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Lecture 15

CSE 123 Lecture Notes - Lecture 15: Routing Information Protocol, Routing Protocol, Enhanced Interior Gateway Routing Protocol

Computer Science and Engineering
Course Code
CSE 123
Alex Snoeren

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Mitigation Strategies
Hold Downs
- As metric increases, “delay” propagating info
- Limitation: delays convergence (deliberating withholding info)
Loop Avoidance
- Full path info in route advertisement
- “Explicit” queries for loops
Note: these do NOT work all the time
Split Horizon
- NEVER advertise a destination through its next hop
- Tell some neighbors one thing, tell others another
- Poison Reverse: Seng negative info when advertising a destination through its next hop
- Some neighbors you say “I CANNOT GET THERE”
- Limtiation: only works for “loop”s of size 2
- DELIBERATELY poisoning
- ONLY for the destinations for which you are using
Split Horizon Limitations
Only one link between A and D….if the link breaks, we’re toast!
A tells B&C that D is unreachable
B computes new route through C
- Tells C that D is unreachable (poison reverse)
- Tells A it has path of cost 3 (split horizon doesn’t apply)
A computes new route through B
- A tells C that D is NOW reachable!
In Practice
RIP (Routing Information Protocol)
- Distance vector where edges are unit costs
- Routers send vectors every 30s
EIGRP (proprietary Cisco protocol)
- DUAL alg (ensures loop-freedom)
- Computes “fancy” edge weights
- ONLY communicates changes
DV Summary
DV computes shortest path!
Weak at adapting to changes out of the box! (loops and count to infinity)
The Internet is Complicated
At school: Large organization → large service providers
At home: consumer ISP → backbone service provider (Verizon, Sprint, AT&T, etc.)
- Connect through “peering points”: very large buildings full of routers (they “peer”)
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