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Let's say, that one has a following simple network topology:

r1 ◄ ► r3

▲      ▲
▼      ▼

r2 ◄ ► r4

All four routers speak RIPv2 on all interfaces. r1 and r2 announce the default route which means that r4 receives the default route from r3(it learned it from r1 via RIP) and r2. For example, here is the packet capture from interface of r4 facing the r3:

18:20:23.759926 02:06:dd:04:ff:f1 > 01:00:5e:00:00:09, ethertype IPv4 (0x0800), length 326: (tos 0xc0, ttl 1, id 50254, offset 0, flags [none], proto UDP (17), length 312)
    10.10.129.1.520 > 224.0.0.9.520: [udp sum ok] 
        RIPv2, Response, length: 284, routes: 14 or less
          Auth header: Packet Len 264, Key-ID 1, Auth Data Len 20, SeqNo 1591543212, MBZ 0, MBZ 0
          AFI IPv4,       200.0.1.0/24, tag 0x0000, metric: 3, next-hop: self
          AFI IPv4,        10.5.5.0/24, tag 0x0000, metric: 1, next-hop: self
          AFI IPv4,     10.10.128.1/32, tag 0x0000, metric: 1, next-hop: self
          AFI IPv4,     192.168.1.0/30, tag 0x0000, metric: 1, next-hop: self
          AFI IPv4,     192.168.3.0/24, tag 0x0000, metric: 1, next-hop: self
          AFI IPv4,         0.0.0.0/0 , tag 0x0064, metric: 2, next-hop: self
          AFI IPv4,      10.1.254.0/24, tag 0x0000, metric: 2, next-hop: self
          AFI IPv4,   10.10.128.100/32, tag 0x0000, metric: 2, next-hop: self
          AFI IPv4,   10.10.128.200/32, tag 0x0000, metric: 3, next-hop: self
          AFI IPv4,       100.0.0.0/24, tag 0x0000, metric: 2, next-hop: self
          AFI IPv4,     200.0.100.0/24, tag 0x0000, metric: 4, next-hop: self
          AFI IPv4,     200.0.200.0/24, tag 0x0000, metric: 5, next-hop: self
          Auth trailer:
          0x0000:  b144 2983 094a 463d f34f 96d3 77e3 9809

As seen above, the r3 indeed announces the default route. However, it is not seen from the r4:

root@r4> show route receive-protocol rip 10.10.129.1 0.0.0.0      

inet.0: 20 destinations, 21 routes (20 active, 0 holddown, 0 hidden)

root@r4> 

I understand that r4 prefers the default route via r2 as it has the lower hop count, but why isn't the default route announced by r3 seen the the output of show route receive-protocol rip or as a non-active route in the output of show route protocol rip in r4?

  • 3
    Because RIP is a simple protocol. If the route it uses fails, then it must wait for the next update period until it can replace it. That is why other routing protocols are preferred, and RIP is rarely used. – Ron Maupin Jun 7 at 16:35
  • What do you mean by if the route it uses fails? Does RIP work in a way that there can't be multiple routes to the same prefix? So in my example, there is already a route to 0.0.0.0/0 via r2 <-> r4 path and thus RIP can't install another 0.0.0.0/0 prefix via r1 <-> r3 <-> r4 path to r4? – Martin Jun 8 at 9:08
  • 1
    "Does RIP work in a way that there can't be multiple routes to the same prefix?" Exactly. RIP is a simple routing protocol. It was not even until RIPv2 that it got triggered updates. RIP was created before there was a separate RIB and FIB. – Ron Maupin Jun 8 at 13:01

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