Network Working Group D. Haskin Request For Comments: 1863 Bay Networks, Inc. Category: Experimental October 1995 A BGP/IDRP Route Server alternative to a full mesh routing
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Abstract
This document describes the u and detailed design of Route Servers for dismination of routing information among BGP/IDRP speaking
whatever的用法
routers.
The intention of the propod technique is to reduce overhead and
management complexity of maintaining numerous direct BGP/IDRP
ssions which otherwi might be required or desired among routers
within a single routing domain as well as among routers in different domains that are connected to a common switched fabric (e.g. an ATM
cloud).
1. Overview
Current deployments of Exterior Routing protocols, such as the Border Gateway Protocol [BGP4] and the adaptation of the ISO Inter-Domain
Routing Protocol [IDRP], require that all BGP/IDRP routers, which
participate in inter-domain routing (border routers) and belong to
the same routing domain, establish a full mesh connectivity with each other for purpo of exchang
ing routing information acquired from
other routing domains. In large routing domains the number of intra- domain connections that needs to be maintained by each border route
can be significant.
In addition, it may be desired for a border router to establish
routing ssions with all border routers in other domains which are
reachable via a shared communication media. We refer to routers that are directly reachable via a shared media as adjacent routers. Such direct peering allows a router to acquire "first hand" information
about destinations which are directly reachable through adjacent
routers and lect the optimum direct paths to the destinations.
Establishment of BGP/IDRP ssions among all adjacent border routers would result in a full mesh routing connectivity. Unfortunately for Haskin Experimental [Page 1]
a switched media as ATM, SMDS or Frame Relay network which may
inter-connect a large number of routers, due to the number of
connections that would be needed to maintain a full mesh direct
peering between the routers, makes this approach impractical.
In order to alleviate the "full mesh" problem, this paper propos to u IDRP/BGP Route Servers which would relay external routes with all of their attributes between client routers. The clients would
maintain IDRP/BGP ssions only with the assigned route rvers
(ssions with more than one rver would be needed if redundancy is desired). All routes that are received from a client router would be propagated to other clients by the Route Server. Since all external routes and their attributes are relayed unmodified between the client routers, the client routers would acquire the same routing
information as they would via direct peering. We refer to such
arrangement as virtual peering. Virtual peering allows client
routers independently apply lection criteria to the acquired
external routes according to their local policies as they would if a direct peering were established.
The routing approach described in this paper assumes that border
routers posss a mechanism to resolve the media access address of
the next hop router for any route acquired from a virtual peer.
It is fair to note that the approach prented in this paper only
reduces the number of routing connection each border router needs to maintain. It does not reduce the volume of routing information that
needs to maintained at each border router.
Besides addressing the "full mesh" problems, the proposal attempts
to achieve the following goals:
- to minimize BGP/IDRP changes that need to be implemented in client routers in order to inter-operate with route rvers;
- to provide for redundancy of distribution of routing information to route rver clients;
- to minimize the amount of routing updates that have to be nt to
route rver clients;
- to provide load distribution between route rvers;
- to avoid an excessive complexity of the interactions between Route Servers themlves.
Haskin Experimental [Page 2]
2. Terms And Acronyms
The following terms and acronyms are ud in this paper:
Routing Domain - a collection of routers with the same t of
routing policies. For IPv4 it can be identified with an Autonomous System Number, for IPv6
it can be identified with a Routing Domain
Identifier.
Border Router (BR) - a router that acquires external routes, i.e.
routes to internet points outside its routing
domain.
Route Server (RS) - a process that collects routing information
from border routers and distributes this
information to ’client routers’.
RS Client (RC) - a router than peers with an RS in order to
acquire routing information. A rver’s client can be a router or another route rver.
RS Cluster (RSC) - two or more of route rvers that share the same subt of clients. A RS Cluster provides
redundancy of routing information to its
clients, i.e. routing information is provided
to all RS Cluster clients as long as there is
at least one functional route rver in the RS
Cluster.
RCID - Cluster ID
3. RS Model
In the propod scheme a Route Server (RS) does not apply any
lection criteria to the routes received from border routers for the purpo of distributing the routes to its clients. All routes
acquired from border routers or other Route Servers are relayed to
the client border routers.
