best route.
McPherson, et al. Informational [Page 9]
RFC 3345 BGP Persistent Route Oscillation Condition August 2002
6) Ra receives the UPDATE from Rd. Ra now has the following in
its BGP table:
NEXT_HOP
AS_PATH MED IGP Cost
-------------------------------
* 10 100 10 3
(65001) 6 100 0 7
6 100 1 2
At this point we have made a full cycle and are back to step 1.
This is an example of Type I Churn with AS Confederations.
2.3. Potential Workarounds for Type I Churn
There are a number of alternatives that can be employed to avoid this
problem:
1) When using Route Reflection make sure that the inter-Cluster
links have a higher IGP metric than the intra-Cluster links.
This is the preferred choice when using Route Reflection. Had
the inter-Cluster IGP metrics been much larger than the intra-
Cluster IGP metrics, the above would not have occurred.
2) When using AS Confederations ensure that the inter-Sub-AS links
have a higher IGP metric than the intra-Sub-AS links. This is
the preferred option when using AS Confederations. Had the
inter-Sub-AS IGP metrics been much larger than the intra-Sub-AS
IGP metrics, the above would not have occurred.
3) Do not accept MEDs from peers (this may not be a feasible
alternative).
4) Utilize other BGP attributes higher in the decision process so
that the BGP decision algorithm never reaches the MED step. As
using this completely overrides MEDs, Option 3 may make more
sense.
5) Always compare BGP MEDs, regardless of whether or not they were
obtained from a single AS. This is probably a bad idea since
MEDs may be derived in a number of ways, and are typically done
so as a matter of operator-specific policy. As such, comparing
MED values for a single prefix learned from multiple ASs is
ill-advised. Of course, this mostly defeats the purpose of
MEDs, and as such, Option 3 may be a more viable alternative.
6) Use a full IBGP mesh. This is not a feasible solution for ASs
with a large number of BGP speakers.
McPherson, et al. Informational [Page 10]
RFC 3345 BGP Persistent Route Oscillation Condition August 2002
3. Discussion of Type II Churn
In the following subsection we provide configurations under which
Type II Churn will occur when using AS Confederations. For the sake
of brevity, we avoid similar discussion of the occurrence when using
Route Reflection.
In general, Type II churn occurs only when BOTH of the following
conditions are met:
1) More than one tier of Route Reflection or Sub-ASs is used in
the network AND
2) the network accepts the BGP MULTI_EXIT_DISC (MED) attribute
from two or more ASs for a single prefix and the MED values are
unique.
McPherson, et al. Informational [Page 11]
RFC 3345 BGP Persistent Route Oscillation Condition August 2002
3.1. AS Confederations and Type II Churn
Let's now examine the occurrence of Type II Churn as it relates to AS
Confederations. Figure 3 provides our sample topology:
---------------------------------------------------------------
/ ------------------- \
| AS 1 / Sub-AS 65500 \ |
| | | |
| | Rc . . . . Rd | |
| | . *2 . | |
| \ . . / |
| .-----------------. |
| .*40 .*40 |
| --------------.----- --.----------------- |
| / . \ / . \ |
| | Sub-AS . | | . Sub-AS | |
| | 65501 . | | . 65502 | |
| | Rb | | Re | |
| | . | | . . | |
| | .*10 | | *2. .*3 | |
| | . | | . . | |
| | Ra | | . Rg . . . Rf | |
| \ . / . . / |
| ----------.---------- . -------------.------- |
\ .(0) .(1) .() /
----------------.---------------.-------------------.----------
. . .
--------- . ---------
|AS 200 | |AS 300 |
--------- ---------
. .
. .
-------------------
| AS 400 | - 10.0.0.0/8
-------------------
Figure 3: Example AS Confederations Topology
In Figure 3 AS 1 contains three Sub-ASs, 65500, 65501 and 65502. No
RR is used within the Sub-AS, and as such, all routers within each
Sub-AS are fully meshed. Ra and Rb are members of Sub-AS 65501. Rc
and Rd are members of Sub-AS 65500. Ra and Rg are EBGP peering with
AS 200, router Rf has an EBGP peering with AS 300. AS 200 and AS 300
provide transit for AS 400, and in particular, the 10/8 network. The
dotted lines are used to represent BGP peering sessions.
McPherson, et al. Informational [Page 12]
RFC 3345 BGP Persistent Route Oscillation Condition August 2002
The number following each '*' on the BGP peering sessions represents
the additive IGP metrics that are to be associated with the BGP
NEXT_HOP. The number contained in parentheses on each AS 1 EBGP
peering session represents the MED value advertised by the peer to be
associated with the network reachability advertisement (10.0.0.0/8).
Rc, Rd and Re are the primary routers involved in the churn, and as
such, will be the only BGP tables that we will monitor step by step.
For the following steps 1 through 8 each router's best route will be
marked with a '*'.
1) Re receives the AS 400 10.0.0.0/8 route advertisement via AS
200 from Rg and AS 300 from Rf. Re selects the path via Rg and
AS 200 because of IGP metric (Re didn't consider MED because
the advertisements were received from different ASs).
NEXT_HOP
Router AS_PATH MED IGP Cost
------------------------------
Re * 200 400 1 2
300 400 3
Re sends an UPDATE message to Rd advertising its new best path
'200 400, 1'.
2) The '200 400, 0' path was advertised from Ra to Rb, and then
from Rb to Rc. Rd learns the '200 400, 1' path from Re.
NEXT_HOP
Router AS_PATH MED IGP Cost
-------------------------------
Rc * 200 400 0 50
Rd * 200 400 1 42
⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -