Administrative Distance; Stub Routing - Cisco Nexus 3000 Series Configuration Manual
Nx-os unicast routing configuration guide,
nx-os release 5.0(3)u1(1)
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Also See for Nexus 3000 Series:
- Command reference manual (356 pages) ,
- Configuration manual (182 pages) ,
- Installation manual (112 pages)
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Chapter 1
Overview
S e n d d o c u m e n t c o m m e n t s t o n e x u s 3 k - d o c f e e d b a c k @ c i s c o . c o m
Administrative Distance
An
administrative distance
the value, the lower the trust rating. Typically, a route can be learned through more than one protocol.
Administrative distance is used to discriminate between routes learned from more than one protocol. The
route with the lowest administrative distance is installed in the IP routing table.
Stub Routing
You can use stub routing in a hub-and-spoke network topology, where one or more end (stub) networks
are connected to a remote router (the spoke) that is connected to one or more distribution routers (the
hub). The remote router is adjacent only to one or more distribution routers. The only route for IP traffic
to follow into the remote router is through a distribution router. This type of configuration is commonly
used in WAN topologies in which the distribution router is directly connected to a WAN. The distribution
router can be connected to many more remote routers. Often, the distribution router is connected to 100
or more remote routers. In a hub-and-spoke topology, the remote router must forward all nonlocal traffic
to a distribution router, so it becomes unnecessary for the remote router to hold a complete routing table.
Generally, the distribution router sends only a default route to the remote router.
Only specified routes are propagated from the remote (stub) router. The stub router responds to all
queries for summaries, connected routes, redistributed static routes, external routes, and internal routes
with the message "inaccessible." A router that is configured as a stub sends a special peer information
packet to all neighboring routers to report its status as a stub router.
Any neighbor that receives a packet informing it of the stub status does not query the stub router for any
routes, and a router that has a stub peer does not query that peer. The stub router depends on the
distribution router to send the proper updates to all peers.
Figure 1-2
Figure 1-2
Stub routing does not prevent routes from being advertised to the remote router.
the remote router can access the corporate network and the Internet through the distribution router only.
A full route table on the remote router, in this example, serves no functional purpose because the path to
the corporate network and the Internet would always be through the distribution router. A larger route
table would reduce only the amount of memory required by the remote router. The bandwidth and
memory used can be lessened by summarizing and filtering routes in the distribution router. In this
network topology, the remote router does not need to receive routes that have been learned from other
networks because the remote router must send all nonlocal traffic, regardless of its destination, to the
distribution router. To configure a true stub network, you should configure the distribution router to send
only a default route to the remote router.
is a rating of the trustworthiness of a routing information source. The higher
shows a simple hub-and-spoke configuration.
Simple Hub-and-Spoke Network
Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, NX-OS Release 5.0(3)U1(1)
Information About Layer 3 Unicast Routing
Figure 1-2
shows that
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