Routing

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30 Οκτ 2013 (πριν από 3 χρόνια και 7 μήνες)

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Redundancy



Single point of failure


Hierarchical design produces many single
points of failure


Redundancy provides alternate paths, but
may undermine stability


Fig 3
-
1 Retana



Issues and strategies of
redundancy


Redundant paths should be used only when
the normal path is broken


Load sharing must be carefully engineered
to prevent instability


Traffic shouldn’t pass through devices and
links not designed for through traffic


Redundant core design


Reduce hop count


Increase the number of failures the core can
withstand

Ring core design


Two paths for each pair of routers


Diameter is N/2


Losing a link increases the diameter


Losing any two links isolates a piece of network

(Analysis)


Reduced number of available paths


Number of hops increases dramatically on a sing
link failure
--
> session timeout

Ring core



Redundant fiber ring
technology


SONET: a pair of fiber optic links of OC
-
48(2.4G), 2nd fiber is used as backup


FDDI: two rings passing data in opposite
directions. If fiber fails, the ring will wrap
using dual attached nodes, healing the break


Redundancy in layer 2

Full mesh core design


Large number of alternate paths


Two hop path to any destination


N hops maximum in the worst case


Highest redundancy

(Analysis)


Smallest hop count


Maximum redundancy


Too many alternate paths
--
> larger convergence
time, difficult to engineer traffic

Full mesh core



Partial mesh core design


Good compromise in hop count, redundancy and
number of paths


No more than three hops for Fig. 3
-
6


If any single link fails, maximum number of hops
will increase to four


Provides full connectivity with three links down as
long as no single router loses both of its
connections


Some routing protocols don’t handle multipoint
partial mesh

Partial mesh core



OSPF on NBMA networks


PVCs on Frame relay, ATM has non
-
broadcast multiple access(NBMA)

Solutions


Problem: non
-
broadcast, since a packet on a
subnet might not be seen by all routers connected
to it, making it hard to choose a DR(designated
router)


Solution:

1) Configure priorities to select Router A as DR

2) Treat all as point
-
to
-
point link
--
> subnet id space
wasted

3) make all the links in the multipoint network to
have same netid
--
> should add a lot of
host

routes
to each hosts’ routing table

Distribution redundancy


Dual homing


Backup links to other distribution layer


Consider unexpected traffic patterns

Dual homing to the core


Doubles the number of available paths to
core routers, slowing network convergence



Dual homing to the core


Double avail. paths:
--
> Advertise only over
one link unless that link becomes unusable


Distribution routers may be drawn into core
role, passing through traffic:
-
> prevent D
from advertising routes learned from C back
to B

Redundant links to other
distribution layer devices



Drawbacks


Double the core’s routing table size


Possible use of redundant paths for core
traffic


Prefer redundant links to passing core


Routing info will leak from A and spread to
other distribution layer routers, causing
instability

Access redundancy


Dual homed
--
> size of routing table in the
core increases: prevent advertising
redundant links

Access redundancy


Provide redundant links in access layer:
saves one link and reduces number of
available paths
--
> should provide enough
bandwidth to handle traffic from both
remote sites to core

Access redundancy

Redundancy through same
distribution layer branch


Load sharing and redundancy


Redundancy through same
distribution layer branch


Filter in router G, H prevents advertising
networks not below them in hierarchy such
as D, F to C and E


Or use dial
-
up

Connections to common services



Case: what’s the best route?


How to determine best routes from a mix of
different routing metrics?

Ex) RIP uses hop count, EIGRP uses bandwidth and
delay


Longest match > adminstrative distance > metric


Each routing protocol has a default adminstrative
distance (lower prefered)


connected: 0 static: 1, OSPF: 110, RIP: 120


floating static routes: a static route with high
adminstrative distance, 200 or more used for
backing up or conditionally advertised (BGP)

Case: Redundancy at layer 2
using switches


(Using spanning tree avoiding loops at layer
2, block ports)


Dial
-
up backup with a single
router


Configure ISDN link as a backup interface