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