Jaycee's Networking

August 21, 2009

Router Design Concept

Filed under: IOS, Routing Design — Tags: , — Jaycee @ 3:58 pm
GRES (graceful Routing Engine switchover) – In a router that contains a master and a backup Routing Engine, allows the backup Routing Engine to assume mastership automatically, with no disruption of packet forwarding.
Graceful switchover — JUNOS software feature that allows a change from the primary device, such as a Routing Engine, to the backup device without interruption of packet forwarding.

(lecture by Tim Chung)

1. Basic Router and Routing:


a. R1 and R2 has routing protocol (i.e RIP or OSPF), so the computer can reach the destination server

b. R2 is a single CPU router which is like a Linux server doing a routing job.

c. A single CPU needs to process all of the packets whichever goes through the router. If the computer is sending too many data packets through R2, then the CPU of R2 is going to be occupied by the data packets.

d. When CPU is too busy (up to 99%~100% usage) on processing the data packets, other important packets for control, such as routing protocols, SNMP, wouldn’t be processed in time which would cause routing adjacency dropped. All of the data packets would not reach the destination.

e. Thus, Cisco 2800 series can only do T1 since it’s a single RISC processor, and Juniper J-series is also single IBM CPU. They both couldn’t handle high traffic. They are both software based routers.

2. For modern routers, they have more than 1 CPU doing data packet forwarding and processing control information.


a. Take Juniper router as an example, a router has 2 plane: RE and PFE. All of the data packets going through PFE and goes out.

b. PFE passes all important control packets to RE.

c. In this way, router wouldn’t drop the adjacency which wont lose the routes. Data packets can be sent to the destination.

3. For Redundancy:


a. Uses fabric between RE and PFE and PIC for high traffic transmissions.

b. Uses full-mesh x-bar for PFEs.

4. For more redundancy with GRES:



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