There can be two class of Route Servers: Route Servers that relay
external routes between routers in a single routing domain and Route Servers that relay external routes between border routers in
different routing domains. The former are Intra-Domain Route Servers and the latter are Inter-Domain Route Servers.
In the RS model propod in this document there is no routing
exchange between Intra-Domain Route Servers and Inter-Domain Route Haskin Experimental [Page 3]
Servers. Routes that cross a domain boundary must always pass
through a border router of such a domain which may apply
administrative filters to such routes.
Operations of Intra-Domain Route Servers and Inter-Domain Route
Servers are identical.
One or more Route Servers form an RS Cluster (RSC). For redundancy’s sake two or more RSs can be configured to operate in an RS Cluster.
All route rvers in an RSC share the same clients, i.e. cluster
sitvclients establish connections to all route rvers in such an RSC for the purpo of exchanging routing information. Each cluster issound
assigned an unique RSC Identifier (RCID) reprented by a 2-octet
always on my mindunsigned integer.
2014高考试卷Clusters which provide virtual connectivity between their clients
would be normally exchanging routing information among themlves so that all external routes ar
e propagated to all participating clients. Though a Route Server Client (RC) can be associated with multiple
RSC, it ems that there is no real advantage of doing so except for a short transition period to provide a graceful re-assignment from
one RSC to another or, if for some reason, there are multiple RS
groups that don’t exchange routing information with each other.
The inter-cluster route exchange can be accomplished by forming a
full mesh routing adjacency between clusters. In this approach,
illustrated in the diagram below, each RS in each RSC would maintain a routing connection with every RS in other RS clusters. Only routes that are acquired from border routers are propagated to RSs in other RS clusters.
BR11 BR12 BR1n BR21 BR22 BR2n
2012高考时间
| | ... | | | ... |
----------------- ------------------
! RS11 RS12 ! --- ! RS21 RS22 !
----------------- ------------------
<RSC#1> \ / <RSC#2>
\ /
-----------------
! RS31 RS32 ! <RSC#3>
-----------------
| | ... |
BR31 BR32 BR3n
Another way to propagate routing information between clusters would
be to form a cluster hierarchy in which an RS in one cluster
maintains ssions only with RSs in designated clusters. In this Haskin Experimental [Page 4]
approach an RS must adverti all acquired routes to an RS in another cluster except the routes that are acquired from that cluster.
Nevertheless, it allows for minimizing the number of routing
ssions which can be highly desirable in some network. It is
important for the hierarchical scheme that the inter-cluster route
step
exchange links form a tree, i.e. there is only one route propagation path between any two clusters, otherwi routing loops may result.
For detection and pruning of routing loops in a hierarchical cluster topology, it is advisable to include the "RCID Path" attribute (e
4.3.4) in all routing updates nt between route rvers. This
attribute lists IDs of all clusters in the route propagation path.
When a duplicate ID is detected in this attribute an offending route needs to be discarded.
The diagram below which illustrates the hierarchical approach is
created from the diagram above by removing the route exchange link
请假条英文between clusters 2 and 3.
BR11 BR12 BR1n BR21 BR22 BR2n
| | ... | | | ... |
银行流水翻译
----------------- ------------------
! RS11 RS12 ! --- ! RS21 RS22 !
----------------- ------------------
<RSC#1> \ <RSC#2>
\
-----------------
! RS31 RS32 ! <RSC#3>
-----------------
| | ... |
BR31 BR32 BR3n
It ems that the only disadvantage of the hierarchical model, is the management headache of avoiding routing loops and redundant
information flow by insuring that inter-cluster links always form a
tree. But more study is needed to fully evaluate the comparative
merits of the full-mesh and hierarchical models.
Since RSs in the same cluster maintain routing ssions with the same t of clients, it may em that there is no need to exchange routing information between RSs in the same cluster. Nevertheless, such a
route exchange may help to maintain identical routing databas in
圣诞节祝福语英文版the rvers during client acquisition periods and when a partial
failure may affect some routing ssions.
Route rvers in the same RS cluster exchange control messages in
attempt to subdivide the responsibilities of providing routing
information to their clients. In order to simplify the RS design,
the RS messaging is implemented on top of exterior protocol which is Haskin Experimental [Page 5]
